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Chapter 1. Introduction

1 .1 Historical Perspective

Materials are so important in the development of civilization that we associate Ages with them. In the origin of human life on Earth, the Stone Age, people used only natural materials, like stone, clay, skins, and wood. When people found copper and how to make it harder by alloying, the Bronze Age started about 3000 BC. The use of iron and steel, a stronger material that gave advantage in wars started at about 1200 BC. The next big step was the discovery of a cheap process to make steel around 1850, which enabled the railroads and the building of the modern infrastructure of the industrial world.

1.2 Materials Science and Engineering

Understanding of how materials behave like they do, and why they differ in properties was only possible with the atomistic understanding allowed by quantum mechanics, that first explained atoms and then solids starting in the 1930s. The combination of physics, chemistry, and the focus on the relationship between the properties of a material and its microstructure is the domain of Materials Science. The development of this science allowed designing materials and provided a knowledge base for the engineering applications (Materials Engineering).

Structure:

Properties are the way the material responds to the environment. For instance, the mechanical, electrical and magnetic properties are the responses to mechanical, electrical and magnetic forces, respectively. Other important properties are thermal (transmission of heat, heat capacity), optical (absorption, transmission and scattering of light), and the chemical stability in contact with the environment (like corrosion resistance).

Processing of materials is the application of heat (heat treatment), mechanical forces, etc. to affect their microstructure and, therefore, their properties.

1.3 Why Study Materials Science and Engineering?

All engineering disciplines need to know about materials. Even the most "immaterial", like software or system engineering depend on the development of new materials, which in turn alter the economics, like software-hardware trade-offs. Increasing applications of system engineering are in materials manufacturing (industrial engineering) and complex environmental systems.

1.4 Classification of Materials

Like many other things, materials are classified in groups, so that our brain can handle the complexity. One could classify them according to structure, or properties, or use. The one that we will use is according to the way the atoms are bound together:

Metals: valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions together. Metals are usually strong, conduct electricity and heat well and are opaque to light (shiny if polished). Examples: aluminum, steel, brass, gold.

Semiconductors: the bonding is covalent (electrons are shared between atoms). Their electrical properties depend extremely strongly on minute proportions of contaminants. They are opaque to visible light but transparent to the infrared. Examples: Si, Ge, GaAs.

Ceramics: atoms behave mostly like either positive or negative ions, and are bound by Coulomb forces between them. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Examples: glass, porcelain, many minerals.

Polymers: are bound by covalent forces and also by weak van der Waals forces, and usually based on H, C and other non-metallic elements. They decompose at moderate temperatures (100 – 400 C), and are lightweight. Other properties vary greatly. Examples: plastics (nylon, Teflon, polyester) and rubber.

Other categories are not based on bonding. A particular microstructure identifies composites, made of different materials in intimate contact (example: fiberglass, concrete, wood) to achieve specific properties. Biomaterials can be any type of material that is biocompatible and used, for instance, to replace human body parts.

1.5 Advanced Materials

Materials used in "High-Tec" applications, usually designed for maximum performance, and normally expensive. Examples are titanium alloys for supersonic airplanes, magnetic alloys for computer disks, special ceramics for the heat shield of the space shuttle, etc.

1.6 Modern Material's Needs


 
 

2.2 Fundamental Concepts

Atoms are composed of electrons, protons, and neutrons. Electron and protons are negative and positive charges of the same magnitude, 1.6 × 10-19 Coulombs.

The mass of the electron is negligible with respect to those of the proton and the neutron, which form the nucleus of the atom. The unit of mass is an atomic mass unit (amu) = 1.66 × 10-27 kg, and equals 1/12 the mass of a carbon atom. The Carbon nucleus has Z=6, and A=6, where Z is the number of protons, and A the number of neutrons. Neutrons and protons have very similar masses, roughly equal to 1 amu. A neutral atom has the same number of electrons and protons, Z.

A mole is the amount of matter that has a mass in grams equal to the atomic mass in amu of the atoms. Thus, a mole of carbon has a mass of 12 grams. The number of atoms in a mole is called the Avogadro number, Nav = 6.023 × 1023. Note that Nav = 1 gram/1 amu.

Calculating n, the number of atoms per cm3 in a piece of material of density d (g/cm3).

n = Nav × d / M

where M is the atomic mass in amu (grams per mol). Thus, for graphite (carbon) with a density d = 1.8 g/cm3, M =12, we get 6 × 1023 atoms/mol × 1.8 g/cm3 / 12 g/mol) = 9 × 1022 C/cm3.

For a molecular solid like ice, one uses the molecular mass, M(H2O) = 18. With a density of 1 g/cm3, one obtains n = 3.3 × 1022 H2O/cm3. Note that since the water molecule contains 3 atoms, this is equivalent to 9.9 × 1022 atoms/cm3.

Most solids have atomic densities around 6 × 1022 atoms/cm3. The cube root of that number gives the number of atoms per centimeter, about 39 million. The mean distance between atoms is the inverse of that, or 0.25 nm. This is an important number that gives the scale of atomic structures in solids.

2.3 Electrons in Atoms

The forces in the atom are repulsions between electrons and attraction between electrons and protons. The neutrons play no significant role. Thus, Z is what characterizes the atom.

The electrons form a cloud around the neutron, of radius of 0.05 – 2 nanometers. Electrons do not move in circular orbits, as in popular drawings, but in 'fuzzy' orbits. We cannot tell how it moves, but only say what is the probability of finding it at some distance from the nucleus. According to quantum mechanics, only certain orbits are allowed (thus, the idea of a mini planetary system is not correct). The orbits are identified by a principal quantum number n, which can be related to the size, n = 0 is the smallest; n = 1, 2 .. are larger. (They are "quantized" or discrete, being specified by integers). The angular momentum l is quantized, and so is the projection in a specific direction m. The structure of the atom is determined by the Pauli exclusion principle, only two electrons can be placed in an orbit with a given n, l, m – one for each spin. Table 2.1 in the textbook gives the number of electrons in each shell (given by n) and subshells (given by l).

2.4 The Periodic Table

Elements are categorized by placing them in the periodic table. Elements in a column share similar properties. The noble gases have closed shells, and so they do not gain or lose electrons near another atom. Alkalis can easily lose an electron and become a closed shell; halogens can easily gain one to form a negative ion, again with a closed shell. The propensity to form closed shells occurs in molecules, when they share electrons to close a molecular shell. Examples are H2, N2, and NaCl.

The ability to gain or lose electrons is termed electronegativity or electropositivity, an important factor in ionic bonds.

2.5 Bonding Forces and Energies

The Coulomb forces are simple: attractive between electrons and nuclei, repulsive between electrons and between nuclei. The force between atoms is given by a sum of all the individual forces, and the fact that the electrons are located outside the atom and the nucleus in the center.

When two atoms come very close, the force between them is always repulsive, because the electrons stay outside and the nuclei repel each other. Unless both atoms are ions of the same charge (e.g., both negative) the forces between atoms is always attractive at large internuclear distances r. Since the force is repulsive at small r, and attractive at small r, there is a distance at which the force is zero. This is the equilibrium distance at which the atoms prefer to stay.

The interaction energy is the potential energy between the atoms. It is negative if the atoms are bound and positive if they can move away from each other. The interaction energy is the integral of the force over the separation distance, so these two quantities are directly related. The interaction energy is a minimum at the equilibrium position. This value of the energy is called the bond energy, and is the energy needed to separate completely to infinity (the work that needs to be done to overcome the attractive force.) The strongest the bond energy, the hardest is to move the atoms, for instance the hardest it is to melt the solid, or to evaporate its atoms.

2.6 Primary Interatomic Bonds

Ionic Bonding

This is the bond when one of the atoms is negative (has an extra electron) and another is positive (has lost an electron). Then there is a strong, direct Coulomb attraction. An example is NaCl. In the molecule, there are more electrons around Cl, forming Cl- and less around Na, forming Na+. Ionic bonds are the strongest bonds. In real solids, ionic bonding is usually combined with covalent bonding. In this case, the fractional ionic bonding is defined as %ionic = 100 × [1 – exp(-0.25 (XA – XB)2], where XA and XB are the electronegativities of the two atoms, A and B, forming the molecule.

Covalent Bonding

In covalent bonding, electrons are shared between the molecules, to saturate the valency. The simplest example is the H2 molecule, where the electrons spend more time in between the nuclei than outside, thus producing bonding.

Metallic Bonding

In metals, the atoms are ionized, loosing some electrons from the valence band. Those electrons form a electron sea, which binds the charged nuclei in place, in a similar way that the electrons in between the H atoms in the H2 molecule bind the protons.

2.7 Secondary Bonding (Van der Waals)

Fluctuating Induced Dipole Bonds

Since the electrons may be on one side of the atom or the other, a dipole is formed: the + nucleus at the center, and the electron outside. Since the electron moves, the dipole fluctuates. This fluctuation in atom A produces a fluctuating electric field that is felt by the electrons of an adjacent atom, B. Atom B then polarizes so that its outer electrons are on the side of the atom closest to the + side (or opposite to the – side) of the dipole in A. This bond is called van der Waals bonding.

Polar Molecule-Induced Dipole Bonds

A polar molecule like H2O (Hs are partially +, O is partially – ), will induce a dipole in a nearby atom, leading to bonding.

Permanent Dipole Bonds

This is the case of the hydrogen bond in ice. The H end of the molecule is positively charged and can bond to the negative side of another dipolar molecule, like the O side of the H2O dipole.

2.8 Molecules

If molecules formed a closed shell due to covalent bonding (like H2, N2) then the interaction between molecules is weak, of the van der Waals type. Thus, molecular solids usually have very low melting points.
 
 

Crystal Structures

3.2 Fundamental Concepts

Atoms self-organize in crystals, most of the time. The crystalline lattice, is a periodic array of the atoms. When the solid is not crystalline, it is called amorphous. Examples of crystalline solids are metals, diamond and other precious stones, ice, graphite. Examples of amorphous solids are glass, amorphous carbon (a-C), amorphous Si, most plastics

To discuss crystalline structures it is useful to consider atoms as being hard spheres, with well-defined radii. In this scheme, the shortest distance between two like atoms is one diameter.

 

3.3 Unit Cells

The unit cell is the smallest structure that repeats itself by translation through the crystal. We construct these symmetrical units with the hard spheres. The most common types of unit cells are the faced-centered cubic (FCC), the body-centered cubic (FCC) and the hexagonal close-packed (HCP). Other types exist, particularly among minerals. The simple cube (SC) is often used for didactical purpose, no material has this structure.

3.4 Metallic Crystal Structures

Important properties of the unit cells are

Unit Cell n CN a/R APF
SC 1 6 2 0.52
BCC 2 8 4Ö 3 0.68
FCC 4 12 2Ö 2 0.74
HCP 6 12   0.74

The closest packed direction in a BCC cell is along the diagonal of the cube; in a FCC cell is along the diagonal of a face of the cube.

 

3.5 Density Computations

The density of a solid is that of the unit cell, obtained by dividing the mass of the atoms (n atoms x Matom) and dividing by Vc the volume of the cell (a3 in the case of a cube). If the mass of the atom is given in amu (A), then we have to divide it by the Avogadro number to get Matom. Thus, the formula for the density is:
 

3.6 Polymorphism and Allotropy

Some materials may exist in more than one crystal structure, this is called polymorphism. If the material is an elemental solid, it is called allotropy. An example of allotropy is carbon, which can exist as diamond, graphite, and amorphous carbon.

3.7 – 3.10 Crystallography – Not Covered

3.11 Close-Packed Crystal Structures

The FCC and HCP are related, and have the same APF. They are built by packing spheres on top of each other, in the hollow sites (Fig. 3.12 of book). The packing is alternate between two types of sites, ABABAB.. in the HCP structure, and alternates between three types of positions, ABCABC… in the FCC crystals.

 

Crystalline and Non-Crystalline Materials

3.12 Single Crystals

Crystals can be single crystals where the whole solid is one crystal. Then it has a regular geometric structure with flat faces.

3.13 Polycrystalline Materials

A solid can be composed of many crystalline grains, not aligned with each other. It is called polycrystalline. The grains can be more or less aligned with respect to each other. Where they meet is called a grain boundary.

3.14 Anisotropy

Different directions in the crystal have a different packing. For instance, atoms along the edge FCC crystals are more separated than along the face diagonal. This causes anisotropy in the properties of crystals; for instance, the deformation depends on the direction in which a stress is applied.

3.16 Non-Crystalline Solids

In amorphous solids, there is no long-range order. But amorphous does not mean random, since the distance between atoms cannot be smaller than the size of the hard spheres. Also, in many cases there is some form of short-range order. For instance, the tetragonal order of crystalline SiO2 (quartz) is still apparent in amorphous SiO2 (silica glass.)
 
 

Imperfections in Solids

4.1 Introduction

Materials are often stronger when they have defects. The study of defects is divided according to their dimension:
0D (zero dimension) – point defects: vacancies and interstitials. Impurities.
1D – linear defects: dislocations (edge, screw, mixed)
2D – grain boundaries, surfaces.
3D – extended defects: pores, cracks.

Point Defects

4.2 Vacancies and Self-Interstitials

A vacancy is a lattice position that is vacant because the atom is missing. It is created when the solid is formed. There are other ways of making a vacancy, but they also occur naturally as a result of thermal vibrations.

An interstitial is an atom that occupies a place outside the normal lattice position. It may be the same type of atom as the others (self interstitial) or an impurity atom.

In the case of vacancies and interstitials, there is a change in the coordination of atoms around the defect. This means that the forces are not balanced in the same way as for other atoms in the solid, which results in lattice distortion around the defect.

The number of vacancies formed by thermal agitation follows the law:

NV = NA × exp(-QV/kT)

where NA is the total number of atoms in the solid, QV is the energy required to form a vacancy, k is Boltzmann constant, and T the temperature in Kelvin (note, not in oC or oF).

When QV is given in joules, k = 1.38 × 10-23 J/atom-K. When using eV as the unit of energy, k = 8.62 × 10-5 eV/atom-K.

Note that kT(300 K) = 0.025 eV (room temperature) is much smaller than typical vacancy formation energies. For instance, QV(Cu) = 0.9 eV/atom. This means that NV/NA at room temperature is exp(-36) = 2.3 × 10-16, an insignificant number. Thus, a high temperature is needed to have a high thermal concentration of vacancies. Even so, NV/NA is typically only about 0.0001 at the melting point.

4.3 Impurities in Solids

All real solids are impure. A very high purity material, say 99.9999% pure (called 6N – six nines) contains ~ 6 × 1016 impurities per cm3.

Impurities are often added to materials to improve the properties. For instance, carbon added in small amounts to iron makes steel, which is stronger than iron. Boron impurities added to silicon drastically change its electrical properties.

Solid solutions are made of a host, the solvent or matrix) which dissolves the solute (minor component). The ability to dissolve is called solubility. Solid solutions are:

Factors for high solubility Composition can be expressed in weight percent, useful when making the solution, and in atomic percent, useful when trying to understand the material at the atomic level.

Miscellaneous Imperfections

4.4 Dislocations—Linear Defects

Dislocations are abrupt changes in the regular ordering of atoms, along a line (dislocation line) in the solid. They occur in high density and are very important in mechanical properties of material. They are characterized by the Burgers vector, found by doing a loop around the dislocation line and noticing the extra interatomic spacing needed to close the loop. The Burgers vector in metals points in a close packed direction.

Edge dislocations occur when an extra plane is inserted. The dislocation line is at the end of the plane. In an edge dislocation, the Burgers vector is perpendicular to the dislocation line.

Screw dislocations result when displacing planes relative to each other through shear. In this case, the Burgers vector is parallel to the dislocation line.

4.5 Interfacial Defects

The environment of an atom at a surface differs from that of an atom in the bulk, in that the number of neighbors (coordination) decreases. This introduces unbalanced forces which result in relaxation (the lattice spacing is decreased) or reconstruction (the crystal structure changes).

The density of atoms in the region including the grain boundary is smaller than the bulk value, since void space occurs in the interface.

Surfaces and interfaces are very reactive and it is usual that impurities segregate there. Since energy is required to form a surface, grains tend to grow in size at the expense of smaller grains to minimize energy. This occurs by diffusion, which is accelerated at high temperatures.

Twin boundaries: not covered

4.6 Bulk or Volume Defects

A typical volume defect is porosity, often introduced in the solid during processing. A common example is snow, which is highly porous ice.

4.7 Atomic Vibrations

Atomic vibrations occur, even at zero temperature (a quantum mechanical effect) and increase in amplitude with temperature. Vibrations displace transiently atoms from their regular lattice site, which destroys the perfect periodicity we discussed in Chapter 3.
 
 

5.1 Introduction

Many important reactions and processes in materials occur by the motion of atoms in the solid (transport), which happens by diffusion.

Inhomogeneous materials can become homogeneous by diffusion, if the temperature is high enough (temperature is needed to overcome energy barriers to atomic motion.

5.2 Diffusion Mechanisms

Atom diffusion can occur by the motion of vacancies (vacancy diffusion) or impurities (impurity diffusion). The energy barrier is that due to nearby atoms which need to move to let the atoms go by. This is more easily achieved when the atoms vibrate strongly, that is, at high temperatures.

There is a difference between diffusion and net diffusion. In a homogeneous material, atoms also diffuse but this motion is hard to detect. This is because atoms move randomly and there will be an equal number of atoms moving in one direction than in another. In inhomogeneous materials, the effect of diffusion is readily seen by a change in concentration with time. In this case there is a net diffusion. Net diffusion occurs because, although all atoms are moving randomly, there are more atoms moving in regions where their concentration is higher.

5.3 Steady-State Diffusion

The flux of diffusing atoms, J, is expressed either in number of atoms per unit area and per unit time (e.g., atoms/m2-second) or in terms of mass flux (e.g., kg/m2-second).

Steady state diffusion means that J does not depend on time. In this case, Fick’s first law holds that the flux along direction x is:

J = – D dC/dx

Where dC/dx is the gradient of the concentration C, and D is the diffusion constant. The concentration gradient is often called the driving force in diffusion (but it is not a force in the mechanistic sense). The minus sign in the equation means that diffusion is down the concentration gradient.

5.4 Nonsteady-State Diffusion

This is the case when the diffusion flux depends on time, which means that a type of atoms accumulates in a region or that it is depleted from a region (which may cause them to accumulate in another region). The relationship of the flux and the concentration gradient is given by a differential equation, Fick’s second law:

Ct = D 2C/¶ x2

The solution depends on the particular value of the boundary conditions (initial values of the concentration and its derivatives.)

5.5 Factors That Influence Diffusion

As stated above, there is a barrier to diffusion created by neighboring atoms that need to move to let the diffusing atom pass. Thus, atomic vibrations created by temperature assist diffusion. Also, smaller atoms diffuse more readily than big ones, and diffusion is faster in open lattices or in open directions. Similar to the case of vacancy formation, the effect of temperature in diffusion is given by a Boltzmann factor: D = D0 × exp(–Qd/kT).

5.6 Other Diffusion Paths

Diffusion occurs more easily along surfaces, and voids in the material (short circuits like dislocations and grain boundaries) because less atoms need to move to let the diffusing atom pass. Short circuits are often unimportant because they constitute a negligible part of the total area of the material normal to the diffusion flux. .
 
 

Mechanical Properties of Metals

    1. Introduction

    2.  

       

      Often materials are subject to forces (loads) when they are used. Mechanical engineers calculate those forces and material scientists how materials deform (elongate, compress, twist) or break as a function of applied load, time, temperature, and other conditions.

      Materials scientists learn about these mechanical properties by testing materials. Results from the tests depend on the size and shape of material to be tested (specimen), how it is held, and the way of performing the test. That is why we use common procedures, or standards, which are published by the ASTM.

    3. Concepts of Stress and Strain

    4.  

       

      To compare specimens of different sizes, the load is calculated per unit area, also called normalization to the area. Force divided by area is called stress. In tension and compression tests, the relevant area is that perpendicular to the force. In shear or torsion tests, the area is perpendicular to the axis of rotation.

      s = F/A0 tensile or compressive stress
      t = F/A0 shear stress
      The unit is the Megapascal = 106 Newtons/m2.

      There is a change in dimensions, or deformation elongation, DL as a result of a tensile or compressive stress. To enable comparison with specimens of different length, the elongation is also normalized, this time to the length L. This is called strain, e.

      e = DL/L
      The change in dimensions is the reason we use A0 to indicate the initial area since it changes during deformation. One could divide force by the actual area, this is called true stress (see Sec. 6.7).

      For torsional or shear stresses, the deformation is the angle of twist, q (Fig. 6.1) and the shear strain is given by:

      g = tg q
    5. Stress—Strain Behavior

    6.  

       

      Elastic deformation. When the stress is removed, the material returns to the dimension it had before the load was applied. Valid for small strains (except the case of rubbers).

      Deformation is reversible, non permanent

      Plastic deformation. When the stress is removed, the material does not return to its previous dimension but there is a permanent, irreversible deformation.

      In tensile tests, if the deformation is elastic, the stress-strain relationship is called Hooke's law:

      s = E e
      That is, E is the slope of the stress-strain curve. E is Young's modulus or modulus of elasticity. In some cases, the relationship is not linear so that E can be defined alternatively as the local slope:
      E = ds/de
      Shear stresses produce strains according to:
      t = G g
      where G is the shear modulus.

      Elastic moduli measure the stiffness of the material. They are related to the second derivative of the interatomic potential, or the first derivative of the force vs. internuclear distance (Fig. 6.6). By examining these curves we can tell which material has a higher modulus. Due to thermal vibrations the elastic modulus decreases with temperature. E is large for ceramics (stronger ionic bond) and small for polymers (weak covalent bond). Since the interatomic distances depend on direction in the crystal, E depends on direction (i.e., it is anisotropic) for single crystals. For randomly oriented policrystals, E is isotropic.

    7. Anelasticity

    8.  

       

      Here the behavior is elastic but not the stress-strain curve is not immediately reversible. It takes a while for the strain to return to zero. The effect is normally small for metals but can be significant for polymers.

    9. Elastic Properties of Materials

    10.  

       

      Materials subject to tension shrink laterally. Those subject to compression, bulge. The ratio of lateral and axial strains is called the Poisson's ratio n.

      n = elateral/eaxial
      The elastic modulus, shear modulus and Poisson's ratio are related by E = 2G(1+n)
    11. Tensile Properties

    12.  

       

      Yield point. If the stress is too large, the strain deviates from being proportional to the stress. The point at which this happens is the yield point because there the material yields, deforming permanently (plastically).

      Yield stress. Hooke's law is not valid beyond the yield point. The stress at the yield point is called yield stress, and is an important measure of the mechanical properties of materials. In practice, the yield stress is chosen as that causing a permanent strain of 0.002 (strain offset, Fig. 6.9.)

      The yield stress measures the resistance to plastic deformation.

      The reason for plastic deformation, in normal materials, is not that the atomic bond is stretched beyond repair, but the motion of dislocations, which involves breaking and reforming bonds.

      Plastic deformation is caused by the motion of dislocations.

      Tensile strength. When stress continues in the plastic regime, the stress-strain passes through a maximum, called the tensile strength (sTS) , and then falls as the material starts to develop a neck and it finally breaks at the fracture point (Fig. 6.10).

      Note that it is called strength, not stress, but the units are the same, MPa.

      For structural applications, the yield stress is usually a more important property than the tensile strength, since once the it is passed, the structure has deformed beyond acceptable limits.

      Ductility. The ability to deform before braking. It is the opposite of brittleness. Ductility can be given either as percent maximum elongation emax or maximum area reduction.

      %EL = emax x 100 %
      %AR = (A0 - Af)/A0
      These are measured after fracture (repositioning the two pieces back together).

      Resilience. Capacity to absorb energy elastically. The energy per unit volume is the

      area under the strain-stress curve in the elastic region.

      Toughness. Ability to absorb energy up to fracture. The energy per unit volume is the total area under the strain-stress curve. It is measured by an impact test (Ch. 8).

    13. True Stress and Strain

    14.  

       

      When one applies a constant tensile force the material will break after reaching the tensile strength. The material starts necking (the transverse area decreases) but the stress cannot increase beyond sTS. The ratio of the force to the initial area, what we normally do, is called the engineering stress. If the ratio is to the actual area (that changes with stress) one obtains the true stress.

    15. Elastic Recovery During Plastic Deformation

    16.  

       

      If a material is taken beyond the yield point (it is deformed plastically) and the stress is then released, the material ends up with a permanent strain. If the stress is reapplied, the material again responds elastically at the beginning up to a new yield point that is higher than the original yield point (strain hardening, Ch. 7.10). The amount of elastic strain that it will take before reaching the yield point is called elastic strain recovery (Fig. 6. 16).

    17. Compressive, Shear, and Torsional Deformation

    18.  

       

      Compressive and shear stresses give similar behavior to tensile stresses, but in the case of compressive stresses there is no maximum in the s-e curve, since no necking occurs.

    19. Hardness

    20.  

       

      Hardness is the resistance to plastic deformation (e.g., a local dent or scratch). Thus, it is a measure of plastic deformation, as is the tensile strength, so they are well correlated. Historically, it was measured on an empirically scale, determined by the ability of a material to scratch another, diamond being the hardest and talc the softer. Now we use standard tests, where a ball, or point is pressed into a material and the size of the dent is measured. There are a few different hardness tests: Rockwell, Brinell, Vickers, etc. They are popular because they are easy and non-destructive (except for the small dent).

    21. Variability of Material Properties

    22.  

       

      Tests do not produce exactly the same result because of variations in the test equipment, procedures, operator bias, specimen fabrication, etc. But, even if all those parameters are controlled within strict limits, a variation remains in the materials, due to uncontrolled variations during fabrication, non homogenous composition and structure, etc. The measured mechanical properties will show scatter, which is often distributed in a Gaussian curve (bell-shaped), that is characterized by the mean value and the standard deviation (width).

    23. Design/Safety Factors

    24.  

       

      To take into account variability of properties, designers use, instead of an average value of, say, the tensile strength, the probability that the yield strength is above the minimum value tolerable. This leads to the use of a safety factor N > 1 (typ. 1.2 - 4). Thus, a working value for the tensile strength would be sW = sTS / N.

      Chapter 7. Dislocations and Strengthening Mechanisms

    25. Introduction

    26.  

       

      The key idea of the chapter is that plastic deformation is due to the motion of a large number of dislocations. The motion is called slip. Thus, the strength (resistance to deformation) can be improved by putting obstacles to slip.

    27. Basic Concepts

    28.  

       

      Dislocations can be edge dislocations, screw dislocations and exist in combination of the two (Ch. 4.4). Their motion (slip) occurs by sequential bond breaking and bond reforming (Fig. 7.1). The number of dislocations per unit volume is the dislocation density, in a plane they are measured per unit area.

    29. Characteristics of Dislocations

    30.  

       

      There is strain around a dislocation which influences how they interact with other dislocations, impurities, etc. There is compression near the extra plane (higher atomic density) and tension following the dislocation line (Fig. 7.4)

      Dislocations interact among themselves (Fig. 7.5). When they are in the same plane, they repel if they have the same sign and annihilate if they have opposite signs (leaving behind a perfect crystal). In general, when dislocations are close and their strain fields add to a larger value, they repel, because being close increases the potential energy (it takes energy to strain a region of the material).

      The number of dislocations increases dramatically during plastic deformation. Dislocations spawn from existing dislocations, and from defects, grain boundaries and surface irregularities.

    31. Slip Systems

    32.  

       

      In single crystals there are preferred planes where dislocations move (slip planes). There they do not move in any direction, but in preferred crystallographic directions (slip direction). The set of slip planes and directions constitute slip systems.

      The slip planes are those of highest packing density. How do we explain this? Since the distance between atoms is shorter than the average, the distance perpendicular to the plane has to be longer than average. Being relatively far apart, the atoms can move more easily with respect to the atoms of the adjacent plane. (We did not discuss direction and plane nomenclature for slip systems.)

      BCC and FCC crystals have more slip systems, that is more ways for dislocation to propagate. Thus, those crystals are more ductile than HCP crystals (HCP crystals are more brittle).

    33. Slip in Single Crystals
A tensile stress s will have components in any plane that is not perpendicular to the stress. These components are resolved shear stresses. Their magnitude depends on orientation (see Fig. 7.7).
tR = s cos f cos l
If the shear stress reaches the critical resolved shear stress tCRSS, slip (plastic deformation) can start. The stress needed is:
sy= tCRSS / (cos f cos l)max
at the angles at which tCRSS is a maximum. The minimum stress needed for yielding is when f = l = 45 degrees: sy= 2tCRSS. Thus, dislocations will occur first at slip planes oriented close to this angle with respect to the applied stress (Figs. 7.8 and 7.9).
    1. Plastic Deformation of Polycrystalline Materials

    2.  

       

      Slip directions vary from crystal to crystal. When plastic deformation occurs in a grain, it will be constrained by its neighbors which may be less favorably oriented. As a result, polycrystalline metals are stronger than single crystals (the exception is the perfect single crystal, as in whiskers.)

    3. Deformation by Twinning

    4.  

       

      This topic is not included.

      Mechanisms of Strengthening in Metals

      General principles. Ability to deform plastically depends on ability of dislocations to move. Strengthening consists in hindering dislocation motion. We discuss the methods of grain-size reduction, solid-solution alloying and strain hardening. These are for single-phase metals. We discuss others when treating alloys. Ordinarily, strengthening reduces ductility.

    5. Strengthening by Grain Size Reduction

    6.  

       

      This is based on the fact that it is difficult for a dislocation to pass into another grain, especially if it is very misaligned. Atomic disorder at the boundary causes discontinuity in slip planes. For high-angle grain boundaries, stress at end of slip plane may trigger new dislocations in adjacent grains. Small angle grain boundaries are not effective in blocking dislocations.

      The finer the grains, the larger the area of grain boundaries that impedes dislocation motion. Grain-size reduction usually improves toughness as well. Usually, the yield strength varies with grain size d according to:

      sy = s0 + ky/ d2
      Grain size can be controlled by the rate of solidification and by plastic deformation.
    7. Solid-Solution Strengthening

    8.  

       

      Adding another element that goes into interstitial or substitutional positions in a solution increases strength. The impurity atoms cause lattice strain (Figs. 7.17 and 7.18) which can "anchor" dislocations. This occurs when the strain caused by the alloying element compensates that of the dislocation, thus achieving a state of low potential energy. It costs strain energy for the dislocation to move away from this state (which is like a potential well). The scarcity of energy at low temperatures is why slip is hindered.

      Pure metals are almost always softer than their alloys.

    9. . Strain Hardening

    10.  

       

      Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point (cold working). (This is different from hot working is the shaping of materials at high temperatures where large deformation is possible.) Strain hardening (work hardening) is the reason for the elastic recovery discussed in Ch. 6.8.

      The reason for strain hardening is that the dislocation density increases with plastic deformation (cold work) due to multiplication. The average distance between dislocations then decreases and dislocations start blocking the motion of each one.

      The measure of strain hardening is the percent cold work (%CW), given by the relative reduction of the original area, A0 to the final value Ad :

      %CW = 100 (A0Ad)/A0
      Recovery, recrystallization and Grain Growth

      Plastic deformation causes 1) change in grain size, 2) strain hardening, 3) increase in the dislocation density. Restoration to the state before cold-work is done by heating through two processes: recovery and recrystallization. These may be followed by grain growth.

    11. . Recovery

    12.  

       

      Heating à increased diffusion à enhanced dislocation motion à relieves internal strain energy and reduces the number of dislocation. The electrical and thermal conductivity are restored to the values existing before cold working.

    13. . Recrystallization

    14.  

       

      Strained grains of cold-worked metal are replaced, upon heating, by more regularly-spaced grains. This occurs through short-range diffusion enabled by the high temperature. Since recrystallization occurs by diffusion, the important parameters are both temperature and time.

      The material becomes softer, weaker, but more ductile (Fig. 7.22).

      Recrystallization temperature: is that at which the process is complete in one hour. It is typically 1/3 to 1/2 of the melting temperature. It falls as the %CW is increased. Below a "critical deformation", recrystallization does not occur.

    15. . Grain Growth
The growth of grain size with temperature can occur in all polycrystalline materials. It occurs by migration of atoms at grain boundaries by diffusion, thus grain growth is faster at higher temperatures. The "driving force" is the reduction of energy, which is proportional to the total area. Big grains grow at the expense of the small ones.
Chapter 8. Failure
  1. Introduction

  2.  

     

    Failure of materials may have huge costs. Causes included improper materials selection or processing, the improper design of components, and improper use.

  3. Fundamentals of Fracture
Fracture is a form of failure where the material separates in pieces due to stress, at temperatures below the melting point. The fracture is termed ductile or brittle depending on whether the elongation is large or small.
Steps in fracture (response to stress):
Ductile vs. brittle fracture
Ductile Brittle
deformation extensive little
track propagation slow, needs stress fast
type of materials most metals (not too cold) ceramics, ice, cold metals
warning permanent elongation none
strain energy higher lower
fractured surface rough smoother
necking yes no
    1. Ductile Fracture
Stages of ductile fracture
The interior surface is fibrous, irregular, which signify plastic deformation.
    1. Brittle Fracture
There is no appreciable deformation, and crack propagation is very fast. In most brittle materials, crack propagation (by bond breaking) is along specific crystallographic planes (cleavage planes). This type of fracture is transgranular (through grains) producing grainy texture (or faceted texture) when cleavage direction changes from grain to grain. In some materials, fracture is intergranular.
    1. Principles of Fracture Mechanics

    2.  

       

      Fracture occurs due to stress concentration at flaws, like surface scratches, voids, etc. If a is the length of the void and r the radius of curvature, the enhanced stress near the flaw is:

      sm » 2 s0 (a/r)1/2

      where s0 is the applied macroscopic stress. Note that a is 1/2 the length of the flaw, not the full length for an internal flaw, but the full length for a surface flaw. The stress concentration factor is:

      Kt = sm/s0 » 2 (a/r)1/2

      Because of this enhancement, flaws with small radius of curvature are called stress raisers.

    3. Impact Fracture Testing
Normalized tests, like the Charpy and Izod tests measure the impact energy required to fracture a notched specimen with a hammer mounted on a pendulum. The energy is measured by the change in potential energy (height) of the pendulum. This energy is called notch toughness.
Ductile to brittle transition occurs in materials when the temperature is dropped below a transition temperature. Alloying usually increases the ductile-brittle transition temperature (Fig. 8.19.) For ceramics, this type of transition occurs at much higher temperatures than for metals.
Fatigue
Fatigue is the catastrophic failure due to dynamic (fluctuating) stresses. It can happen in bridges, airplanes, machine components, etc. The characteristics are:
    1. Cyclic Stresses
These are characterized by maximum, minimum and mean stress, the stress amplitude, and the stress ratio (Fig. 8.20).
    1. The S—N Curve
S—N curves (stress-number of cycles to failure) are obtained using apparatus like the one shown in Fig. 8.21. Different types of S—N curves are shown in Fig. 8.22.

Fatigue limit (endurance limit) occurs for some materials (like some ferrous and Ti allows). In this case, the S—N curve becomes horizontal at large N . This means that there is a maximum stress amplitude (the fatigue limit) below which the material never fails, no matter how large the number of cycles is.

For other materials (e.g., non-ferrous) the S—N curve continues to fall with N.

Failure by fatigue shows substantial variability (Fig. 8.23).

Failure at low loads is in the elastic strain regime, requires a large number of cycles (typ. 104 to 105). At high loads (plastic regime), one has low-cycle fatigue (N < 104 - 105 cycles).
 
 

    1. Crack Initiation and Propagation
Stages is fatigue failure:
I. crack initiation at high stress points (stress raisers)
II. propagation (incremental in each cycle)
III. final failure by fracture
Nfinal = Ninitiation + Npropagation
Stage I - propagation
Stage II - propagation
crack propagates by repetive plastic blunting and sharpening of the crack tip. (Fig. 8.25.)
    1. . Crack Propagation Rate (not covered)
    1. . Factors That Affect Fatigue Life
    1. . Environmental Effects
Creep
Creep is the time-varying plastic deformation of a material stressed at high temperatures. Examples: turbine blades, steam generators. Keys are the time dependence of the strain and the high temperature.
    1. . Generalized Creep Behavior
At a constant stress, the strain increases initially fast with time (primary or transient deformation), then increases more slowly in the secondary region at a steady rate (creep rate). Finally the strain increases fast and leads to failure in the tertiary region. Characteristics:
    1. . Stress and Temperature Effects
Creep becomes more pronounced at higher temperatures (Fig. 8.37). There is essentially no creep at temperatures below 40% of the melting point.

Creep increases at higher applied stresses.

The behavior can be characterized by the following expression, where K, n and Qc are constants for a given material:

de/dt = K sn exp(-Qc/RT)

    1. . Data Extrapolation Methods (not covered.)
    1. . Alloys for High-Temperature Use
These are needed for turbines in jet engines, hypersonic airplanes, nuclear reactors, etc. The important factors are a high melting temperature, a high elastic modulus and large grain size (the latter is opposite to what is desirable in low-temperature materials).
Some creep resistant materials are stainless steels, refractory metal alloys (containing elements of high melting point, like Nb, Mo, W, Ta), and superalloys (based on Co, Ni, Fe.)

9.1 Introduction

Definitions

Component: pure metal or compound (e.g., Cu, Zn in Cu-Zn alloy, sugar, water, in a syrup.)

Solvent: host or major component in solution.

Solute: dissolved, minor component in solution.

System: set of possible alloys from same component (e.g., iron-carbon system.)

Solubility Limit: Maximum solute concentration that can be dissolved at a given temperature.

Phase: part with homogeneous physical and chemical characteristics

9.2 Solubility Limit

Effect of temperature on solubility limit. Maximum content: saturation. Exceeding maximum content (like when cooling) leads to precipitation.

9.3 Phases

One-phase systems are homogeneous. Systems with two or more phases are heterogeneous, or mixtures. This is the case of most metallic alloys, but also happens in ceramics and polymers.

A two-component alloy is called binary. One with three components, ternary.

9.4 Microstructure

The properties of an alloy do not depend only on concentration of the phases but how they are arranged structurally at the microscopy level. Thus, the microstructure is specified by the number of phases, their proportions, and their arrangement in space.
A binary alloy may be
    1. a single solid solution
    2. two separated, essentially pure components.
    3. two separated solid solutions.
    4. a chemical compound, together with a solid solution.
The way to tell is to cut the material, polish it to a mirror finish, etch it a weak acid (components etch at a different rate) and observe the surface under a microscope.

9.5 Phase Equilibriam

Equilibrium is the state of minimum energy. It is achieved given sufficient time. But the time to achieve equilibrium may be so long (the kinetics is so slow) that a state that is not at an energy minimum may have a long life and appear to be stable. This is called a metastable state.

A less strict, operational, definition of equilibrium is that of a system that does not change with time during observation.

Equilibrium Phase Diagrams

Give the relationship of composition of a solution as a function of temperatures and the quantities of phases in equilibrium. These diagrams do not indicate the dynamics when one phase transforms into another. Sometimes diagrams are given with pressure as one of the variables. In the phase diagrams we will discuss, pressure is assumed to be constant at one atmosphere.

9.6 Binary Isomorphous Systems

This very simple case is one complete liquid and solid solubility, an isomorphous system. The example is the Cu-Ni alloy of Fig. 9.2a. The complete solubility occurs because both Cu and Ni have the same crystal structure (FCC), near the same radii, electronegativity and valence.
The liquidus line separates the liquid phase from solid or solid + liquid phases. That is, the solution is liquid above the liquidus line.
The solidus line is that below which the solution is completely solid (does not contain a liquid phase.)
Interpretation of phase diagrams
Concentrations: Tie-line method
    1. locate composition and temperature in diagram
    2. In two phase region draw tie line or isotherm
    3. note intersection with phase boundaries. Read compositions.
Fractions: lever rule
    1. construct tie line (isotherm)
    2. obtain ratios of line segments lengths.
Note: the fractions are inversely proportional to the length to the boundary for the particular phase. If the point in the diagram is close to the phase line, the fraction of that phase is large.
Development of microstructure in isomorphous alloys
a) Equilibrium cooling
Solidification in the solid + liquid phase occurs gradually upon cooling from the liquidus line. The composition of the solid and the liquid change gradually during cooling (as can be determined by the tie-line method.) Nuclei of the solid phase form and they grow to consume all the liquid at the solidus line.
b) Non-equilibrium cooling
Solidification in the solid + liquid phase also occurs gradually. The composition of the liquid phase evolves by diffusion, following the equilibrium values that can be derived from the tie-line method. However, diffusion in the solid state is very slow. Hence, the new layers that solidify on top of the grains have the equilibrium composition at that temperature but once they are solid their composition does not change. This lead to the formation of layered (cored) grains (Fig. 9.14) and to the invalidity of the tie-line method to determine the composition of the solid phase (it still works for the liquid phase, where diffusion is fast.)

9.7 Binary Eutectic Systems

Interpretation: Obtain phases present, concentration of phases and their fraction (%).
Solvus line: limit of solubility
Eutectic or invariant point. Liquid and two solid phases exist in equilibrium at the eutectic composition and the eutectic temperature.
Note:
Development of microstructure in eutectic alloys
Case of lead-tin alloys, figures 9.9–9.14. A layered, eutectic structure develops when cooling below the eutectic temperature. Alloys which are to the left of the eutectic concentration (hipoeutectic) or to the right (hypereutectic) form a proeutectic phase before reaching the eutectic temperature, while in the solid + liquid region. The eutectic structure then adds when the remaining liquid is solidified when cooling further. The eutectic microstructure is lamellar (layered) due to the reduced diffusion distances in the solid state.
To obtain the concentration of the eutectic microstructure in the final solid solution, one draws a vertical line at the eutectic concentration and applies the lever rule treating the eutectic as a separate phase (Fig. 9.16).

9.8 Equilibrium Diagrams Having Intermediate Phases or Compounds

A terminal phase or terminal solution is one that exists in the extremes of concentration (0 and 100%) of the phase diagram. One that exists in the middle, separated from the extremes, is called an intermediate phase or solid solution.

An important phase is the intermetallic compound, that has a precise chemical compositions. When using the lever rules, intermetallic compounds are treated like any other phase, except they appear not as a wide region but as a vertical line.

9.9 Eutectoid and Peritectic Reactions

The eutectoid (eutectic-like) reaction is similar to the eutectic reaction but occurs from one solid phase to two new solid phases. It also shows as V on top of a horizontal line in the phase diagram. There are associated eutectoid temperature (or temperature), eutectoid phase, eutectoid and proeutectoid microstructures.

Solid Phase 1 à Solid Phase 2 + Solid Phase 3

The peritectic reaction also involves three solid in equilibrium, the transition is from a solid + liquid phase to a different solid phase when cooling. The inverse reaction occurs when heating.

Solid Phase 1 + liquid à Solid Phase 2

9.10 Congruent Phase Transformations

Another classification scheme. Congruent transformation is one where there is no change in composition, like allotropic transformations (e.g., a-Fe to g-Fe) or melting transitions in pure solids.

9.11 Ceramic and Ternary Phase Diagrams

Ternary phase diagrams are three-dimensional. Ceramic phase diagrams will be discussed in Ch. 13.

9.12 The Gibbs Phase Rule (not discussed)

The Iron–Carbon Diagram

9.13 The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram

This is one of the most important alloys for structural applications. The diagram Fe—C is simplified at low carbon concentrations by assuming it is the Fe—Fe3C diagram. Concentrations are usually given in weight percent. The possible phases are:
The maximum solubility of C in a- ferrite is 0.022 wt%. d-ferrite is only stable at high temperatures. It is not important in practice. Austenite has a maximum C concentration of 2.14 wt %. It is not stable below the eutectic temperature (727 C) unless cooled rapidly (Chapter 10). Cementite is in reality metastable, decomposing into a-Fe and C when heated for several years between 650 and 770 C.
For their role in mechanical properties of the alloy, it is important to note that:
Ferrite is soft and ductile
Cementite is hard and brittle
Thus, combining these two phases in solution an alloy can be obtained with intermediate properties. (Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed.)

9.14 Development of Microstructures in Iron—Carbon Alloys

The eutectoid composition of austenite is 0.76 wt %. When it cools slowly it forms perlite, a lamellar or layered structure of two phases: a-ferrite and cementite (Fe3C).

Hypoeutectoid alloys contain proeutectoid ferrite plus the eutectoid perlite. Hypereutectoid alloys contain proeutectoid cementite plus perlite.

Since reactions below the eutectoid temperature are in the solid phase, the equilibrium is not achieved by usual cooling from austenite. The new microstructures that form are discussed in Ch. 10.

9.15 The Influence of Other Alloying Elements

As mentioned in section 7.9, alloying strengthens metals by hindering the motion of dislocations. Thus, the strength of Fe–C alloys increase with C content and also with the addition of other elements.

10.1 Introduction

The goal is to obtain specific microstructures that will improve the mechanical properties of a metal, in addition to grain-size refinement, solid-solution strengthening, and strain-hardening.

10.2 Basic Concepts

Phase transformations that involve a change in the microstructure can occur through:

10.3 The Kinetics of Solid-State Reactions

Change in composition implies atomic rearrangement, which requires diffusion. Atoms are displaced by random walk. The displacement of a given atom, d, is not linear in time t (as would be for a straight trajectory) but is proportional to the square root of time, due to the tortuous path: d = c(Dt) 1/2 where c is a constant and D the diffusion constant. This time-dependence of the rate at which the reaction (phase transformation) occurs is what is meant by the term reaction kinetics.

D is called a constant because it does not depend on time, but it depends on temperature as we have seen in Ch. 5. Diffusion occurs faster at high temperatures.

Phase transformation requires two processes: nucleation and growth. Nucleation involves the formation of very small particles, or nuclei (e.g., grain boundaries, defects). This is similar to rain happening when water molecules condensed around dust particles. During growth, the nuclei grow in size at the expense of the surrounding material.

The kinetic behavior often has the S-shape form of Fig. 10.1, when plotting percent of material transformed vs. the logarithm of time. The nucleation phase is seen as an incubation period, where nothing seems to happen. Usually the transformation rate has the form r = A e-Q/RT (similar to the temperature dependence of the diffusion constant), in which case it is said to be thermally activated.

10.4 Multiphase Transformations

To describe phase transformations that occur during cooling, equilibrium phase diagrams are inadequate if the transformation rate is slow compared to the cooling rate. This is usually the case in practice, so that equilibrium microstructures are seldom obtained. This means that the transformations are delayed (e.g., case of supercooling), and metastable states are formed. We then need to know the effect of time on phase transformations.

10.5 Isothermal Transformation Diagrams

We use as an example the cooling of an eutectoid alloy (0.76 wt% C) from the austenite (g- phase) to perlite, that contains ferrite (a) plus cementite (Fe3C or iron carbide). When cooling proceeds below the eutectoid temperature (727 oC) nucleation of perlite starts. The S-shaped curves (fraction of perlite vs. log. time, fig. 10.3) are displaced to longer times at higher temperatures showing that the transformation is dominated by nucleation (the nucleation period is longer at higher temperatures) and not by diffusion (which occurs faster at higher temperatures).

The family of S-shaped curves at different temperatures can be used to construct the TTT (Time-Temperature-Transformation) diagrams (e.g., fig. 10.4.) For these diagrams to apply, one needs to cool the material quickly to a given temperature To before the transformation occurs, and keep it at that temperature over time. The horizontal line that indicates constant temperature To intercepts the TTT curves on the left (beginning of the transformation) and the right (end of the transformation); thus one can read from the diagrams when the transformation occurs. The formation of perlite shown in fig. 10.4 also indicates that the transformation occurs sooner at low temperatures, which is an indication that it is controlled by the rate of nucleation. At low temperatures, nucleation occurs fast and grain growth is reduced (since it occurs by diffusion, which is hindered at low temperatures). This reduced grain growth leads to fine-grained microstructure (fine perlite). At higher temperatures, diffusion allows for larger grain growth, thus leading to coarse perlite.

At lower temperatures nucleation starts to become slower, and a new phase is formed, bainite. Since diffusion is low at low temperatures, this phase has a very fine (microscopic) microstructure.

Spheroidite is a coarse phase that forms at temperatures close to the eutectoid temperature. The relatively high temperatures caused a slow nucleation but enhances the growth of the nuclei leading to large grains.

A very important structure is martensite, which forms when cooling austenite very fast (quenching) to below a maximum temperature that is required for the transformation. It forms nearly instantaneously when the required low temperature is reached; since no thermal activation is needed, this is called an athermal transformation. Martensite is a different phase, a body-centered tetragonal (BCT) structure with interstitial C atoms. Martensite is metastable and decomposes into ferrite and perlite but this is extremely slow (and not noticeable) at room temperature.

In the examples, we used an eutectoid composition. For hypo- and hypereutectoid alloys, the analysis is the same, but the proeutectoid phase that forms before cooling through the eutectoid temperature is also part of the final microstructure.

 

10.7 Mechanical Behavior of Fe-C Alloys

The strength and hardness of the different microstructures is inversely related to the size of the microstructures. Thus, spheroidite is softest, fine perlite is stronger than coarse perlite, bainite is stronger than perlite and martensite is the strongest of all. The stronger and harder the phase the more brittle it becomes.

10.8 Tempered Martensite

Martensite is so brittle that it needs to be modified in many practical cases. This is done by heating it to 250-650 oC for some time (tempering) which produces tempered martensite, an extremely fine-grained and well dispersed cementite grains in a ferrite matrix.
 
 

Annealing Processes

11.1 Introduction

Annealing is a heat treatment where the material is taken to a high temperature, kept there for some time and then cooled. High temperatures allow diffusion processes to occur fast. The time at the high temperature (soaking time) is long enough to allow the desired transformation to occur. Cooling is done slowly to avoid the distortion (warping) of the metal piece, or even cracking, caused by stresses induced by differential contraction due to thermal inhomogeneities. Benefits of annealing are:

11.2 Process Annealing

Deforming a piece that has been strengthened by cold working requires a lot of energy. Reverting the effect of cold work by process annealing eases further deformation. Heating allows recovery and recrystallization but is usually limited to avoid excessive grain growth and oxidation.

11.3 Stress Relief

Stresses resulting from machining operations of non-uniform cooling can be eliminated by stress relief annealing at moderately low temperatures, such that the effect of cold working and other heat treatments is maintained.

11.4 Annealing of Ferrous Alloys

Normalizing (or austenitizing) consists in taking the Fe-C alloy to the austenitic phase which makes the grain size more uniform, followed by cooling in air.

Full anneal involves taking hypoeutectoid alloys to the austenite phase and hypereutectoid alloys over the eutectoid temperature (Fig. 11.1) to soften pieces which have been hardened by plastic deformation, and which need to be machined.

Spheroidizing consists in prolongued heating just below the eutectoid temperature, which results in the soft spheroidite structure discussed in Sect. 10.5. This achieves maximum softness that minimizes the energy needed in subsequent forming operations.

Heat Treatment of Steels

11.5 Hardenability

To achieve a full conversion of austenite into hard martensite, cooling needs to be fast enough to avoid partial conversion into perlite or bainite. If the piece is thick, the interior may cool too slowly so that full martensitic conversion is not achieved. Thus, the martensitic content, and the hardness, will drop from a high value at the surface to a lower value in the interior of the piece. Hardenability is the ability of the material to be hardened by forming martensite.

Hardenability is measured by the Jominy end-quench test (Fig. 11.2). Hardenability is then given as the dependence of hardness on distance from the quenched end. High hardenability means that the hardness curve is relatively flat.
 

11.6 Influence of Quenching Medium, Specimen Size, and Geometry

The cooling rate depends on the cooling medium. Cooling is fastest using water, then oil, and then air. Fast cooling brings the danger of warping and formation of cracks, since it is usually accompanied by large thermal gradients.

The shape and size of the piece, together with the heat capacity and heat conductivity are important in determining the cooling rate for different parts of the metal piece. Heat capacity is the energy content of a heated mass, which needs to be removed for cooling. Heat conductivity measures how fast this energy is transported to the colder regions of the piece.
 
 

Precipitation Hardening

Hardening can be enhanced by extremely small precipitates that hinder dislocation motion. The precipitates form when the solubility limit is exceeded. Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time.

11.7 Heat Treatments

Precipitation hardening is achieved by:
a) solution heat treatment where all the solute atoms are dissolved to form a single-phase solution.
b) rapid cooling across the solvus line to exceed the solubility limit. This leads to a supersaturated solid solution that remains stable (metastable) due to the low temperatures, which prevent diffusion.
c) precipitation heat treatment where the supersaturated solution is heated to an intermediate temperature to induce precipitation and kept there for some time (aging).


If the process is continued for a very long time, eventually the hardness decreases. This is called overaging.

The requirements for precipitation hardening are:

11.8 Mechanism of Hardening

Strengthening involves the formation of a large number of microscopic nuclei, called zones. It is accelerated at high temperatures. Hardening occurs because the deformation of the lattice around the precipitates hinder slip. Aging that occurs at room temperature is called natural aging, to distinguish from the artificial aging caused by premeditated heating.

11.9 Miscellaneous Considerations

Since forming, machining, etc. uses more energy when the material is hard, the steps in the processing of alloys are usually: Exposure of precipitation-hardened alloys to high temperatures may lead to loss of strength by overaging.
 
 

Earliest development

Gold can be agglomerated into larger pieces by cold hammering, but native copper cannot, and an essential step toward the Metal Age was the discovery that metals such as copper could be fashioned into shapes by melting and casting in molds; among the earliest known products of this type are copper axes cast in the Balkans in the 4th millennium BC. Another step was the discovery that metals could be recovered from metal-bearing minerals. These had been collected and could be distinguished on the basis of colour, texture, weight, and flame colour and smell when heated. The notably greater yield obtained by heating native copper with associated oxide minerals may have led to the smelting process, since these oxides are easily reduced to metal in a charcoal bed at temperatures in excess of 700 C (1,300 F), as the reductant, carbon monoxide, becomes increasingly stable. In order to effect the agglomeration and separation of melted or smelted copper from its associated minerals, it was necessary to introduce iron oxide as a flux. This further step forward can be attributed to the presence of iron oxide gossan minerals in the weathered upper zones of copper sulfide deposits.

Bronze

In many regions, copper-arsenic alloys, of superior properties to copper in both cast and wrought form, were produced in the next period. This may have been accidental at first, owing to the similarity in colour and flame colour between the bright green copper carbonate mineral malachite and the weathered products of such copper-arsenic sulfide minerals as enargite, and it may have been followed later by the purposeful selection of arsenic compounds based on their garlic odour when heated.

Arsenic contents varied from 1 to 7 percent, with up to 3 percent tin. Essentially arsenic-free copper alloys with higher tin content--in other words, true bronze--seem to have appeared between 3000 and 2500 BC, beginning in the Tigris-Euphrates delta. The discovery of the value of tin may have occurred through the use of stannite, a mixed sulfide of copper, iron, and tin, although this mineral is not as widely available as the principal tin mineral, cassiterite, which must have been the eventual source of the metal. Cassiterite is strikingly dense and occurs as pebbles in alluvial deposits together with arsenopyrite and gold; it also occurs to a degree in the iron oxide gossans mentioned above.

While there may have been some independent development of bronze in varying localities, it is most likely that the bronze culture spread through trade and the migration of peoples from the Middle East to Egypt, Europe, and possibly China. In many civilizations the production of copper, arsenical copper, and tin bronze continued together for some time. The eventual disappearance of copper-arsenic alloys is difficult to explain. Production may have been based on minerals that were not widely available and became scarce, but the relative scarcity of tin minerals did not prevent a substantial trade in that metal over considerable distances. It may be that tin bronzes were eventually preferred owing to the chance of contracting arsenic poisoning from fumes produced by the oxidation of arsenic-containing minerals.

As the weathered copper ores in given localities were worked out, the harder sulfide ores beneath were mined and smelted. The minerals involved, such as chalcopyrite, a copper-iron sulfide, needed an oxidizing roast to remove sulfur as sulfur dioxide and yield copper oxide. This not only required greater metallurgical skill but also oxidized the intimately associated iron, which, combined with the use of iron oxide fluxes and the stronger reducing conditions produced by improved smelting furnaces, led to higher iron contents in the bronze.

Iron

It is not possible to mark a sharp division between the Bronze Age and the Iron Age. Small pieces of iron would have been produced in copper smelting furnaces as iron oxide fluxes and iron-bearing copper sulfide ores were used. In addition, higher furnace temperatures would have created more strongly reducing conditions (that is to say, a higher carbon monoxide content in the furnace gases). An early piece of iron from a trackway in the province of Drenthe, Neth., has been dated from 1350 BC, a date normally taken as the Middle Bronze Age for this area. In Anatolia, on the other hand, iron was in use as early as 2000 BC. There are also occasional references to iron in even earlier periods, but this material was of meteoric origin.

Once a relationship had been established between the new metal found in copper smelts and the ore added as flux, the operation of furnaces for the production of iron alone naturally followed. Certainly by 1400 BC in Anatolia, iron was assuming considerable importance, and by 1200-1000 BC it was being fashioned on quite a large scale into weapons, initially dagger blades. For this reason, 1200 BC has been taken as the beginning of the Iron Age. Evidence from excavations indicates that the art of iron making originated in the mountainous country to the south of the Black Sea, an area dominated by the Hittites. Later the art apparently spread to the Palestinians, for crude furnaces dating from 1200 BC have been unearthed at Gerar, together with a number of iron objects.

Smelting of iron oxide with charcoal demanded a high temperature, and, since the melting temperature of iron at 1,540 C (2,800 F) was not attainable then, the product was merely a spongy mass of pasty globules of metal intermingled with a semiliquid slag. This product, later known as bloom, was hardly usable as it stood, but repeated reheating and hot hammering eliminated much of the slag, creating wrought iron, a much better product.

The properties of iron are much affected by the presence of small amounts of carbon, with large increases in strength associated with contents of less than 0.5 percent. At the temperatures then attainable--about 1,200 C (2,200 F)--reduction by charcoal produced an almost pure iron, which was soft and of limited use for weapons and tools, but when the ratio of fuel to ore was increased and furnace drafting improved with the invention of better bellows, more carbon was absorbed by the iron. This resulted in blooms and iron products with a range of carbon contents, making it difficult to determine the period in which iron may have been purposely strengthened by carburizing, or reheating the metal in contact with excess charcoal.

Carbon-containing iron had the further great advantage that, unlike bronze and carbon-free iron, it could be made still harder by quenching--i.e., rapid cooling by immersion in water. There is no evidence for the use of this hardening process during the early Iron Age, so that it must have been either unknown then or not considered advantageous, in that quenching renders iron very brittle and has to be followed by tempering, or reheating at a lower temperature, to restore toughness. What seems to have been established early on was a practice of repeated cold forging and annealing at 600-700 C (1,100-1,300 F), a temperature naturally achieved in a simple fire. This practice is common in parts of Africa even today.

By 1000 BC iron was beginning to be known in central Europe. Its use spread slowly westward; iron making was fairly widespread in Great Britain at the time of the Roman invasion in 55 BC. In Asia iron was also known in ancient times, in China by about 700 BC.

To make iron you start with iron ore. Iron ore is nothing more than rock that happens to contain a high concentration of iron. One thing that gave certain countries an edge between the 15th and 20th centuries was the availability of iron ore deposits. For example, England, the U.S., France, Germany, Spain and Russia all have good iron ore deposits. When you think of the historical importance of all of these countries, you can see the correlation!

Common iron ores include:

Usually you find these minerals mixed into rocks containing silica.

Creating Iron

You can see in the previous section that all of the iron ores contain iron combined with oxygen. To make iron from iron ore, you need to eliminate the oxygen to create pure iron.

The most primitive facility used to refine iron from iron ore is called a bloomery. In a bloomery you burn charcoal with iron ore and a good supply of oxygen (provided by a bellows or blower). Charcoal is essentially pure carbon. The carbon combines with oxygen to create carbon dioxide and carbon monoxide (releasing lots of heat in the process). Carbon and carbon monoxide combine with the oxygen in the iron ore and carry it away, leaving iron metal.

In a bloomery the fire does not get hot enough to melt the iron completely, so you are left with a spongy mass containing iron and silicates from the ore (the bloom). By heating and hammering the bloom, the glassy silicates mix into the iron metal to create wrought iron. Wrought iron is tough and easy to work, making it perfect for creating tools in a blacksmith shop.

The more advanced way to smelt iron is in a blast furnace. A blast furnace is charged with iron ore, charcoal or coke (coke being charcoal made from coal) and limestone (CaCO4). Huge quantities of air blast in at the bottom of the furnace. The calcium in the limestone combines with the silicates to form slag. At the bottom of the blast furnace, liquid iron collects along with a layer of slag on top. Periodically you let the liquid iron flow out and cool. Typically the liquid iron flows into a channel and indentations in a bed of sand. Once it cools, this metal is known as pig iron.

To create a ton of pig iron you start with 2 tons of ore, 1 ton of coke and half a ton of limestone, and the fire consumes 5 tons of air. The temperature reaches almost 3000 degrees F (1600 or so degrees C) at the core of the blast furnace!

Pig iron contains 4% to 5% carbon and is so hard and brittle it is almost useless. You do one of two things with pig iron:

  1. You melt it, mix it with slag and hammer it to eliminate most of the carbon (down to 0.3%) and create wrought iron. Wrought iron is the stuff a blacksmith works with to create tools, horseshoes, and so on. When you heat wrought iron, it is malleable, bendable, weldable and very easy to work with.
  2. You create steel (see the next section).
The Advantages of Iron
Iron is an incredibly useful substance for several reasons: When you compare iron and steel with something like aluminum, you can see why it was so important historically. To refine aluminum you must have access to huge amounts of electricity. To shape aluminum you must either cast it or extrude it. Iron is much easier to deal with by comparison. Iron has been useful to man for thousands of years, while aluminum really did not exist in any meaningful way until the 20th century (fun fact - the tip of the Washington Monument is made of a 10 inch high aluminum pyramid rather than gold, because gold was less valuable than aluminum in 1884!).

An object like the flintlock rifle would be impossible to create without iron. Fortunately, iron can be created relatively easily with tools available to primitive societies. There will likely come a day when we become so technologically advanced that iron is completely replaced by aluminum, plastics and things like carbon and glass fibers. But right now the economic equation gives inexpensive iron and steel a huge advantage over these much more expensive alternatives.

Brass

While some zinc appears in bronzes dating from the Bronze Age, this was almost certainly an accidental inclusion, although it may foreshadow the complex ternary alloys of the early Iron Age, in which substantial amounts of zinc as well as tin may be found. Brass, as an alloy of copper and zinc without tin, did not appear in Egypt until about 30 BC, but after this it was rapidly adopted throughout the Roman world, for example, for currency. It was made by the calamine process, in which zinc carbonate or zinc oxide were added to copper and melted under a charcoal cover in order to produce reducing conditions. The general establishment of a brass industry was one of the important metallurgical contributions made by the Romans.

Precious metals

Bronze, iron, and brass were, then, the metallic materials on which successive peoples built their civilizations and of which they made their implements for both war and peace. In addition, by 500 BC, rich lead-bearing silver mines had opened in Greece. Reaching depths of several hundred metres, these mines were vented by drafts provided by fires lit at the bottom of the shafts. Ores were hand-sorted, crushed, and washed with streams of water to separate valuable minerals from the barren, lighter materials. Because these minerals were principally sulfides, they were roasted to form oxides and were then smelted to recover a lead-silver alloy.

Lead was removed from the silver by cupellation, a process of great antiquity in which the alloy was melted in a shallow porous clay or bone-ash receptacle called a cupel. A stream of air over the molten mass preferentially oxidized the lead. Its oxide was removed partially by skimming the molten surface; the remainder was absorbed into the porous cupel. Silver metal and any gold were retained on the cupel. The lead from the skimmings and discarded cupels was recovered as metal upon heating with charcoal.

Native gold itself often contained quite considerable quantities of silver. These silver-gold alloys, known as electrum, may be separated in a number of ways, but presumably the earliest was by heating in a crucible with common salt. In time and with repetitive treatments, the silver was converted into silver chloride, which passed into the molten slag, leaving a purified gold. Cupellation was also employed to remove from the gold such contaminates as copper, tin, and lead. Gold, silver, and lead were used for artistic and religious purposes, personal adornment, household utensils, and equipment for the chase.

From 500 BC To AD 1500

In the thousand years between 500 BC and AD 500, a vast number of discoveries of significance to the growth of metallurgy were made. The Greek mathematician and inventor Archimedes, for example, demonstrated that the purity of gold could be measured by determining its weight and the quantity of water displaced upon immersion--that is, by determining its density. In the pre-Christian portion of the period, the first important steel production was started in India, using a process already known to ancient Egyptians. Wootz steel, as it was called, was prepared as sponge (porous) iron in a unit not unlike a bloomery. The product was hammered while hot to expel slag, broken up, then sealed with wood chips in clay containers and heated until the pieces of iron absorbed carbon and melted, converting it to steel of homogeneous composition containing 1 to 1.6 percent carbon. The steel pieces could then be heated and forged to bars for later use in fashioning articles, such as the famous Damascus swords made by medieval Arab armourers.

Arsenic, zinc, antimony, and nickel may well have been known from an early date but only in the alloy state. By 100 BC mercury was known and was produced by heating the sulfide mineral cinnabar and condensing the vapours. Its property of amalgamating (mixing or alloying) with various metals was employed for their recovery and refining. Lead was beaten into sheets and pipes, the pipes being used in early water systems. The metal tin was available and Romans had learned to use it to line food containers. Although the Romans made no extraordinary metallurgical discoveries, they were responsible for, in addition to the establishment of the brass industry, contributing toward improved organization and efficient administration in mining.

Beginning about the 6th century, and for the next thousand years, the most meaningful developments in metallurgy centred on iron making. Great Britain, where iron ore was plentiful, was an important iron-making region. Iron weapons, agricultural implements, domestic articles, and even personal adornments were made. Fine-quality cutlery was made near Sheffield. Monasteries were often centres of learning of the arts of metalworking. Monks became well known for their iron making and bell founding, the products made either being utilized in the monasteries, disposed of locally, or sold to merchants for shipment to more distant markets. In 1408 the bishop of Durham established the first water-powered bloomery in Britain, with the power apparently operating the bellows. Once power of this sort became available, it could be applied to a range of operations and enable the hammering of larger blooms.

In Spain, another iron-making region, the Catalan forge had been invented, and its use later spread to other areas. A hearth type of furnace, it was built of stone and was charged with iron ore, flux, and charcoal. The charcoal was kept ignited with air from a bellows blown through a bottom nozzle, or tuyere (see figure). The bloom that slowly collected at the bottom was removed and upon frequent reheating and forging was hammered into useful shapes. By the 14th century the furnace was greatly enlarged in height and capacity.
 
 
 
 

If the fuel-to-ore ratio in such a furnace was kept high, and if the furnace reached temperatures sufficiently hot for substantial amounts of carbon to be absorbed into the iron, then the melting point of the metal would be lowered and the bloom would melt. This would dissolve even more carbon, producing a liquid cast iron of up to 4 percent carbon and with a relatively low melting temperature of 1,150 C (2,100 F). The cast iron would collect in the base of the furnace, which technically would be a blast furnace rather than a bloomery in that the iron would be withdrawn as a liquid rather than a solid lump.

While the Iron Age peoples of Anatolia and Europe on occasion may have accidently made cast iron, which is chemically the same as blast-furnace iron, the Chinese were the first to realize its advantages. Although brittle and lacking the strength, toughness, and workability of steel, it was useful for making cast bowls and other vessels. In fact, the Chinese, whose Iron Age began about 500 BC, appear to have learned to oxidize the carbon from cast iron in order to produce steel or wrought iron indirectly, rather than through the direct method of starting from low-carbon iron.

After 1500

During the 16th century, metallurgical knowledge was recorded and made available. Two books were especially influential. One, by the Italian Vannoccio Biringuccio, was entitled De la pirotechnia (Eng. trans., The Pirotechnia of Vannoccio Biringuccio, 1943). The other, by the German Georgius Agricola, was entitled De re metallica. Biringuccio was essentially a metalworker, and his book dealt with smelting, refining, and assay methods (methods for determining the metal content of ores) and covered metal casting, molding, core making, and the production of such commodities as cannons and cast-iron cannonballs. His was the first methodical description of foundry practice.

Agricola, on the other hand, was a miner and an extractive metallurgist; his book considered prospecting and surveying in addition to smelting, refining, and assay methods. He also described the processes used for crushing and concentrating the ore and then, in some detail, the methods of assaying to determine whether ores were worth mining and extracting. Some of the metallurgical practices he described are retained in principle today.

Ferrous metals

From 1500 to the 20th century, metallurgical development was still largely concerned with improved technology in the manufacture of iron and steel. In England, the gradual exhaustion of timber led first to prohibitions on cutting of wood for charcoal and eventually to the introduction of coke, derived from coal, as a more efficient fuel. Thereafter the iron industry expanded rapidly in Great Britain, which became the greatest iron producer in the world. The crucible process for making steel, introduced in England in 1740, by which bar iron and added materials were placed in clay crucibles heated by coke fires, resulted in the first reliable steel made by a melting process.

One difficulty with the bloomery process for the production of soft bar iron was that, unless the temperature was kept low (and the output therefore small), it was difficult to keep the carbon content low enough so that the metal remained ductile. This difficulty was overcome by melting high-carbon pig iron from the blast furnace in the puddling process, invented in Great Britain in 1784. In it, melting was accomplished by drawing hot gases over a charge of pig iron and iron ore held on the furnace hearth. During its manufacture the product was stirred with iron rabbles (rakes), and, as it became pasty with loss of carbon, it was worked into balls, which were subsequently forged or rolled to a useful shape. The product, which came to be known as wrought iron, was low in elements that contributed to the brittleness of pig iron and contained enmeshed slag particles that became elongated fibres when the metal was forged. Later, the use of a rolling mill equipped with grooved rolls to make wrought-iron bars was introduced.

The most important development of the 19th century was the large-scale production of cheap steel. Prior to about 1850, the production of wrought iron by puddling and of steel by crucible melting had been conducted in small-scale units without significant mechanization. The first change was the development of the open-hearth furnace by William and Friedrich Siemens in Britain and by Pierre and Émile Martin in France. Employing the regenerative principle, in which outgoing combusted gases are used to heat the next cycle of fuel gas and air, this enabled high temperatures to be achieved while saving on fuel. Pig iron could then be taken through to molten iron or low-carbon steel without solidification, scrap could be added and melted, and iron ore could be melted into the slag above the metal to give a relatively rapid oxidation of carbon and silicon--all on a much enlarged scale. Another major advance was Henry Bessemer's process, patented in 1855 and first operated in 1856, in which air was blown through molten pig iron from tuyeres set into the bottom of a pear-shaped vessel called a converter. Heat released by the oxidation of dissolved silicon, manganese, and carbon was enough to raise the temperature above the melting point of the refined metal (which rose as the carbon content was lowered) and thereby maintain it in the liquid state. Very soon Bessemer had tilting converters producing 5 tons in a heat of one hour, compared with four to six hours for 50 kilograms (110 pounds) of crucible steel and two hours for 250 kilograms of puddled iron.

Neither the open-hearth furnace nor the Bessemer converter could remove phosphorus from the metal, so that low-phosphorus raw materials had to be used. This restricted their use from areas where phosphoric ores, such as those of the Minette range in Lorraine, were a main European source of iron. The problem was solved by Sidney Gilchrist Thomas, who demonstrated in 1876 that a basic furnace lining consisting of calcined dolomite, instead of an acidic lining of siliceous materials, made it possible to use a high-lime slag to dissolve the phosphates formed by the oxidation of phosphorus in the pig iron. This principle was eventually applied to both open-hearth furnaces and Bessemer converters.

As steel was now available at a fraction of its former cost, it saw an enormously increased use for engineering and construction. Soon after the end of the century it replaced wrought iron in virtually every field. Then, with the availability of electric power, electric-arc furnaces were introduced for making special and high-alloy steels. The next significant stage was the introduction of cheap oxygen, made possible by the invention of the Linde-Frankel cycle for the liquefaction and fractional distillation of air. The Linz-Donawitz process, invented in Austria shortly after World War II, used oxygen supplied as a gas from a tonnage oxygen plant, blowing it at supersonic velocity into the top of the molten iron in a converter vessel. As the ultimate development of the Bessemer/Thomas process, oxygen blowing became universally employed in bulk steel production.

There are two process routes for making steel in the UK today: the electric arc furnace and the basic oxygen converter. The latter requires a charge of molten iron, which is produced in blast furnaces.

The raw materials for producing molten iron are iron ore, coking coal and fluxes (materials that help the chemical process) - mainly limestone. The iron ore and coal used in the UK is imported (primarily from the USA, Canada, Brazil, Australia and Scandinavia), because the UK's resources of good quality coking coal and ore are limited and not economically viable.

The coal and ore arrives by sea in very large ships and is off-loaded at deepwater harbours close to the four steelworks that use it. These steelworks are at Teesside and Scunthorpe on the North East coast, and Llanwern and Port Talbot in South Wales. The iron ores arrive in a number of forms: lumps of ore in the form in which they were mined; fine-sized iron ores; and pellets - fine ores which have been processed to stick together to form hard spheres of iron ore. The coals and ores are transported by conveyor belt or rail to stockyards where they are stored and carefully blended.

Blended coal is first heated in coke ovens to produce coke. This process is known as carbonisation. The gas produced during carbonisation is extracted and used for fuel elsewhere in the steelworks. Other by-products (such as tar and benzole) are also extracted for further refining and sale. Once carbonised, the coke is pushed out of the ovens and allowed to cool.

Fine-sized ore is first mixed with coke and fluxes and heated in a sinter plant. This is a continuous moving belt on which the coke is ignited. The high temperatures generated fuse the ore particles and fluxes together to form a porous clinker called sinter. The use of sinter in the blast furnace helps make the ironmaking process more efficient.

Iron ore lumps and pellets, coke, sinter and possibly extra flux are carried to the top of the blast furnace on a conveyor or in skips and then tipped, or charged, into the furnace. Hot air (900 degrees C etc) is blasted into the bottom of the furnace through nozzles called tuyeres. The oxygen in the air combusts with the coke to form carbon monoxide gas, and this generates a great deal of heat. Frequently oil or coal is injected with the air, which enables less (relatively expensive) coke to be used. The carbon monoxide flows up through the blast furnace and removes oxygen from the iron ores on their way down, thereby leaving iron. The heat in the furnace melts the iron, and the resulting liquid iron (or hot metal as it is called in the industry) is tapped at regular intervals by opening a hole in the bottom of the furnace and allowing it to flow out. The fluxes combine with the impurities in the coke and ore to form a molten slag, which floats on the iron and is also removed (tapped) at regular intervals.

The hot metal flows into torpedo ladles. These are specially constructed railway containers which transport iron, still in liquid form, to the steel furnace.

The process described above goes on continuously for ten years or more. (This is known as a campaign.) If the furnace were allowed to cool, damage could be caused to its lining of refractory bricks as a result of their contracting as they cooled. Eventually the refractory brick linings are worn away, and at that stage the process is stopped and the furnace relined with new bricks, ready to begin its next campaign.

The iron produced by the blast furnace has a carbon content of 4 to 4.5% as well as a number of other "impurities". This makes it relatively brittle. Steelmaking refines iron, amongst other things by reducing its carbon content, to make it a stronger and more manipulable product.
 
 

The Basic Oxygen Converter

The BOS (Basic Oxygen Steelmaking) process is the major modern process for making bulk steels. In the UK, apart from special quality steels (such as stainless steel), all flat products (see relevant information page), and long products (see relevant information page) over a certain size, are rolled from steel made by the BOS process.

The BOS vessel is first tilted to allow materials to be tipped into it (charged). Scrap steel is first charged into the vessel, followed by hot metal (liquid iron) from the blast furnace. A water-cooled lance is lowered into the vessel through which very pure oxygen is blown at high pressure. The oxygen, through a process known as oxidation, combines with the carbon, and with other unwanted elements, separating them from the metal, leaving steel. Lime-based fluxes (materials that help the chemical process) are charged, and they combine with the "impurities" to form slag.

The main gas formed as a by-product of the oxidation process is carbon-monoxide, and this is sometimes collected for use as a fuel elsewhere in the works.

A careful balance between the amounts of hot metal and scrap charged into the converter is maintained as a means of controlling the temperature and to ensure that steel of the required specification is produced. After a sample has been taken to check that the chemical content of the steel is correct, the vessel is again tilted to allow the molten steel to flow out. This is known as tapping. The steel is tapped into a ladle, in which secondary steelmaking (see relevant information page) frequently takes place. During tapping small quantities of other metals and fluxes are often added to control the state of oxidation and to meet customer requirements for particular grades of steel.

Finally the vessel is turned upside down and the slag tipped out into a container. Steelmaking slag is sometimes recycled to make road building materials.

The modern BOS vessel makes up to 350 tonnes of steel at a time, and the whole process takes about 40 minutes.
 
Definition
Steel is the common name for a large family of iron alloys which are easily malleable after the molten stage. Steels are commonly made from iron ore, coal, and limestone. When these raw materials are put into the blast furnace, the result is a "pig iron" which has a composition of iron, carbon, manganese, sulfur, phosphorus, and silicon. 

As pig iron is hard and brittle, steelmakers must refine the material by purifying it and then adding other elements to strengthen the material. The steel is next deoxidized by a carbon and oxygen reaction. A strongly deoxidized steel is called "killed", and a lesser degrees of deoxodized steels are called "semikilled", "capped", and "rimmed". 

Steels can either be cast directly to shape, or into ingots which are reheated and hot worked into a wrought shape by forging, extrusion, rolling, or other processes. Wrought steels are the most common engineering material used, and come in a variety of forms with different finishes and properties. 

Standard Steels
According to the chemical compositions, standard steels can be classified into three major groups: carbon steels, alloy steels, and stainless steels
Steels Compositions
Carbon Steels Alloying elements do not exceed these limits: 1% carbon, 0.6% copper, 1.65% manganese, 0.4% phosphorus, 0.6% silicon, and 0.05% sulfur. 
Alloy Steels Steels that exceed the element limits for carbon steels. Also includes steels that contain elements not found in carbon steels such as nickel, chromium (up to 3.99%), cobalt, etc.
Stainless Steels Contains at least 10% chromium, with or without other elements. Based on the structures, stainless steels can be grouped into three grades: 
 
Austenitic: Typically contains 18% chromium and 8% nickel and is widely known as 18-8. Nonmagnetic in annealed condition, this grade can only be hardened by cold working.
   
Ferritic: Contains very little nickel and either 17% chromium or 12% chromium with other elements such as aluminum or titanium. Always magnetic, this grade can be hardened only by cold working.
   
Martensitic: Typically contains 12% chromium and no nickel. This grade is magnetic and can be hardened by heat treatment.
Tool Steels
Tool steels typically have excess carbides (carbon alloys) which make them hard and wear-resistant. Most tool steels are used in a heat-treated state, generally hardened and tempered. 

There are a number of categories assigned by AISI (American Iron and Steel Institute), each with an identifying letter: 

 
 
W: Water-Hardening
S: Shock-Resisting
O: Cold-Work (Oil-Hardening)
A: Cold-Work (Medium-Alloy, Air-Hardening)
D: Cold-Work (High-Carbon, High-Chromium)
L: Low-Alloy
F: Carbon-Tungsten
P: P1-P19: Low-Carbon Mold Steels
  P20-P39: Other Mold Steels
H: H1-H19: Chromium-Base Hot Work
  H20-H29: Tungsten-Base Hot Work
  H40-H59: Molybdenum-Base Hot Work
T: High-Speed (Tungsten-Base)
M: High-Speed (Molybdenum-Base)

 

Stainless Steel

Stainless Steel is HBM's material of choice. The biggest advantage is in the corrosive protection. Stainless Steel does not require any protective coatings that may chip or flake off during use. In addition we use strain gages that are matched for the material in use. With this technique the performance of either material will meet all of our customers expectations.
 
 

Iron Based Superalloys

Iron, nickel, and cobalt-based alloys used primarily for high-temperature applications are known as superalloys. The iron-based grades, which are less expensive than cobalt or nickel-based grades, are of three types: alloys that can be strengthened by a martensitic type of transformation, alloys that are austenitic and are strengthened by a sequence of hot and cold working (usually, forging at 2,000 to 2,100°F followed by finishing at 1,200 to 1,600°F), and austenitic alloys strengthened by precipitation hardening.

Some metallurgists consider the last group only as superalloys, the others being categorized as high-temperature, high-strength alloys. In general, the martensitic types are used at temperatures below 1,000°F; the austenitic types, above 1,000°F.

The AISI 600 series of superalloys consists of six subclasses of iron-based alloys:

Iron-based superalloys are characterized by high temperature as well as room-temperature strength and resistance to creep, oxidation, corrosion, and wear. Wear resistance increases with carbon content. Maximum wear resistance is obtained in alloys 611, 612, and 613, which are used in high-temperature aircraft bearings and machinery parts subjected to sliding contact. Oxidation resistance increases with chromium content. The martensitic chromium steels, particularly alloy 616, are used for steam-turbine blades.

The superalloys are available in all conventional mill forms -- billet, bar, sheet, and forgings -- and special shapes are available for most alloys. In general, austenitic alloys are more difficult to machine than martensitic types, which machine best in the annealed condition. Austenitic alloys are usually "gummy" in the solution-treated condition and machine best after being partially aged or fully hardened.

Crack sensitivity makes most of the martensitic steels difficult to weld by conventional methods. These alloys should be annealed or tempered prior to welding; even then, preheating and postheating are recommended. Welding drastically lowers the mechanical properties of alloys that depend on hot/cold work for strength.

All of the martensitic low-alloy steels machine satisfactorily and are readily fabricated by hot working and cold working. The martensitic secondary-hardening and chromium alloys are all hot worked by preheating and hot forging. Austenitic alloys are more difficult to forge than the martensitic grades.

Light metals

Another important development of the late 19th century was the separation from their ores, on a substantial scale, of aluminum and magnesium. In the earlier part of the century, several scientists had made small quantities of these light metals, but the most successful was Henri-Étienne Sainte-Claire Deville, who by 1855 had developed a method by which cryolite, a double fluoride of aluminum and sodium, was reduced by sodium metal to aluminum and sodium fluoride. The process was very expensive, but cost was greatly reduced when the American chemist Hamilton Young Castner developed an electrolytic cell for producing cheaper sodium in 1886. At the same time, however, Charles M. Hall in the United States and Paul-Louis-Toussaint Héroult in France announced their essentially identical processes for aluminum extraction, which were also based on electrolysis. Use of the Hall-Héroult process on an industrial scale depended on the replacement of storage batteries by rotary power generators; it remains essentially unchanged to this day.

Welding

One of the most significant changes in the technology of metals fabrication has been the introduction of fusion welding during the 20th century. Before this, the main joining processes were riveting and forge welding. Both had limitations of scale, although they could be used to erect substantial structures. In 1895 Henry-Louis Le Chatelier stated that the temperature in an oxyacetylene flame was 3,500 C (6,300 F), some 1,000 C higher than the oxyhydrogen flame already in use on a small scale for brazing and welding. The first practical oxyacetylene torch, drawing acetylene from cylinders containing acetylene dissolved in acetone, was produced in 1901. With the availability of oxygen at even lower cost, oxygen cutting and oxyacetylene welding became established procedures for the fabrication of structural steel components.

The metal in a join can also be melted by an electric arc, and a process using a carbon as a negative electrode and the workpiece as a positive first became of commercial interest about 1902. Striking an arc from a coated metal electrode, which melts into the join, was introduced in 1910. Although it was not widely used until some 20 years later, in its various forms it is now responsible for the bulk of fusion welds.
 
 

Metallography

The 20th century has seen metallurgy change progressively, from an art or craft to a scientific discipline and then to part of the wider discipline of materials science. In extractive metallurgy, there has been the application of chemical thermodynamics, kinetics, and chemical engineering, which has enabled a better understanding, control, and improvement of existing processes and the generation of new ones. In physical metallurgy, the study of relationships between macrostructure, microstructure, and atomic structure on the one hand and physical and mechanical properties on the other has broadened from metals to other materials such as ceramics, polymers, and composites.

This greater scientific understanding has come largely from a continuous improvement in microscopic techniques for metallography, the examination of metal structure. The first true metallographer was Henry Clifton Sorby of Sheffield, Eng., who in the 1860s applied light microscopy to the polished surfaces of materials such as rocks and meteorites. Sorby eventually succeeded in making photomicrographic records, and by 1885 the value of metallography was appreciated throughout Europe, with particular attention being paid to the structure of steel. For example, there was eventual acceptance, based on micrographic evidence and confirmed by the introduction of X-ray diffraction by William Henry and William Lawrence Bragg in 1913, of the allotropy of iron and its relationship to the hardening of steel. During subsequent years there were advances in the atomic theory of solids; this led to the concept that, in nonplastic materials such as glass, fracture takes place by the propagation of preexisting cracklike defects and that, in metals, deformation takes place by the movement of dislocations, or defects in the atomic arrangement, through the crystalline matrix. Proof of these concepts came with the invention and development of the electron microscope; even more powerful field ion microscopes and high-resolution electron microscopes now make it possible to detect the position of individual atoms.

Another example of the development of physical metallurgy is a discovery that revolutionized the use of aluminum in the 20th century. Originally, most aluminum was used in cast alloys, but the discovery of age hardening by Alfred Wilm in Berlin about 1906 yielded a material that was twice as strong with only a small change in weight. In Wilm's process, a solute such as magnesium or copper is trapped in supersaturated solid solution, without being allowed to precipitate out, by quenching the aluminum from a higher temperature rather than slowly cooling it. The relatively soft aluminum alloy that results can be mechanically formed, but, when left at room temperature or heated at low temperatures, it hardens and strengthens. With copper as the solute, this type of material came to be known by the trade name Duralumin. The advances in metallography described above eventually provided the understanding that age hardening is caused by the dispersion of very fine precipitates from the supersaturated solid solution; this restricts the movement of the dislocations that are essential to crystal deformation and thus raises the strength of the metal. The principles of precipitation hardening have been applied to the strengthening of a large number of alloys.

(C.H.L./J.A.C.)

Extractive metallurgy

Following separation and concentration by mineral processing, metallic minerals are subjected to extractive metallurgy, in which their metallic elements are extracted from chemical compound form and refined of impurities.

Metallic compounds are frequently rather complex mixtures (those treated commercially are for the most part sulfides, oxides, carbonates, arsenides, or silicates), and they are not often types that permit extraction of the metal by simple, economical processes. Consequently, before extractive metallurgy can effect the separation of metallic elements from the other constituents of a compound, it must often convert the compound into a type that can be more readily treated. Common practice is to convert metallic sulfides to oxides, sulfates, or chlorides; oxides to sulfates or chlorides; and carbonates to oxides. The processes that accomplish all this can be categorized as either pyrometallurgy or hydrometallurgy. Pyrometallurgy involves heating operations such as roasting, in which compounds are converted at temperatures just below their melting points, and smelting, in which all the constituents of an ore or concentrate are completely melted and separated into two liquid layers, one containing the valuable metals and the other the waste rock. Hydrometallurgy consists of such operations as leaching, in which metallic compounds are selectively dissolved from an ore by an aqueous solvent, and electrowinning, in which metallic ions are deposited onto an electrode by an electric current passed through the solution.

Extraction is often followed by refining, in which the level of impurities is brought lower or controlled by pyrometallurgical, electrolytic, or chemical means. Pyrometallurgical refining usually consists of the oxidizing of impurities in a high-temperature liquid bath. Electrolysis is the dissolving of metal from one electrode of an electrolytic cell and its deposition in a purer form onto the other electrode. Chemical refining involves either the condensation of metal from a vapour or the selective precipitation of metal from an aqueous solution. (See electrometallurgy, electrorefining.)

The processes to be used in extraction and refining are selected to fit into an overall pattern, with the product from the first process becoming the feed material of the second process, and so on. It is quite common for hydrometallurgical, pyrometallurgical, and electrolytic processes to be used one after another in the treatment of a single metal. The choices depend on several conditions. One of these is that certain types of metallic compounds lend themselves to easiest extraction by certain methods; for example, oxides and sulfates are readily dissolved in leach solutions, while sulfides are only slightly soluble. Another condition is the degree of purity, which can vary from one type of extraction to another. Zinc production illustrates this, in that zinc metal produced by pyrometallurgical retort or blast-furnace operations is 98 percent pure, with traces of lead, iron, and cadmium. This is adequate for galvanizing, but the preferred purity for die-casting (99.99 percent) must be obtained hydrometallurgically, from the electrolysis of a zinc sulfate solution. Also to be considered in choosing a processing method is the recovery of particular impurities that may have value themselves as by-products. One example is copper refining: the pyrometallurgical refining of blister copper removes many impurities, but it does not recover or remove silver or gold; the precious metals are recovered, however, by subsequent electrolytic refining.

Pyrometallurgy

Two of the most common pyrometallurgical processes, in both extraction and refining, are oxidation and reduction. In oxidation, metals having a great affinity for oxygen selectively combine with it to form metallic oxides; these can be treated further in order to obtain a pure metal or can be separated and discarded as a waste product. Reduction can be viewed as the reverse of oxidation. In this process, a metallic oxide compound is fed into a furnace along with a reducing agent such as carbon. The metal releases its combined oxygen, which recombines with the carbon to form a new carbonaceous oxide and leaves the metal in an uncombined form.

Oxidation and reduction reactions are either exothermic (energy-releasing) or endothermic (energy-absorbing). One example of an exothermic reaction is the oxidation of iron sulfide (FeS) to form iron oxide (FeO) and sulfur dioxide (SO2) gas:

This process gives off large quantities of heat beyond that required to initiate the reaction. One endothermic reaction is the smelting reduction of zinc oxide (ZnO) by carbon monoxide (CO) to yield zinc (Zn) metal and carbon dioxide (CO2):

For this reaction to proceed at a reasonable rate, external heat must be supplied to maintain the temperature at 1,300 to 1,350 C (2,375 to 2,450 F).

Roasting

As stated above, for those instances in which a metal-bearing compound is not in a chemical form that permits the metal to be easily and economically removed, it is necessary first to change it into some other compound. The preliminary treatment that is commonly used to do this is roasting.

Processes

There are several different types of roast, each one intended to produce a specific reaction and to yield a roasted product (or calcine) suitable for the particular processing operation to follow. The roasting procedures are:
 
 

1. Oxidizing roasts, which remove all or part of the sulfur from sulfide metal compounds, replacing the sulfides with oxides. (The sulfur removed goes off as sulfur dioxide gas.) Oxidizing roasts are exothermic.
 
 
 
 

2. Sulfatizing roasts, which convert certain metals from sulfides to sulfates. Sulfatizing roasts are exothermic.

3. Reducing roasts, which lower the oxide state or even completely reduce an oxide to a metal. Reducing roasts are exothermic.

4. Chloridizing roasts, or chlorination, which change metallic oxides to chlorides by heating with a chlorine source such as chlorine gas, hydrochloric acid gas, ammonium chloride, or sodium chloride. These reactions are exothermic.

5. Volatilizing roasts, which eliminate easily volatilized oxides by converting them to gases.

6. Calcination, in which solid material is heated to drive off either carbon dioxide or chemically combined water. Calcination is an endothermic reaction.

Roasters

Each of the above processes can be carried out in specialized roasters. The types most commonly in use are fluidized-bed, multiple-hearth, flash, chlorinator, rotary kiln, and sintering machine (or blast roaster).

Fluidized-bed roasters (see figure) have found wide acceptance because of their high capacity and efficiency. They can be used for oxidizing, sulfatizing, and volatilizing roasts. The roaster is a refractory-lined, upright cylindrical steel shell with a grate bottom through which air is blown in sufficient volume to keep fine, solid feed particles in suspension and give excellent gas-solid contact. The ore feed can be introduced dry or as a water suspension through a downpipe into the turbulent layer zone of the roaster. Discharge of the roasted calcines is through a side overflow pipe.

Multiple-hearth roasters also have found wide acceptance in that they can be used for oxidizing, sulfatizing, chloridizing, volatilizing, reducing, and calcining processes. The roaster is a refractory-lined, vertical cylindrical steel shell in which are placed a number of superimposed refractory hearths. A slowly rotating central shaft turns rabble arms on each hearth both to stir the roasting material and to push it into drop holes leading to the hearth below. Feed material is fed to the top hearth, and, as it follows a zig-zag path across the hearths and downward, it meets the rising gas stream that effects the roasting. The calcines are discharged from the bottom hearth.

Flash roasters are used only for oxidizing roasts and are, in effect, multiple-hearth roasters with the central hearths removed. This design came with the realization that much of the oxidizing takes place as the particles are actually dropping from hearth to hearth.

Chlorinators are used for roasting oxides to chlorides. They are tall, circular steel shells lined with refractory brick to prevent chlorine attack on the steel. The top of each chlorinator has a sealed hopper for periodic feed charging, and gaseous or liquid chlorine is added at the bottom of the unit. Heat is supplied by electrical resistance through the shell wall and by any exothermic reaction that may occur. The product depends on the chloridizing reaction taking place, with magnesium dichloride, for example, forming as a watery liquid and titanium tetrachloride coming off as a gas.

Calcination of carbonates to oxides is done in a horizontal rotary kiln, which is a mild-steel circular shell lined with refractory material and having a length 10 to 12 times the diameter. Sloping slightly downward from feed to discharge ends, the kiln slowly rotates while fuel-fired burners located inside the kiln provide the required heat.

A sintering machine, or blast roaster, can conduct oxidizing or reducing roasts and then agglomerate the roasted calcines, or it can be used for agglomeration alone. (Agglomeration is the fusing of fine feed material into larger chunks that can be fed into a blast furnace or retort, eliminating the problem of losing the fine feed in the hot air blast.) The oxidizing or reducing reaction is exothermic, but in order for agglomeration alone to be conducted, a fuel such as fine coke must be mixed with the charge.

The sintering machine consists of an endless belt of moving metal pallets with grate bottoms on which a fine feed charge is spread and passed under a burner. As the charge ignites, the pallet passes over a suction wind box, so that air being drawn through the feed layer causes combustion (i.e., oxidation) of sulfur or carbon to continue from top to bottom. Because the temperature is high and there is no agitation of the feed, a partial fusion takes place on the surface of the particles, leaving them adhering together in the form of a porous, cellular clinker known as sinter.

Smelting

Smelting is a process that liberates the metallic element from its compound as an impure molten metal and separates it from the waste rock part of the charge, which becomes a molten slag. There are two types of smelting, reduction smelting and matte smelting. In reduction smelting, both the metallic charge fed into the smelter and the slag formed from the process are oxides; in matte smelting, the slag is an oxide while the metallic charge is a combination of metallic sulfides that melt and recombine to give a homogeneous metallic sulfide called matte.

Reduction smelting

Many types of furnace are used for reduction smelting. The blast furnace is universally used in the reduction of such compounds as iron oxide, zinc oxide, and lead oxide, though there are great differences between the furnace designs used in each case. Iron, found naturally in the oxide ores hematite and magnetite, is smelted in a tall, circular, sealed blast furnace (see figure). A sintered or pelletized feed consisting of coke (for fuel), limestone (as a flux for slag making), and iron oxide is charged into the top of the furnace through a double bell or rotating chute, and heated air is blown in through nozzles, or tuyeres, close to the furnace bottom. In the ensuing combustion reaction, oxygen in the air combines with carbon in the coke, generating enough heat to melt the furnace charge and forming carbon monoxide, which, in turn, reduces the iron oxide to metallic iron. The furnace is sealed to prevent the escape of carbon monoxide gas, which is recovered and burned as fuel to heat the tuyere air. In the hearth at the furnace bottom, molten slag and iron collect in two layers, the lighter slag on top. Both are periodically drawn off, with the slag being discarded and the iron going on to be refined into steel.

The zinc blast furnace also is a sealed furnace, with a charge of sintered zinc oxide and preheated coke added through a sealed charging bell. The furnace is rectangular, with a shorter shaft than the iron blast furnace. A blast of hot air through the tuyeres provides oxygen to burn the coke for heat and to supply carbon monoxide reducing gas. The reduced zinc passes out of the furnace as vapour, and this is drawn off to a spray bath of molten lead and condensed to liquid zinc metal. Slag and any lead present in the charge are tapped as liquids from the furnace hearth. (For an illustration of a zinc-lead blast furnace, see figure).

The lead blast furnace is similar in size and shape to the zinc blast furnace, but it is not a sealed furnace, and it does not use preheated tuyere air. A charge of lead oxide sinter, coke, and flux is poured into the open top of the furnace, and the strong reducing atmosphere in the furnace shaft reduces the oxide to metal. Liquid lead and slag collect in two layers in the furnace hearth, with the lead in the bottom layer and slag above.

Two newer processes for the direct reduction of unroasted lead sulfide concentrate are the QSL (Queneau-Schuhmann-Lurgi) and the KIVCET (a Russian acronym for "flash-cyclone-oxygen-electric smelting"). In the QSL reactor a submerged injection of shielded oxygen oxidizes lead sulfide to lead metal, while the KIVCET is a type of flash-smelting furnace in which fine, dried lead sulfide concentrate combines with oxygen in a shaft to give lead metal.

Matte smelting

The primary purpose of matte smelting is to melt and recombine the charge into a homogeneous matte of metallic copper, nickel, cobalt, and iron sulfides and to give an iron and silicon oxide slag. It is done in many types of furnace on both roasted or unroasted sulfide feed material.

The reverberatory furnace is essentially a rectangular refractory-brick box equipped with end-wall burners to provide heat for melting. The furnace is relatively quiet, and it does not blow out much fine feed (which is added through roof ports) with the exhaust gases. The matte is tapped periodically from a centre taphole, while the slag runs off continuously at the furnace flue end. Oxygen lances inserted through the roof, or oxygen added through the burners, can increase smelting capacity considerably.

Electric furnaces are similar to reverberatory furnaces except for the method of heating--in this case a row of electrodes projecting through the roof into the slag layer on the furnace hearth and heating by resistance.

Flash smelting is a relatively recent development that has found worldwide acceptance. It is an autogenous process, using the oxidation of sulfides in an unroasted charge to supply the heat required to reach reaction temperatures and melt the feed material. The most widely used furnace has a vertical reaction shaft at one end of a long, low settling hearth and a vertical gas-uptake shaft at the other end. Fine, unroasted feed is blown into the reaction shaft along with preheated air; these react instantaneously, and liquid droplets fall onto the settling hearth, separating into layers of slag and matte. The off-gas, high in sulfur dioxide, is ideal for sulfur-recovery processes.

The second stage of matte smelting is converting the sulfides to metal. For many years the standard vessel for this operation has been the Peirce-Smith converter. This is a rotatable, refractory-lined, horizontal steel drum with an opening at the centre of the top for charging and discharging and a row of tuyeres across the back through which air, oxygen-enriched air, or oxygen can be blown into the liquid bath. Molten matte from the smelting furnace is poured into the converter, after which gas is blown through the tuyeres to oxidize first iron and then sulfur. The sulfur goes off as sulfur dioxide gas and the iron as iron oxide slag, leaving semipure metal. Considerable heat is generated by this exothermic reaction, keeping the bath liquid and maintaining the required reaction temperature.

More recent processes take advantage of exothermic heat evolution to accomplish both the smelting of unroasted sulfides and the conversion of matte in one combined operation. These are the Noranda, TBRC (top-blown rotary converter), and Mitsubishi processes. The Noranda reactor is a horizontal cylindrical furnace with a depression in the centre where the metal collects and a raised hearth at one end where the slag is run off. Pelletized unroasted sulfide concentrate is poured into the molten bath at one end, where tuyeres inject an air-oxygen mixture. This causes an intense mixing action that aids the melting, smelting, and oxidation steps, which follow one another in sequence, by taking advantage of the exothermic heat. The TBRC also is cylindrical in shape but is inclined at 17 to the horizontal, has an open mouth at the high end for charging and pouring, and revolves at 5 to 40 rotations per minute. A lance inserted through the mouth can give any combination of oxygen, air, or natural gas to impinge on the molten bath and create the conditions required for smelting and oxidizing. The combination of surface blowing and bath rotation improves the performance of the converter. The Mitsubishi process is a continuous smelting-converting operation that uses three stationary furnaces in series. The first furnace is for smelting, with oxygen lances and a fuel-fired burner inserted through the roof. Slag and matte flow from here to a slag-cleaning furnace (heated by electric arc), and high-grade matte flows from this to the converting furnace, where oxygen-enriched air is blown into the bath through roof lances. Exothermic heat produced here is sufficient to keep the bath up to reaction temperature.

Electrolytic smelting

Smelting is also carried out by the electrolytic dissociation, at high temperatures, of a liquid metallic chloride compound (as is done with magnesium) or of a metallic oxide powder dissolved in molten electrolyte (as is done with aluminum). In each case, electric current is passed through the bath to dissociate the metallic compound; the metal released collects at the cathode, while a gas is given off at the anode.

The magnesium smelting cell consists of a steel pot that serves as the cathode; two rows of graphite electrodes are inserted through a refractory cover as anodes. The electrolyte is a mixture of chlorides, with magnesium chloride making up 20 percent, and the cell is maintained at 700 C (1,300 F). The passage of current breaks down the magnesium chloride into chlorine gas and magnesium metal, which go to the anode and cathode, respectively.

In the Hall-Héroult smelting process, a nearly pure aluminum oxide compound called alumina is dissolved at 950 C (1,750 F) in a molten electrolyte composed of aluminum, sodium, and fluorine; this is electrolyzed to give aluminum metal at the cathode and oxygen gas at the anode. The smelting cell is a carbon-lined steel box, which acts as the cathode, and a row of graphite electrodes inserted into the bath serves as anodes.

Refining

Refining is the final procedure for removing (and often recovering as by-products) the last small amounts of impurities left after the major extraction steps have been completed. It leaves the major metallic element in a practically pure state for commercial application. The procedure is accomplished in three ways: refining by fire, by electrolytic, or by chemical methods.

Fire refining

Iron, copper, and lead are fire-refined by selective oxidation. In this process, oxygen or air is added to the impure liquid metal; the impurities oxidize before the metal and are removed as an oxide slag or a volatile oxide gas. (See pyrometallurgy.)

The basic oxygen furnace (BOF) is a vessel used to convert pig iron, of about 94 percent iron and 6 percent combined impurities such as carbon, manganese, and silicon, into steel with as little as 1 percent combined impurities. The BOF is a large pear-shaped unit that can be tilted to charge and pour. Molten blast-furnace iron and steel scrap are charged into the furnace; then it is turned to an upright position and a lance inserted to blow high-tonnage oxygen gas into the bath. Oxidation reactions occur rapidly, with silicon and manganese oxidizing first and combining to form an oxide slag, then carbon oxidizing to carbon monoxide gas and burning to carbon dioxide as it leaves the furnace mouth. These reactions are strongly exothermic and keep the vessel up to its reaction temperature without any external heat or fuel being added.

Converter-produced blister copper and blast-furnace lead also are treated by fire refining, with both processes depending on the weaker affinity for oxygen of the metals than the impurities they contain. Molten copper in a small reverberatory-type furnace has compressed air blown into it through steel pipes below the surface. This oxidizes zinc, tin, iron, lead, arsenic, antimony, and sulfur; the sulfur goes off as sulfur dioxide gas, while the other impurities form an oxide slag that is skimmed off. Lead is refined in much the same way, with compressed air blown into a molten lead bath and the major impurities of tin, antimony, and arsenic oxidizing in that order, rising to the surface as skims and being scraped off.

Other fire-refining operations use fractional distillation. By this method, zinc metal of 98-percent purity can be upgraded to 99.995-percent purity. The main impurities in blast-furnace zinc are lead and cadmium, with lead boiling at 1,744 C (3,171 F), zinc at 907 C (1,665 F), and cadmium at 765 C (1,409 F). In the first stage zinc and cadmium are boiled off, leaving liquid lead, and in the second stage cadmium is boiled off to leave special high-purity zinc metal.

Electrolytic refining

This method gives the highest-purity metal product as well as the best recovery of valuable impurities. It is used for copper, nickel, lead, gold, and silver. The metal to be refined is cast into a slab, which becomes the anode of an electrolytic cell; another sheet of metal is the cathode. Both electrodes are immersed in an aqueous electrolyte capable of conducting an electric current. As a direct current is impressed on the cell, metal ions dissolve from the anode and deposit at the cathode. The insoluble sludge left in the cell is treated to recover any valuable by-product metals.

Chemical refining

An example of chemical refining is the nickel carbonyl process, in which impure nickel metal is selectively reacted with carbon monoxide gas to form nickel carbonyl gas. This gas is then decomposed to give high-purity nickel metal.

Hydrometallurgy

Hydrometallurgy is concerned with the selective leaching of metallic compounds to form a solution from which the metals can be precipitated and recovered. Leaching processes are used when it is the simplest method or when the ore is of too low a grade for more expensive extractive procedures.

Conversion

Because not all ores and concentrates are found naturally in a form that is satisfactory for leaching, they must often be subjected to preliminary operations. For example, sulfide ores, which are relatively insoluble in sulfuric acid, can be converted to quite soluble forms by oxidizing or sulfatizing roasts. On the other hand, oxide ores and concentrates can be given a controlled reducing roast in order to produce a calcine containing a reduced metal that will dissolve easily in the leaching solution. These treatments are described in more detail above (see Pyrometallurgy: Roasting).

A second popular treatment for converting sulfides is pressure oxidation, in which the sulfides are oxidized to a porous structure that provides good access for the leaching solution. This treatment was developed for the recovery of gold from sulfide ores, which are not suitable for cyanide leaching without first being oxidized. A finely ground concentrate slurry is preheated to 175 C (350 F) and pumped into a four- or five-compartment autoclave, each compartment containing an agitator. Gaseous oxygen is added to each compartment, and retention time in the autoclave is two hours in order to achieve the desired oxidation.

Leaching

Oxides are leached with a sulfuric acid or sodium carbonate solvent, while sulfates can be leached with water or sulfuric acid. Ammonium hydroxide is used for native ores, carbonates, and sulfides, and sodium hydroxide is used for oxides. Cyanide solutions are a solvent for the precious metals, while a sodium chloride solution dissolves some chlorides. In all cases the leach solvent should be cheap and available, strong, and preferably selective for the values present.

Leaching is carried out by two main methods: simple leaching at ambient temperature and atmospheric pressure; and pressure leaching, in which pressure and temperature are increased in order to accelerate the operation. The method chosen depends on the grade of the feed material, with richer feed accommodating a costlier, more extensive treatment.

Leaching in-place, or in situ leaching, is practiced on ores that are too far underground and of too low a grade for surface treatment. A leach solution is circulated down through a fractured ore body to dissolve the values and is then pumped to the surface, where the values are precipitated.

Heap leaching is done on ores of semilow grade--that is, high enough to be brought to the surface for treatment. This method is increasing in popularity as larger tonnages of semilow-grade ore are mined. The ore is piled in heaps on pads and sprayed with leach solution, which trickles down through the heaps while dissolving the values. The pregnant solution is drained away and taken to precipitation tanks.

Higher-grade ores are treated by tank leaching, which is carried out in two ways. One method is of very large scale, with several thousand tons of ore treated at a time in large concrete tanks with a circulating solution. In the second method, small amounts of finely ground high-grade ore are agitated in tanks by air or by mechanical impellers. Both solutions pass to precipitation after leaching is completed.

Pressure leaching shortens the treatment time by improving the solubility of solids that dissolve only very slowly at atmospheric pressure. For this process autoclaves are used, in both vertical and horizontal styles. After leaching, the pregnant solution is separated from the insoluble residue and sent to precipitation.

Recovery

Pregnant solution from leaching operations is treated in a variety of ways to precipitate the dissolved metal values and recover them in solid form. These include electrolytic deposition, transfer of metal ions, chemical precipitation, solvent extraction in combination with electrolytic and chemical methods, and carbon adsorption combined with electrolytic treatment.

Electrolytic deposition, also called electrowinning, gives a pure product and is a preferred method. However, it is expensive, owing to the cost of electricity, and must have a solution of high metal content. Insoluble anodes, and cathodes made of either a strippable inert material or a thin sheet of the deposited metal, are inserted into a tank containing leach solution. As current is passed, the solution dissociates, and metal ions deposit at the cathode. This common method is used for copper, zinc, nickel, and cobalt.

Solvent extraction combined with electrolytic deposition takes dilute, low-value metal solutions and concentrates them into small volumes and high metal contents, rendering them satisfactory for electrolytic treatment. Low-grade copper ores are processed in this manner. First, a large volume of a low-value copper leach solution (2.5 grams per litre, or 0.33 ounces per gallon) is contacted with a small volume of water-immiscible organic solvent in kerosene. The metal values pass from the leach solution into the extraction solution, the two phases are separated, and the extraction solution goes on to the stripping circuit. Here another fluid is added that has a still greater affinity for the metal values, picking them out of the extraction solution. The two solutions are separated, with the small volume of stripping solution having a metal content high enough (50 grams per litre, or 6.6 ounces per gallon) to be suitable for electrolytic precipitation.

An adsorption circuit is used to strip pregnant solutions of gold cyanide with activated carbon. The carbon is in turn stripped of the metal by a solution, which then goes to an electrolytic cell where the gold content is deposited at the cathode.

Chemical precipitation can be accomplished in a number of ways. In one method, a displacement reaction takes place in which a more active metal replaces a less active metal in solution. For example, in copper cementation iron replaces copper ions in solution, solid particles of copper precipitating while iron goes into solution. This is an inexpensive method commonly applied to weak, dilute leach solutions. Another displacement reaction uses gas, with hydrogen sulfide, for example, added to a solution containing nickel sulfate and precipitating nickel sulfide. Finally, changing the acidity of a solution is a common method of precipitation. Yellow cake, a common name for sodium diuranate, is precipitated from a concentrated uranium leach solution by adding sodium hydroxide to raise the pH to 7.

Physical metallurgy

Physical metallurgy is the science of making useful products out of metals. Metal parts can be made in a variety of ways, depending on the shape, properties, and cost desired in the finished product. The desired properties may be electrical, mechanical, magnetic, or chemical in nature; all of them can be enhanced by alloying and heat treatment. The cost of a finished part is often determined more by its ease of manufacture than by the cost of the material. This has led to a wide variety of ways to form metals and to an active competition among different forming methods, as well as among different materials. Large parts may be made by casting. Thin products such as automobile fenders are made by forming metal sheets, while small parts are often made by powder metallurgy (pressing powder into a die and sintering it). Usually a metal part has the same properties throughout. However, if only the surface needs to be hard or corrosion-resistant, the desired performance can be obtained through a treatment that changes only the composition and strength of the surface.

 

Structures and properties of metals

Metallic crystal structures

Metals are used in engineering structures (e.g., automobiles, bridges, pressure vessels) because, in contrast to glass or ceramic, they can undergo appreciable plastic deformation before breaking. This plasticity stems from the simplicity of the arrangement of atoms in the crystals making up a piece of metal and the nondirectional nature of the bond between the atoms. Atoms can be arranged in many different ways in crystalline solids, but in metals the packing is in one of three simple forms. In the most ductile metals, atoms are arranged in a close-packed manner. If atoms were visualized as identical spheres and if these spheres were packed into planes in the closest possible manner, there would be two ways to stack close-packed planes one above another (see figure). One would lead to a crystal with hexagonal symmetry (called hexagonal close-packed, or hcp); the other would lead to a crystal with cubic symmetry that could also be visualized as an assembly of cubes with atoms at the corners and at the centre of each face (known as face-centred cubic, or fcc). Examples of metals with the hcp type of structure are magnesium, cadmium, zinc, and alpha titanium. Metals with the fcc structure include aluminum, copper, nickel, gamma iron, gold, and silver.

The third common crystal structure in metals can be visualized as an assembly of cubes with atoms at the corners and an atom in the centre of each cube; this is known as body-centred cubic, or bcc. Examples of metals with the bcc structure are alpha iron, tungsten, chromium, and beta titanium.

Some metals, such as titanium and iron, exhibit different crystal structures at different temperatures. The lowest-temperature structure is labeled alpha (), and higher-temperature structures beta (), gamma (), and delta (). This allotropy, or transformation from one structure to another with changing temperature, leads to the marked changes in properties that can come from heat treatment (see below Heat treating).

When a metal undergoes a phase change from liquid to solid or from one crystal structure to another, the transformation begins with the nucleation and growth of many small crystals of the new phase. All these crystals, or grains, have the same structure but different orientations, so that, when they finally grow together, boundaries form between the grains. These boundaries play an important role in determining the properties of a piece of metal. At room temperature they strengthen the metal without reducing its ductility, but at high temperatures they often weaken the structure and lead to early failure. They can be the site of localized corrosion, which also leads to failure.

Mechanical properties

When a metal rod is lightly loaded, the strain (measured by the change in length divided by the original length) is proportional to the stress (the load per unit of cross-sectional area). This means that, with each increase in load, there is a proportional increase in the rod's length, and, when the load is removed, the rod shrinks to its original size. The strain here is said to be elastic, and the ratio of stress to strain is called the elastic modulus. If the load is increased further, however, a point called the yield stress will be reached and exceeded. Strain will now increase faster than stress, and, when the sample is unloaded, a residual plastic strain (or elongation) will remain. The elastic strain at the yield stress is typically 0.1 to 1 percent, whereas, with the sample pulled to rupture, the plastic strain is typically 20 to 40 percent for an alloy (it may exceed 100 percent in some cases).

The most important mechanical properties of a metal are its yield stress, its ductility (measured by the elongation to fracture), and its toughness (measured by the energy absorbed in tearing the metal). The yield stress of a metal is determined by the resistance to slipping of one plane of atoms over another. Various barriers to slip can be produced by heat treatment and alloying; examples of such barriers are grain boundaries, fine precipitates, distortion introduced by cold working the metal, and alloying elements dissolved in the metal.

When a metal is made very strong through one or more of these methods, it may suddenly fracture under a load instead of yielding. This is particularly true when the metal contains notches or cracks that locally raise the stress and localize the yielding. The property of interest then becomes the fracture toughness, measured by the energy required to extend an existing crack in a piece of metal. In almost all cases, the fracture toughness of an alloy can be improved only by reducing its yield strength. The only exception to this is a smaller grain size, which increases both toughness and strength.

Electrical properties

The electrical conductivity of a metal (or its reciprocal, electrical resistivity) is determined by the ease of movement of electrons past the atoms under the influence of an electric field. This movement is particularly easy in copper, silver, gold, and aluminum--all of which are well-known conductors of electricity. The conductivity of a given metal is decreased by phenomena that deflect, or scatter, the moving electrons. These can be anything that destroys the local perfection of the atomic arrangement--for example, impurity atoms, grain boundaries, or the random oscillation of atoms induced by thermal energy. This last example explains why the conductivity of a metal increases substantially with falling temperature: in a pure metal at room temperature, most resistance to the motion of free electrons comes from the thermal vibration of the atoms; if the temperature is reduced to almost absolute zero, where thermal motion essentially stops, conductivity can increase several thousandfold.

Magnetic properties

When an electric current is passed through a coil of metal wire, a magnetic field is developed around the coil. When a piece of copper is placed inside the coil, this field increases by less than 1 percent, but, when a piece of iron, cobalt, or nickel is placed inside the coil, the external field can increase 10,000 times. This strong magnetic property is known as ferromagnetism, and the three metals listed above are the most prominent ferromagnetic metals. When the piece of ferromagnetic metal is removed from the coil, it retains some of this magnetism (that is, it is magnetized). If the metal is hard, as in a hardened piece of steel, the loss, or reversal, of magnetization will be slow, and the sample will be useful as a permanent magnet. If the metal is soft, it will quickly lose its magnetism; this will make it useful in electrical transformers, where rapid reversal of magnetization is essential.

In many types of solids, the atoms possess a permanent magnetic moment (they act like small bar magnets). In most solids, the direction of these moments is arranged at random. What is exceptional about ferromagnetic solids is that the interatomic forces cause the moments of neighbouring atoms spontaneously to align in the same direction. If the moments of all of the atoms in a single sample lined up in the same direction, the sample would be an exceptionally strong magnet with exceptionally high energy. That energy would be reduced if the sample broke up into domains, with all atomic moments in each domain being aligned but the direction of magnetization in adjacent domains being in opposite directions and thus tending to cancel one another. This is what happens when a ferromagnetic metal is magnetized: all domains do not take on the same orientation, but domains of one orientation grow at the expense of others. The alignment of atomic magnetic moments within a domain is weakened by thermally induced oscillations, and ferromagnetism is finally lost above the Curie point, which is 770 C (1,420 F) for iron and 358 C (676 F) for nickel.

Chemical properties

Almost any metal will oxidize in air, the only exception being gold. At room temperature a clean metal surface will oxidize very little, since a thin oxide film forms and protects the metal from further oxidation. At elevated temperatures, though, oxidation is faster, and the film is less protective. Many chemicals accelerate this corrosion process (that is, the conversion of a metal to an oxide in air or to a hydroxide in the presence of water).

A special property of metal surfaces is their ability to catalyze chemical reactions. For example, in the exhaust system of most automobiles, combustion gases pass over a dispersion of very fine platinum particles. The surfaces of these particles greatly accelerate the oxidation of carbon monoxide and hydrocarbons to carbon dioxide and water, thus reducing the toxicity of the exhaust gases.
 
 

Alloying

Almost all metals are used as alloys--that is, mixtures of several elements--because these have properties superior to pure metals. Alloying is done for many reasons, typically to increase strength, increase corrosion resistance, or reduce costs.

Processes

In most cases, alloys are mixed from commercially pure elements. Mixing is relatively easy in the liquid state but slow and difficult in the solid state, so that most alloys are made by melting the base metal--for instance, iron, aluminum, or copper--and then adding the alloying agents. Care must be taken to avoid contamination, and in fact purification is often carried out at the same time, since this is also done more easily in the liquid state. Examples can be found in steelmaking, including the desulfurizing of liquid blast-furnace iron in a ladle, the decarburization of the iron during its conversion to steel, the removal of oxygen from the liquid steel in a vacuum degasser, and finally the addition of tiny amounts of alloying agents to bring the steel to the desired composition.

The largest tonnages of alloys are melted in air, with the slag being used to protect the metal from oxidation. However, a large and increasing amount is melted and poured entirely in a vacuum chamber. This allows close control of the composition and minimizes oxidation. Most of the alloying elements needed are placed in the initial charge, and melting is done with electricity, either by induction heating or by arc melting. Induction melting is conducted in a crucible, while in arc melting the melted droplets drip from the arc onto a water-cooled pedestal and are immediately solidified.

Sometimes an inhomogeneous, composite structure is desired, as in cemented tungsten carbide cutting tools. In such cases, the alloy is not melted but is made by powder metallurgical techniques.

 
Increasing strength
The most common reason for alloying is to increase the strength of a metal. This requires that barriers to slip be distributed uniformly throughout the crystalline grains. On the finest scale, this is done by dissolving alloying agents in the metal matrix (a procedure known as solid solution hardening). The atoms of the alloying metals may substitute for matrix atoms on regular sites (in which case they are known as substitutional elements), or, if they are appreciably smaller than the matrix atoms, they may take up places between regular sites (where they are called interstitial elements).

The next coarser type of barrier to slip is a fine, solute-rich precipitate with dimensions of only tens or hundreds of atomic diameters. These particles are formed by heat treatment. The metal is heated to a temperature at which the solute-rich phase dissolves (e.g., 5 percent copper in aluminum at 540 C [1,000 F]), and then it is rapidly cooled to avoid precipitation. The next step is to form a fine precipitate throughout the sample by aging at an elevated temperature that is well below the temperature used for the initial dissolution.

In metals that undergo transformations from one crystal structure to another on heating (e.g., iron or titanium), the difference in solute solubility between the high- and low-temperature phases is often utilized. For example, in the low-alloy steels used for tools and gears, carbon forms the hardening precipitate. Carbon is much more soluble in the high-temperature fcc phase (gamma iron, also called austenite) than in the low-temperature bcc phase (alpha iron, or ferrite). The other alloying elements added (e.g., chromium, nickel, and molybdenum) retard the transformation of austenite on cooling, so that the fcc-to-bcc transformation occurs at a low temperature by a sudden, shear transformation; this allows no time for carbon precipitation and makes the steel harder. A final reheating tends to coarsen the precipitate and thereby increase ductility; this is commonly called tempering.

An array of barriers on the same scale as precipitation hardening can be created by plastically deforming the metal at room temperature. This is often done in a cold-working operation such as rolling, forging, or drawing. The deformation occurs through the generation and motion of line defects, called dislocations, on slip planes spaced only a few hundred atom diameters apart. When slip occurs on different planes, the intersecting dislocations form tangles that inhibit further slip on those planes. Such strain hardening can double or triple the yield stress of a metal.
 
 

Increasing corrosion resistance
Alloys can have much better high-temperature oxidation resistance than pure metals. The alloying elements most commonly used for this purpose are chromium and aluminum, both of which form an adherent film of stable oxide on the surface that protects the metal from further oxidation. Eleven percent or more chromium is added to iron to create a stainless steel, while 10 to 15 percent chromium and 3 to 5 percent aluminum are commonly added to the nickel- or cobalt-based superalloys used in the highest-temperature components of jet engines.

Inhibiting the corrosion of alloys in water is more varied and complex than inhibiting high-temperature oxidation. Nevertheless, one of the most common techniques is to add alloying elements that inhibit the corrosion.

Reducing costs
Gold and silver used in jewelry and coins are alloyed with other metals to increase strength and reduce cost. Sterling silver contains 7.5 percent base metal, commonly copper. The fraction of gold in gold jewelry is designated in karats, with 24-karat being pure gold and 18-karat being 75 percent gold by weight. In coins, alloys with the look and density of silver are commonly substituted for silver; for instance, all U.S. coins that appear to be made of silver actually have a surface layer of 75 percent copper and 25 percent nickel.
 
 
Lowering melting points
Alloying can also be done to lower the melting point of a metal. For example, adding lead to tin lowers the melting point of the tin-rich alloy, and adding tin to lead lowers the melting point of the lead-rich alloy. A 62-percent-tin 38-percent-lead alloy, which is called the eutectic composition, has the lowest melting point of all, much lower than that of either metal. Eutectic lead-tin alloys are used for soldering.
 
 

Casting

Casting consists of pouring molten metal into a mold, where it solidifies into the shape of the mold. The process was well established in the Bronze Age (beginning c. 3000 BC), when it was used to form most of the bronze pieces now found in museums. It is particularly valuable for the economical production of complex shapes, ranging from mass-produced parts for automobiles to one-of-a-kind production of statues, jewelry, or massive machinery. (See founding.)

Processes

Casting processes differ in how the mold is made and in how the metal is forced into the mold. For metals with a high melting temperature, stable refractory material must be used to avoid reaction between the metal and the mold. Most steel and iron castings, for example, are poured into silica sand, though some parts are cast into coated metal molds. For metals of lower melting point, such as aluminum or zinc, molds can be made of another metal or of sand, depending on how many parts are to be produced and other considerations. Gravity is most frequently employed to fill the mold, but some processes use centrifugal force or pressure injection.
 
 
Sand-casting
Sand-casting is widely used for making cast-iron and steel parts of medium to large size in which surface smoothness and dimensional precision are not of primary importance.

The first step in any casting operation is to form a mold that has the shape of the part to be made. In many processes, a pattern of the part is made of some material such as wood, metal, wax, or polystyrene, and refractory molding material is formed around this. For example, in greensand-casting, sand combined with a binder such as water and clay is packed around a pattern to form the mold. The pattern is removed, and on top of the cavity is placed a similar sand mold containing a passage (called a gate) through which the metal flows into the mold. The mold is designed so that solidification of the casting begins far from the gate and advances toward it, so that molten metal in the gate can flow in to compensate for the shrinkage that accompanies solidification. Sometimes additional spaces, called risers, are added to the casting to provide reservoirs to feed this shrinkage. After solidification is complete, the sand is removed from the casting, and the gate is cut off. If cavities are intended to be left in the casting--for example, to form a hollow part--sand shapes called cores are made and suspended in the casting cavity before the metal is poured.

Patterns are also formed for sand-casting out of polymers that are evaporated by the molten metal. Such patterns may be injection molded and can possess a very complex shape. The process is called full-mold or evaporative pattern casting.

A variant of sand-casting is the shell-molding process, in which a mixture of sand and a thermosetting resin binder is placed on a heated metal pattern. The resin sets, binding the sand particles together and forming half of a strong mold. Two halves and any desired cores are then assembled to form the mold, and this mold is backed up with moist sand for casting. Greater dimensional accuracy and a smoother surface are obtained in this process than in greensand-casting.

Metal molds
Other molds are made from metal. Here a die of the desired shape is machined from cast iron or steel. If the metal flows into the mold by gravity, the process is called permanent mold casting. If the molten metal is forced in under pressure, the process is called die casting. Die-casting dies are water-cooled; consequently, they can produce parts with thinner walls at a higher rate than permanent mold machines. The rapid cooling creates a stronger part than sand-casting, but ductility may be poorer owing to entrapped gas and porosity.

Since the initial cost of a die is substantial, metal molds are cost-effective only when many identical parts are to be made. Indeed, a die may be made to produce several parts at once.
 
 

Investment casting
In investment casting a mold is made by drying a refractory slurry on a pattern made of wax or plastic. A series of layers is applied and dried to make a ceramic shell, and the pattern is then melted or burned out to provide the mold. This process allows the mass production of parts with more complex shapes and finer surface detail than can be attained by other processes. It can be used with almost any metal and is customarily employed for casting relatively small parts. The wax pattern can be made by injection molding.
Centrifugal casting
Centrifugal casting forces the metal into a mold by spinning it. It is used for the casting of small precious-metal objects, so that essentially all of the metal goes into the casting instead of the gates and risers. It is also used to produce long, hollow objects without resorting to cores--for example, to cast pipe. Here the long, cylindrical mold is horizontal and is spun about the axis of the cylinder as metal is poured into the mold.
 
 
Continuous casting
Actually not a means of casting parts, continuous casting is practiced in the primary production of metals to form strands for further processing. The metal is poured into a short, reciprocating, water-cooled mold and solidifies even as it is withdrawn from the other side of the mold. The process is widely used in the steel industry because it eliminates the cost of reheating ingots and rolling them to the proportions of the billets, blooms, and slabs made by continuous casting.

Metallurgy

The mechanical properties of castings can be degraded by inhomogeneities in the solidifying metal. These include segregation, porosity, and large grain size.
Grain size
A fine-grained casting can be produced by rapidly cooling the liquid metal to well below its equilibrium freezing temperature--i.e., by pouring into a mold that cools the metal rapidly. For this reason, die castings have a finer grain size than the same alloy cast in a sand mold.

In cast iron, remarkable changes in microstructure result from various alloying additions and casting temperatures. For example, normal cast iron solidified in a sand mold forms what is known as gray iron, an iron matrix containing about 20 percent by volume graphite flakes. This type of iron has limited ductility. However, when a small amount of magnesium is added to the melt before casting, the result is a "spheroidal graphite" iron, in which graphite appears as spherical nodules and ductility is greatly increased. If the molten iron is chill cast (i.e., rapidly cooled), it will form a "white" iron containing about 60 percent cementite, or iron carbide. This material is hard and wear-resistant, but it has no ductility at all. These cast irons are usually given a heat treatment to improve their mechanical properties.

Segregation
Different parts of a casting may have different compositions, stemming from the fact that the solid freezing out of a liquid has a different composition from the liquid with which it is in contact. (For example, when salt water is cooled until ice forms, the ice is essentially pure water while the salt concentration of the water rises.) Minor segregation is unimportant, but large differences can lead to local spots that are exceptionally weak or strong, and both of these can lead to early failure in a part under stress.
 
 
Porosity
A major problem in castings, porosity is principally caused by the shrinkage that accompanies solidification. Molds are designed to feed metal to the casting in order to keep it full as solidification proceeds, but, if this feeding is incomplete, the shrinkage will show up as internal pores or cracks. If these cracks are large, the casting will be useless. If they are small, they will have relatively little effect on the properties.

Another cause of porosity is the presence of gas-forming impurities in the liquid metal that exceed the solubility of the gas in the solid. In such cases, solidification is accompanied by the formation of bubbles as the gas is rejected. To eliminate this problem, gas-forming elements must be removed from the liquid before casting. Bubbling an inert gas such as argon through the liquid before casting is one means of doing this; vacuum degassing is another.
 
 

Metalworking

Processes

Metals are important largely because they can be easily deformed into useful shapes. Literally hundreds of metalworking processes have been developed for specific applications, but these can be divided into five broad groups: rolling, extrusion, drawing, forging, and sheet-metal forming. The first four processes subject a metal to large amounts of strain. However, if deformation occurs at a sufficiently high temperature, the metal will recrystallize--that is, its deformed grains will be consumed by the growth of a set of new, strain-free grains. For this reason, a metal is usually rolled, extruded, drawn, of forged above its recrystallization temperature. This is called hot working, and under these conditions there is virtually no limit to the compressive plastic strain to which the metal can be subjected.

Other processes are performed below the recrystallization temperature. These are called cold working. Cold working hardens metal and makes the part stronger. However, there is a definite limit to the strain that can be put into a cold part before it cracks.
 
 

Rolling
Rolling is the most common metalworking process. More than 90 percent of the aluminum, steel, and copper produced is rolled at least once in the course of production--usually to take the metal from a cast ingot down to a sheet or bar. The most common rolled product is sheet. With high-speed computer control, it is common for several stands of rolls to be combined in series, with thick sheet entering the first stand and thin sheet being coiled from the last stand at linear speeds of more than 100 kilometres (60 miles) per hour. Similar multistand mills are used to form coils of wire rod from bars. Other rolling mills can press large bars from several sides to form I-beams or railroad rails.

Rolling can be done either hot or cold. If the rolling is finished cold, the surface will be smoother and the product stronger.
 
 

Extrusion
Extrusion converts a billet of metal into a length of uniform cross section by forcing the billet to flow through the orifice of a die. In forward extrusion the ram and the die are on opposite sides of the workpiece. Products may have either a simple or a complex cross section; examples of complex extrusions can be found in aluminum window frames.

Tubes or other hollow parts can also be extruded. The initial piece is a thick-walled tube, and the extruded part is shaped between a die on the outside of the tube and a mandrel held on the inside.

In impact extrusion (also called back-extrusion), the workpiece is placed in the bottom of a hole (the die), and a loosely fitting ram is pushed against it. The ram forces the metal to flow back around it, with the gap between the ram and the die determining the wall thickness. When toothpaste tubes were made of a lead alloy, they were formed by this process.

Drawing
Drawing consists of pulling metal through a die. One type is wire drawing. The diameter reduction that can be achieved in such a die is limited, but several dies in series can be used to obtain the desired reduction. Deep drawing starts with a disk of metal and ends up with a cup by pushing the metal through a hole (die). Several drawing operations in sequence may be used for one part. Deep drawing is employed in making aluminum beverage cans and brass rifle cartridges from sheet.
Sheet metal forming
In stretch forming, the sheet is formed over a block while the workpiece is under tension. The metal is stretched just beyond its yield point (2 to 4 percent strain) in order to retain the new shape. Bending can be done by pressing between two dies. (Often a part can be made equally well by either stretch forming or bending; the choice then is made on the basis of cost.) Shearing is a cutting operation similar to that used for cloth. In these methods the thickness of the sheet changes little in processing.

Each of these processes may be used alone, but often all three are used on one part. For example, to make the roof of an automobile from a flat sheet, the edges are gripped and the piece pulled in tension over a lower die. Next a mating die is pressed over the top, finishing the forming operation, and finally the edges are sheared off to give the final dimensions.
 
 

Forging
Forging is the shaping of a piece of metal by pushing with open or closed dies. It is usually done hot in order to reduce the required force and increase the metal's plasticity.

Open-die forging is usually done by hammering a part between two flat faces. It is used to make parts that are too big to be formed in a closed die or in cases where only a few parts are to be made and the cost of a die is therefore unjustified. The earliest forging machines lifted a large hammer that was then dropped on the work, but now air- or steam-driven hammers are used, since these allow greater control over the force and the rate of forming. The part is shaped by moving or turning it between blows. A forged ring can be formed by placing a mandrel through the ring and deforming the metal between the hammer and the mandrel. Rings also can be forged by rolling with one roll inside the ring and the other outside.

Closed-die forging is the shaping of hot metal within the walls of two dies that come together to enclose the workpiece on all sides. The process starts with a rod or bar cut to the length needed to fill the die. Since large, complex shapes and large strains are involved, several dies may be used to go from the initial bar to the final shape. With closed dies, parts can be made to close tolerances so that little finish machining is required.

Two closed-die forging operations given special names are upsetting and coining. Coining takes its name from the final stage of forming metal coins, where the desired imprint is formed on a smooth metal disk that is pressed in a closed die. Coining involves small strains and is done cold to enhance surface definition and smoothness. Upsetting involves a flow of the metal back upon itself. An example of this process is the pushing of a short length of a rod through a hole, clamping the rod, and then hitting the exposed length with a die to form the head of a nail or bolt.

An important benefit of hot working is that it provides control over and improvement of mechanical properties. Hot-rolling or hot-forging eliminate much of the porosity, directionality, and segregation that may be present in cast shapes. The resulting "wrought" product therefore has better ductility and toughness than the unworked casting. During the forging of a bar, the grains of the metal become greatly elongated in the direction of flow. As a result, the toughness of the metal is substantially improved in this direction and somewhat weakened in directions transverse to the flow. Part of the design of a good forging is to assure that the flow lines in the finished part are oriented so as to lie in the direction of maximum stress when the part is placed in service.

The ability of a metal to resist thinning and fracture during cold-working operations plays an important role in alloy selection and process design. In operations that involve stretching, the best alloys are those which grow stronger with strain (strain harden)--for example, the copper-zinc alloy, brass, used for cartridges and the aluminum-magnesium alloys in beverage cans, which exhibit greater strain hardening than do pure copper or aluminum, respectively.

Another useful property that can be controlled by processing and composition is the plastic anisotropy ratio. When a segment of sheet is strained (i.e., elongated) in one direction, the thickness and width of the segment must shrink, since the volume remains constant. In an isotropic sheet the thickness and width show equal strain, but, if the grains of the sheet are oriented properly, the thickness will shrink only about half as much as the width. Since it is thinning that leads to early fracture, this plastic anisotropy imparts better deep-drawing properties to sheet material with the optimum grain orientation.

Fracture of the workpiece during forming can result from flaws in the metal; these often consist of nonmetallic inclusions such as oxides or sulfides that are trapped in the metal during refining. Such inclusions can be avoided by proper manufacturing procedures. Laps are another type of flaw in which part of a metal piece is inadvertently folded over on itself but the two sides of the fold are not completely welded together. If a force tending to open this fold is applied during the forming operation, the metal will fail at the lap.

The ability of different metals to undergo strain varies appreciably. The shape change that can be made in one forming operation is often limited by the tensile ductility of the metal. Metals with the face-centred cubic crystal structure, such as copper and aluminum, are inherently more ductile in such operations than metals with the body-centred cubic structure. To avoid early fracture in the latter type of metals, processes are used that apply primarily compressive stresses rather than tensile stresses.
 
 

Powder metallurgy

Powder metallurgy (P/M) consists of making solid parts out of metal powders. The powder is mixed with a lubricant, pressed into a die to form the desired shape, and then sintered, or heated to a temperature below the melting point of the alloy where solid-state bonding of the particles takes place. In the absence of any external force, sintering typically leaves the sample containing about 5 percent pores by volume, but, when pressure is applied during sintering (a process called hot pressing), virtually zero porosity remains. In some parts made by mixing two different elements, one component melts at the sintering temperature, and this liquid phase aids sintering of the solid particles.

Applications

The following roughly chronological account indicates the types of products that can be made by P/M.

The earliest commercial use of P/M was in the production of such high-melting-point metals as platinum, tungsten, and tantalum. Pure powders of these metals could be made by the low-temperature reduction of powders, usually oxides, and, since these metals melt at extremely high temperatures, it was easier to form solid parts by pressing and sintering the powders than by melting and casting. For example, P/M played an important role in the development of tungsten filaments for electric light bulbs.

Another early P/M product was porous-metal bearings and filters. In such parts sintering is conducted at a relatively low temperature so that the pores between the particles remain open and connected. Disks sintered in this way can serve as filters for liquids, or the sintered part can be impregnated with oil to make a self-lubricating bearing. In the latter case, the oil is held in the pores by surface tension. When the bearing heats up in use, some oil flows out and lubricates the surface, and, when the part cools, surface tension pulls the oil back into the fine channels.

Cemented carbides form another class of sintered product. Pure tungsten carbide (WC) is an extremely hard compound, but it is too brittle to be used in tools. However, useful tools can be made by mixing WC powder with cobalt powder and sintering at a temperature above the melting point of cobalt. Liquid cobalt then reacts with the surface of the WC, and when the part is cooled the cobalt freezes, holding the WC tightly together to form a composite structure with enough toughness to be used for tools and dies.

The greatest volume of P/M parts is now produced from iron powder, a process that was first developed during World War II. Small, complex parts, such as gears, require much work if machined from steel bars, and a significant volume of material is wasted as chips from the machining. However, if the part is made by P/M processes, little or no machining is necessary, there is less wasted material, and the cost is much lower. Many small parts for automobiles and appliances are produced in this manner. The second greatest volume of P/M parts is made from aluminum powder. These parts are light, corrosion-resistant, and (if alloy is used) can be heat treated to appreciably increase the strength. Small parts for automobiles and appliances are the most common applications.

A recent process uses P/M methods to improve the homogeneity and toughness of high-alloy tool steels. Cast ingots of these alloys contain a coarse network of brittle phases that are very difficult to break up by hot working, but if, instead of being cast into ingots, the liquid is atomized (solidified as small droplets), the rapidly solidified particles will be homogeneous. This powder can then be hot pressed into consolidated bars with better mechanical properties than those produced by ingot casting. Consolidation is often achieved by hot isostatic pressing, wrapping the pressed powder in an envelope of steel or glass, and heating it in a hot inert gas at high pressure. The consolidated metal is then worked into finished parts.

Processes

The most common method of producing metal powders is atomization of a liquid. Here a stream of molten metal is broken up into small droplets with a jet of water, air, or inert gas such as nitrogen or argon. Atomization in water yields irregularly shaped particles that can be pressed to a higher initial, or "green," strength and density than can the spherical particles formed by atomizing with an inert gas.

In other atomization processes, centrifugal force is used. The metal can be poured onto a spinning disk that breaks up the stream, or a spinning rod can be melted by an electric arc so that it throws off particles as it spins.

The liquid metal being atomized may be an alloy or a pure metal that will subsequently be blended with other elements to form an alloy. After atomization, the powder must be separated into size ranges by passing it through a series of sieves. Powders of different sizes (and of different metals) are then blended for pressing into parts.

Powders are often produced by chemically reducing a powdered oxide--for example, iron oxide reduced with carbon or hydrogen. The resulting metal aggregate is then milled and sieved to obtain the desired powder. Powder also can be made by electrodepositing the metal at a high current density, followed by milling to break up the deposit.

The above processes often produce powders that are roughly 50 to 200 micrometres in diameter. Powders less than one-tenth this size can be found in the finest fraction of the powder produced by atomization. Such fine powders can be mixed with a wax, injection molded to form several parts at once, and then sintered. The resulting parts require very little machining to yield a finished product.
 
 

Heat treating

The properties of metals can be substantially changed by various heat-treatment processes. Depending on the alloy and its condition, heat treating can harden or soften a metal.
 
 

Hardening treatments

Hardening heat treatments invariably involve heating to a sufficiently high temperature to dissolve solute-rich precipitates. The metal is then rapidly cooled to avoid reprecipitation; often this is done by quenching in water or oil. The concentration of solute dissolved in the metal is now much greater than the equilibrium concentration. This produces what is known as solid-solution hardening, but the alloy can usually be hardened appreciably more by aging to allow a very fine precipitate to form. Aging is done at an elevated temperature that is still well below the temperature at which the precipitate will dissolve. If the alloy is heated still further, the precipitate will coarsen; that is, the finest particles will dissolve so that the average particle size will increase. This will reduce the hardness somewhat but increase the ductility. Precipitation hardening is used to produce most high-strength alloys. In products made of soft, ductile metals such as aluminum or copper, the age-hardened alloy is put into service with the finest precipitate (that is, the highest strength) possible.

When heated to a high temperature, a few metals, principally iron and titanium alloys, transform to a different crystal structure. Often the high-temperature phase has a higher solubility for the solute, thus aiding the dissolution of the precipitate. If the alloy is slowly cooled, the reverse phase transformation will occur at a high temperature, forming a coarse precipitate and yielding a soft structure. This is the principle behind annealing procedures. However, if the alloy is quenched from the high temperature, the reverse transformation occurs at a much lower temperature, so that a very fine precipitate forms. This is the basis for hardening iron-carbon (steel) alloys. The hardness of the low-temperature-transformation phase (known as martensite) increases with carbon content, and this can result in some very strong alloys. Other alloying elements such as nickel, chromium, and manganese are added to steel primarily to slow transformation from the high-temperature phase so that thicker pieces, which cool more slowly on quenching, will harden to martensite on cooling. Steel with fresh martensite is still not tough enough for use without first being heated to an elevated temperature. This tempering process reduces residual stresses produced by the phase transformation, reduces hardness by coarsening the carbide precipitate, and increases toughness. Where high strength is the main concern, the tempering temperature is kept low. When toughness is the primary goal and strength secondary, a relatively high tempering temperature is used.

Softening treatments

In many situations the purpose of heat treating is to soften the alloy and thereby increase its ductility. This may be necessary if a number of cold-forming operations are required to form a part but the metal is so hardened after the first operation that further cold working will cause it to crack. If the metal is recrystallized by annealing at an elevated temperature, it will become soft enough to allow further forming operations. Another case arises when it is necessary to machine high-carbon tools in order to form a die. If the alloy is quenched from a high temperature to form martensite, it will be hard, brittle, and virtually impossible to machine. But if it is slowly cooled, the carbides will be much coarser, and the steel will be machinable.

Processes

Most furnaces designed for heat treatment use natural gas or electricity to raise the temperature. The atmosphere around the work may be air for low-temperature anneals, but at elevated temperatures some atmosphere other than air must be used in order to avoid oxidation. One common atmosphere is obtained by burning natural gas with less than the stoichiometric amount of air. With more reactive metals, annealing can be done in a vacuum furnace.

In some heat treatments only the surface need be heated. With electromagnetic induction or by use of a laser, this can be done so quickly that no special atmosphere is needed to avoid oxidation. Surface heat treating also avoids the distortion that can accompany heating and quenching the entire part. For example, the rear axle of most automobiles is a steel bar roughly 1 metre long and 3 centimetres in diameter (about 3 feet long and 1.25 inches in diameter). The surface can be hardened by passing the bar through an induction coil that quickly heats the surface immediately beneath the coil to red heat, transforming it to austenite. The inside remains cold, however, and, after the coil passes, this cold interior quickly draws heat from the surface, transforming it to martensite. The part is then tempered and put into service. Since most of the cross section of the part is not heated or transformed during this operation, there is no distortion and therefore no need to straighten a hardened part.
 
 

Surface treating

Because it is the surface of a metal that people see and that reacts with the environment around it, special effort is sometimes made to add lustre, colour, or texture to a surface. In addition, special corrosion-resistant layers are placed on the surface for some applications, and in yet others the surface is hardened to add strength and reduce wear. This section discusses surface treatments that add corrosion resistance or hardness.

Corrosion resistance

Barrier protection
When a metal corrodes in water, the atoms lose electrons and become ions that move into the water. This is called an anodic reaction, and for the corrosion process to proceed there must be a corresponding cathodic reaction that adsorbs the electrons. The process can be stopped by isolating the metal from the water with an impermeable barrier. One of the older applications of this idea is the tin can. Unlike steel, tin is not affected by the acids in food, so that a layer of tin placed on steel sheet protects the steel in the can from corrosion.

The exterior surfaces of many large household appliances consist of steel covered with a layer of coloured glass called enamel. Enamel is inert and adheres tightly to the steel, thus protecting it from corrosion as well as providing an attractive appearance. Decorative chromium plating is another example of a protective-barrier coating on steel. Since chromium does not adhere well to steel, the steel is first electroplated with layers of copper and nickel before being plated with a thin layer of chromium.

The protective layers described above are metallic, but the most common protective barriers are organic. Paints, polymers, and thin lacquer films are used for various applications near room temperature.

The oxide layer that forms on metals when they are exposed to air also constitutes a protective barrier. Stainless steel and aluminum form the most stable and protective of such films. The thickness of the oxide film on aluminum is often increased by making the part function as the anode in an electrolytic cell. This process, called anodizing, enhances the corrosion resistance somewhat and makes colouring the surface easier. The films that form on copper and steel as a result of corrosion (commonly known as tarnish and rust) are somewhat thicker and show a characteristic colour that is often incorporated into the design of the part.
 
 

Galvanic protection
Protective films on steel are susceptible to being broken by flying stones or other sharp objects. This is especially true on the parts of an automobile that are close to the road. The barriers described above have limited ability for self-healing after they are broken, but protection of the underlying small exposed area of steel can be maintained if a metal that has a greater tendency than steel to give up electrons in water is attached to the surface. In this way, when the protective barrier is broken, the more reactive metal is corroded preferentially, and the steel is given "galvanic" protection.

A layer of zinc can be placed on a steel surface either by hot-dipping the steel in molten zinc or by electroplating zinc onto the surface. Galvanized steel is much more resistant to corrosion than ungalvanized steel. For this reason, it is often used in the lowest panels of automobiles--i.e., the parts exposed to corrosive salt spray from snowy roads.

Electrodeposits of cadmium are also used for galvanic protection of steel. Hot-dip aluminum-coated steel is used in the exhaust systems of automobiles. At low temperatures its action is primarily galvanic, while at high temperatures it oxidizes to form a barrier layer.

Other coating techniques
Among other methods for applying metal layers to metal is thermal spray coating, a generic term for processes in which a metal wire is melted by a plasma arc or a flame, atomized, and sprayed onto a surface in an inert gas. Another method is vacuum coating, which produces thin, bright, attractive layers on a part by evaporating and depositing a coating metal in a high vacuum.

Enhanced oxidation protection is imparted to high-temperature turbine parts made of superalloys by annealing the parts in a container containing volatile aluminum chloride. This is done at high temperatures, so that aluminum diffuses into the alloy to form an aluminum-rich surface layer. In high-temperature service, such a layer oxidizes to form a protective aluminum oxide coating.
 
 

Hardening

 
Carburizing
The strength of hardened steel increases rapidly as the percentage of carbon is increased, but at the same time the steel's toughness decreases. Often the most useful part is one in which the surface is higher in carbon and thus hard, while the interior is lower in carbon and thus tough. Such a combination of properties can be obtained by carburizing, or annealing the parts in a gas rich in carbon. (The carburizing potential of the gas rises with the ratio of carbon monoxide to carbon dioxide.) The carburizing temperature is high enough to transform the surface of the steel to the high-temperature austenite phase, which has a much higher carbon solubility than the low-temperature ferrite phase. At these temperatures, carbon deposited on the surface diffuses through the steel and into the solid. The thickness of the diffusion layer increases with time, although at a decreasing rate; depths of 1 to 2 millimetres (0.04 to 0.08 inch) in 4 to 16 hours are typical. Following diffusion, the part is quenched in oil. The high-carbon surface transforms into a hard, brittle martensitic structure, while the lower-carbon interior transforms into a tougher, softer structure. The part is then tempered to raise the toughness of the surface layer. Small machine parts, such as gears, are often carburized to increase their strength and resistance to wear.
 
 
Nitriding
Nitriding provides an alternative means of hardening a steel surface. The surface layer is only one-tenth the depth of a carburized layer, but it is appreciably harder. The steel part is heated to a lower temperature, so that its crystal structure remains ferritic. Heating is conducted in an atmosphere of ammonia (NH3) and hydrogen, and nitrogen from the ammonia diffuses into the steel.

Hardening is accomplished in one of two ways. One way is solid-solution hardening, which occurs in all steels. The other way is precipitation hardening. For example, if a steel contains aluminum, the aluminum and nitrogen will combine to form very fine particles that harden the steel quite effectively.

Though it is extremely hard, the nitride layer does not tend to crack, because it is very thin and adheres well to the ductile steel beneath it. The part need not be quenched from the nitriding temperature, nor does it need to be tempered before it is put into service.

Other methods
Surfaces can also be hardened by heat treating with induction or laser heating. In other applications, surface hardening is accomplished by depositing a "hard-facing alloy" on the base part. One example is "hard chromium plating," in which a thick layer of chromium is deposited on a part. Automotive valve stems and piston rings and the bores of diesel engine cylinders are common applications.
 
 

Metallography and metal testing

Metallography

The properties of an alloy of a given composition can change markedly with the microscopic arrangement of its crystalline grains--i.e., its microstructure. To evaluate and control the microstructure of a sample, various types of microscope are used, and the field is called metallography. (See mechanical testing.)
 
 
Optical microscopy
The simplest, and oldest, type of metallography (though hardly a century old) involves polishing the surface to a mirrorlike finish and examining light reflected from it at magnifications of 50 to 1500. If the surface is lightly etched in an appropriate solution (often an acid), the grain boundaries, matrix, and constituent phases will be attacked at different rates and will be discernible. This makes it possible to establish which phases are present as well as their shape, size, and distribution. Similarly, grain size and shape can be observed. With this information it is possible to infer the history of the sample and predict its behaviour. Metallography is of particular value in the analysis of samples that have failed or performed in an unexpected manner.
 
 
Electron microscopy
Great progress has been made in using finely focused beams of energetic electrons to examine metals. Electron microscopes are basically of two types, transmission and scanning. Transmission electron microscopes require the preparation of films so thin that they are transparent to a beam of electrons with energies of roughly 200 kiloelectron volts. This means the film must have a thickness of only one, or a few, hundred nanometres (10-9 metre). Films of lighter elements, such as aluminum, can be thicker, while films of heavier elements, such as gold, must be thinner. Contrast between neighbouring regions is best developed by differences in their diffraction of the electron beam, although differences in density can also be used. Spatial resolution is excellent, going down to atomic resolution in special microscopes, and orientation relations between adjoining regions can be easily discerned. On the other hand, only very small samples can be examined in any given film. This means that the technique is not good for measuring defects larger than the film thickness or those whose number per unit volume is low.

A scanning electron microscope (SEM) uses a narrow beam of electrons (often of about 40 kiloelectron volts) that scans the surface of a sample and forms a corresponding image from the backscattered electrons or secondary electrons. No special surface preparation is necessary, and, since the depth of focus in an SEM is much greater than in an optical microscope, quite irregular surfaces, such as fractures, can be studied successfully. (The strikingly detailed pictures of insects seen in publications are also taken with an SEM.) Useful magnifications range from 100 to 20,000.

The electron beam used in an SEM causes each atom near the surface to emit an X ray that is characteristic of that element. By constructing an image based on the distribution of the intensity of the characteristic X ray of a given element, it is possible to show that element's distribution among the phases in the surface. If the electron beam is not swept but held in one spot, a chemical analysis can be made of the various elements in the region under the electron beam by measuring the intensity of the X rays emitted by each element.

Testing mechanical properties

The most common mechanical properties are yield stress, elongation, hardness, and toughness. The first two are measured in a tensile test, where a sample is loaded until it begins to undergo plastic strain (i.e., strain that is not recovered when the sample is unloaded). This stress is called the yield stress. It is a property that is the same for various samples of the same alloy, and it is useful in designing structures since it predicts the loads beyond which a structure will be permanently bent out of shape.

If the tensile test is continued past yielding, the load reaches a maximum as the strain localizes and a neck develops in the sample. The maximum load, divided by the initial cross-sectional area of the sample, is called the ultimate tensile stress (UTS). The final length minus the initial length, divided by the initial length, is called the elongation. Yield stress, UTS, and elongation are the most commonly tabulated mechanical properties of metals.

The hardness of a metal can be measured in several ways. If a hard indenter (a sphere, cone, or pyramid) is pushed a short distance into a metal with a defined load, the load divided by the contact area becomes the measure of hardness. For testing steel, one of the oldest of such tests, the Brinell hardness test, uses a 10-millimetre-diameter ball and a 3,000-kilogram load. Brinell hardness values correlate well with UTS. Much smaller loads and diamond microindenters also can be used in conjunction with a microscope to measure the hardness of quite small regions (down to a few micrometres, or millionths of a metre, across). (See hardness tester.)

A different type of testing machine, which indicates hardness directly, bears the name Rockwell. Here, instead of measuring the width of the indentation after the indenter has been removed, a sensitive gauge indicates the depth to which the indenter sinks into the surface under a given load. Various sizes of indenters and loads allow a wide range of hardness to be measured. These hardness numbers are useful for quality control in manufacturing, especially in assuring consistency from batch to batch.

Of particular concern in engineering structures is the prevention of sudden, complete failure, as in the fracture of a brittle material. Much to be preferred is a structure that will deform under an overload but not fail. Sudden failure begins at a notch or crack that locally concentrates the stress, and the energy required to extend such a crack in a solid is a measure of the solid's toughness. In a hard, brittle material, toughness is low, while in a strong, ductile metal it is high. A common test of toughness is the Charpy test, which employs a small bar of a metal with a V-shaped groove cut on one side. A large hammer is swung so as to strike the bar on the side opposite the groove. The energy absorbed in driving the hammer through the bar is the toughness.

A test requiring more instrumentation, but measuring material properties that are more useful for analysis, is the pulling apart of two sides of a sample containing a crack that is initially cut about one-third of the way through the sample. The use and analysis of such a test is called fracture mechanics, and the information acquired is used to demonstrate the integrity of structures made of strong materials that contain small flaws--for example, rocket casings, airplanes, and nuclear reactor pressure vessels.

If a part is loaded once to a stress near the yield stress, it will not break. However, if it is loaded repeatedly to this level, it will eventually break. This failure is called fatigue, and avoiding fatigue is an important goal in the design of moving machinery. The more cycles a part will undergo, the lower the allowable stress it must be assigned in order to avoid failure by fatigue.

Another type of failure observable at loads below the yield stress is called creep. If a load is applied and left on the sample for months or years, the sample will slowly extend. In metals with high melting temperatures, creep becomes a problem at higher temperatures. It becomes a limiting consideration in gas turbines that operate at the highest temperature the metal parts can take.
 
 

Iron-Iron Carbide Phase Diagram Example


Fig 1: Fe-Fe3C Phase Diagram (clickable), Materials Science and Metallurgy,
4th ed., Pollack, Prentice-Hall, 1988

Figure 1 shows the equilibrium diagram for combinations of carbon in a solid solution of iron. The diagram shows iron and carbons combined to form Fe-Fe3C at the 6.67%C end of the diagram. The left side of the diagram is pure iron combined with carbon, resulting in steel alloys. Three significant regions can be made relative to the steel portion of the diagram. They are the eutectoid E, the hypoeutectoid A, and the hypereutectoid B. The right side of the pure iron line is carbon in combination with various forms of iron called alpha iron , gamma iron (austenite), and delta iron. The black dots mark clickable sections of the diagram.

Allotropic changes take place when there is a change in crystal lattice structure. From 2802º-2552ºF the delta iron has a body-centered cubic lattice structure. At 2552ºF, the lattice changes from a body-centered cubic to a face-centered cubic lattice type. At 1400ºF, the curve shows a plateau but this does not signify an allotropic change. It is called the Curie temperature, where the metal changes its magnetic properties.

Two very important phase changes take place at 0.83%C and at 4.3% C. At 0.83%C, the transformation is eutectoid, called pearlite.

gamma (austenite) --> alpha + Fe3C (cementite)

At 4.3% C and 2066ºF, the transformation is eutectic, called ledeburite.

L(liquid) --> gamma (austenite) + Fe3C (cementite)