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PREPARATION OF METALLURGIGAL SPECIMEN FOR MICROSCOPIC  EXAMINATION

APPARATUS  AND  REAGENT:   Cutter, grinder, polisher,  dryer,  emery,  papers,  sylvith  cloth, polishing wax or powder and  etching solutions.

THORY:  The  entire  process  of specimen  preparation  for  metallography  and  microscopic  examination  is  to  prepare  a  scratch  free  non-  deformed  surface  employing  a series  of successively  finer  abrasives  .Thus  main  object  of  fine  grinding  and  polishing  is  to  reduce. The  thickness  of  the  deformed  layer   lying  below  the  specimen   surface   and  expose  the  non-deformed  layer  for  examination. However  each  step  of  grinding and  polishing  itself tends   to  produce deformation  hence  it  is  necessary   to use  successively  finer  abrasives.

CUTTING: Where  ever  metallurgical  or material  samples are  prepared  for microscopic   study  there   is  a  need  for  proper  out  of  a suitable piece .Cutting  is  carried  out  by :
a) Cut -off machine
b) Hack saw or band

MOUNTING: Samples which are very small , irregularly shaped, delicate or fragile need to be firmly embedded in some material which will allow convenient handling. Mounting may be cold or hot, for hot mounting compression molding plastic material is used and mounting is performed in mounting presses. For cold mounting a suitable polymeric resin with a hardener is used.

GRINDING: Coarse grinding is essentially the process of preparing a flat surface on the specimen for subsequent fine grinding and polishing. Care should be taken to avoid heating up of the sample and deep surface scratching.  File and emery papers of 120 or lower grades are used.
Fine grinding removes the scratches from any previous grinding operation. Wet grinding is preferred for flushing action of lubricant Emery papers of grit sizes 320, 400 and 600are used.

POLISHING:  It is achieved be several stages using successively finer abrasives of 6 micron to ¼ micron. Diamond dust, powdered aluminum oxide and magnesium oxide are used.

ETCHING: It is done to produce surface relief and contrast among the different phases and grains when seen under the microscope. Etching preferentially attacks the grain boundaries and phase boundaries on the surface of the specimen. Suitable acidic or alkaline  solutions are used . In the current practical polished surface is dipped in 2% Nital solution  wash thoroughly and dry.

Observe the microstructure  under a microscope.

In case of gray cast iron, there is no need for etching but the remaining processes remain same.

MICROEXAMINATION OF THE GIVEN SPECIMEN WITH THE HELP OF METALLURGICAL EXAMINATION.

MICROEXAMINATION  OF METALLIC SPECIMEN: By the examination  of fractured pieces and of small prepared sections of metals the metallurgist can obtain vital information regarding the microstructure, their properties and treatment to which they have been subjected   .
The methods adopted for the visual examination can be divided into two groups.
1) Macro-examination either with the naked eye or under very low magnification (upto  10X)
2) Micro- examination at higher magnifications(20X to 2000X)

APPEARANCE OF MICROSTRUCTURE: The polished surface of a uniform specimen appears bright because the light rays from vertical illuminator strike the surface of the specimen vertically, with the result that they reduce their path finally passing into the eye. The slight chemical attack or etching of the polished surface first reveals the grain boundaries. Further attach produces the shades of varying degrees in the grains. This is due to the fact that the etching reagent has produced. Not a general tarnish of the surface but a series of small and but well-defined facets upon each grain. These facets have the same orientation in each grain . In adjacent grains however, the inclination to the surface changes thus one grain may reflect the light up the microscope tube and appear very bright, while the adjacent grain appear darker in the obliquely reflected light.  In micro-examination every small area of the sample is thoroughly studied hence random sampling of carefully selected specimen should be done. In some case both longitudinal and transverse sections should be examined.
 

METALLURGICAL MICROSCOPE: Metallic specimens, in contrast to transparent specimen used in microscope work on plant and animal life, have to be examined by reflected light. Consequently the sub- stage condenser and mirror underneath the stage are not required. The metallurgical microscope consists essentially of an of an optical system and illumination system. The optical system includes the eyepiece, relay system and the objective lens. Illumination system consist of a high intensity light source, Condenser lenses, and an aperture diaphragm, a dark-field stop and a plane glass reflector. Some metallurgical microscopes are equipped with revolving nosepieces. Which may hold upto four objective lenses. This helps in changing the magnifying power, without removing  other specimen.
The magnifying power of both objective and eyepiece lenses is engraved on the lens mount. Total magnification of a microscope may be determined by finding the product of the eyepiece and objective lens magnifications.
The two most important optical parts of microscope are the objective for resolving the structure of the metal, and the ocular or eyepiece for enlarging the image formed be the objective.

PROCEDURE: Mount the polished and etched specimen on a glass slide with the help of putty and level under a leveling press using moderate pressure. Examine the microstructure under reflected light .Increase the magnification gradually for better  resolution. Changing the objective lens as follow can do this.

OBJECTIVE EYEPIECE MAGNIFICAITON
10X 10X 100
20X 10X 200
40X 10X 400
60X 10X 600
 

PRECAUTION:
1) .Use the fine adjustment for focussing. Do not use the coarse adjustment when looking through the eyepiece onto the specimen.
2) Lenses for microscope must be maintained free from fingerprints dust, oil and corrosive atmosphere.

The polished surface of the specimen should be kept free from dust impurities and  finger  prints.
.
 

DETERMINATION OF GRAIN OF A METAL SPECIMEN

Metallography  is the study of structures in metals and alloys. The structure, which can be seen under a microscope after polishing and etching indicates that crystalline materials consists of grains and in many cases more than one phase is present.
A grain is essentially a single  crystal with almost any external shape but with an internal atomic structure upon the space lattice with which the crystals born. The size of grain depends on a number of factors,
Such as temperature, rate of cooling, solute atoms and insoluble precipitates. The grain size plays vital role in determining the properties of materials. Small grains give better strength and toughness. Large are preferred in materials used at high temperature for creep properties such  as in nuclear reactors. Single
Grains are noted for their electrical and electronic properties.
In order to achieve both strength and toughness , the metals and alloys used in automobiles, building  structures, machine tool and article of day to day use should have  a small grains size.

MEASUREMENT OF GRAINS SIZE: This may be done :
1. Directly measuring under a microscope
2. Projecting the image onto a screen
3. Using photograph of the specimen

In each case the measurement is taken by imposing a line of fixed length or a circle of a known diameter the onto the image. The number of grain boundaries  cutting the line is then counted. An average of 300
To 400 grains should be counted for a precise grain size.

ARTICLES USED: Photographs, ruler

PROCEDURE: Impose a fixed length of 100mm onto the photograph at random. Count the number of grain boundaries, witch cut  across the imposed  length. Repeat the procedure to obtain ten readings. The ruler should be placed at a different angle each time. Tabulate the readings and calculate the average size.

RESULTS AND CONCLUSIONS:

No. of reading boundaries                     Length of line (mm)                         No. of Grain
1                                                                 100                                             Check
2                                                                 100
3                                                                 100
4                                                                 100
5                                                                 100
6                                                                 100
7                                                                 100
8                                                                 100
9                                                                 100
10                                                                 100

Total  No. 10                                            100 x 10
 

100 grains measure ………………………..    1000 mm
1 grain measure …………………………….  1000/100=10 mm

Image of the specimen under the microscope was magnified 100 times.
The print was magnified 4 times
Total  magnification 4x100 = 400

The actual grain size     = 10/400
                                   = 0.25x 10 E –04 m
 

HEAT TREATMENT OF STEELS

THEORY:  A combination of heating and cooling operations timed and applied to a metal or alloy in the solid state in a way that will produce the desired properties in it is referred to as heat treatment.
The first step in the heat treatment of steel is to heat the metal to some predetermined temperature above the critical temperature (in case of tempering below the critical point) where the steel may be transformed into a structure called austenite.
The second step is to cool the metal in various different ways. Usually three methods are employed for cooling, viz. furnace cooling, air cooling and quenching.

1. ANNEALING:  This process consists of heating the steel to the proper temperature and then cooling it slowly through the transformation range. This slow cooling is achieved by switching off the furnace and leaving the part to cool within the furnace. Some times the part can be cooled by putting in an insulating material.

PURPOSE:  To refine the grains, induce softness, improve electrical and      magnetic properties and in some cases to improve machinability.

2. NORMALIZING:  This process consists of heating the steel part to the proper temperature and then cooling in air to room temperature.

PURPOSE:  To produce a harder and stronger steel than produced by annealing, improve machinability and refine grain size.

3. QUENCHING:  It is the process of heating the steel part to the proper temperature and then cooling rapidly in different cooling media.

PURPOSE:  To produce hard and strong steel.

4. TEMPERING:  It is the process of reheating the quenched metal to sub-critical temperature.

PURPOSE:  To reduce hardness and brittleness of the hardened steel and thus increase its ductility and toughness, and remove internal stresses produced by quenching.
 

ANNEALING OF GIVEN STEEL SPECIMEN

EQUIPMENT/APPARATUS/MATERIAL:  Muffle furnace, steel specimen.

THEORY:  It is one of the most widely used operations in heat treatment of steels and is defined as a softening process in which iron based alloys are heated above the transformation range, held there for a proper time called soaking time and then cooled slowly at the rate of 30 to 150 ?C/hr in the furnace itself. The objective of annealing is to soften the metal so that it can be cold worked, to reduce hardness and improve machinability, to refine grain size, to improve ductility, to prepare the steel for subsequent heat treatment, to obtain desired mechanical and magnetic properties, to relieve internal stresses.

PROCEDURE:  First of  all the given specimen is placed in the furnace, the furnace is turned on and the specimen is heated to a predetermined temperature obtained from a standard chart. This temperature depends on the carbon content of the steel. Specimen is kept at that temperature for one hour which is called soaking time. After soaking time is over the specimen is left in the furnace. After cooling the specimen is taken out and its hardness is tested.
 

NORMALIZING OF GIVEN STEEL SPECIMEN

EQUIPMENT/APPARATUS/MATERIAL

Muffle furnace, steel specimen cut in suitable size, pair of hand gloves.

THEORY

In this process iron based alloys are heated at 40-50 deg. above upper critical temperature, hold there for a specific period followed by cooling in still air. The objectives of the normalizing are to eliminate grain structures obtained during forging, rolling and stamping and to produce fine grains, increase machinability and to reduce internal stresses.

PROCEDURE

First of all steel specimen of given size is placed in the furnace and the furnace is turned on. The specimen is heated to a pre determined temperature obtained from the standard chart. This temperature depends upon the carbon content present in the steel. Specimen is kept at that temperature for an hour, which is called soaking time during which a fully austentic structure is produced. After soaking time is over specimen is taken out of the furnace and it is allowed to cool slowly in the still air. Hardness of the specimen is found and noted.
 

HARDENING OF GIVEN STEEL SPECIMEN

EQUIPMENT/APPARATUS/MATERIAL

Muffle furnace, steel specimen cut in suitable size, pair of tongs, pair of gloves, adequate quantity of water and oil in containers.

THEORY
In this process iron based alloys are heated at 30-50 deg. C above the upper critical temperature and held there for a specified period ( to ensure that a fully austentic structure is obtained ) and followed by rapid cooling to room temperature by quenching in water, oil or brine solution. Objectives of hardening is to increase the hardness.
Austenite is changed into a fine needle like microstructure known as martensite. Hardness in steel is due to this very microstructure. The hardness produced by hardening treatment depends upon the carbon content present in the steel.

PROCEDURE
First hardness of the steel specimen is found. Then these specimen are put into furnace and the furnace is turned on. Specimen are heated to a temperature determined from standard charts. Specimen are soaked at this temperature for an hour. After soaking time is over, specimen are taken out of the furnace with the help of a pair of tongs and one specimen is quenched in water and the other in oil.

RESULTS

HARDNESS HRB

                                        Before Hardening             After Hardening                 % increase
Oil quenched                         120                                     130                             8.33%
Water quenched                     120                                     142                            18.33%

CONCLUSION:
From the above result, we see that percentage increase in hardness in water is greater than percentage increase in oil. We know that hardness in steel in mainly due to its micro-structure, so when we cool the steel in water, there is very little time for grain to merge and we get small grains.

TEMPERING OF HARDENED STEEL SPECIMEN

EQUIPMENT/APPARATUS/MATERIAL

Muffle furnace, Hardened steel specimen, pair of tings, pair of gloves

THEORY
Steel hardened by rapid quenching is very hard and brittle. It also contains internal stresses that are severe and unequally distributed to cause cracks or even rupture of hardened steel. Hardness is carried out to increase roughness and ductility at the expense of hardness and strength. Tempering is the reheat process, the reheating being carried out at such critical temperatures. Tempering permits the trapped martensite to transform into troostitr or sorbite depending upon the tempering temperature. This process also relives internal stresses. Tempering reduces brittleness, increases ductility, removes internal stresses and makes steel tough to resist shock and fatigue.

There are three classes of tempering:
Low temperature tempering for cutting and measuring tools ( steel is tempered upto 200 deg. C ).

Medium temperature tempering is generally carried out for springs ( steel is tempered from 250 to 350 deg. C ).

High temperature tempering is carried out for structural steel ( steel is tempered from 350 to 550 deg. C ).

PROCEDURE
Hardened steel specimen are put into furnace and the furnace is turned on. Specimen are soaked for one hour at tempering temperature. After the soaking time is over specimens are taken out of the furnace and allowed to cool in still air.

RESULTS
HARDNESS HRB

                                        Before Hardening                     After Hardening                 % decrease
Oil quenched                             130                                         126                             3.07%
Water quenched                         142                                         137                            3.52%

CONCLUSIONS
By tempering, the hardness and brittleness may be reduced to the desired point. Although this process soften steels, it differs considerably from annealing in that the process lends itself to close control of the physical properties and in most cases does not soften the steel that extent that annealing would.