Ferrous metal are those which have iron as their main constituent. The ferrous metal commonly used in engineering practice are cast iron, wrought iron, steel and alloy steels. The principal raw material for all ferrous metals is pig iron which is obtained by smelting iron ore, with coke and lime stone, in the blast furnace.
The cast iron is obtained by remelting pig iron with coke and limestone in a furnace known as cupola. It is primarily an alloy of iron and carbon. The carbon contents in cast iron varies from 1.7% to 4.5%. It also contains small amounts of silicon, manganese, phosphorus and sulphur. The carbon in a cast iron is present in either of the following two forms:
1) Free carbon or graphite
2) Combined carbon or cementite
Since the cast iron is a brittle material, therefore, it cannot be used in those parts of machines whcih are subjected to shocks. The properties of cast iron which make it a valuable material for engineering purposes are its low cost, good casting characteristics, high compressive strength, wear resistance and excellent machinability. The compressive strength of cast iron is much greater than the tensile strenght. Following are the values of ultimate strength of cast iron.
= 100 to 200 N/mm2
Compressive strength = 400 to 1000 N/mm2
Shear strength = 120 N/mm2
TYPES OF CAST IRON:
1) Grey Cast Iron:
The grey cast iron has a low tensile strenght, high compressive strength and no ductility. It can be easily machined. It is very suitable for those parts where sliding action is desired. The grey iron castings are widely used for machines tool bodies, automotive cylinder blocks, heads, housings, fly-wheels, pipes and pipe fittings.
2) White Cast Iron:
It has high tensile strength and low compressive strength. It is used in car wheels, rolls for crushing grains and jaw crusher plates.
3) Chilled Cast Iron:
It is white cast iron produced by quick cooling of molten iron. Chills are used on any faces of casting which are required to be hard to withstand wear and friction.
4) Mottled Cast Iron:
It is product in between grey and white cast iron in composition, colour and general properties. It is obtained in castings where certain wearing surfaces have been chilled.
5) Malleable Cast Iron:
It is ductile and may be bent without breaking or fracturing the section. It has excellent machining qualities. It is used for machine parts for which the steel forgings would be too expensive and in which the metal should have a fair degree of accuracy, e.g. hubs of wagon wheels, small fittings, for railway rolling stock, brake supports, parts of agricultural machinery, pipe fittings, door higes etc.
6) Nodular Cast Iron:
This type of cast iron is obtained by adding small amounts of magnesium (0.1 to 0.8%) to the molten grey cast iron. It has high fluidity, castability, tensile strength, toughness, wear resistance, pressure tightness, weldability and machinability. It is used in hydraulic cylinders, cylinder heads, rolls for rolling mill and centrifugally cast products.
It is the purest iron which contains atleast 99.5% iron but may contain upto 99.9% iron. The typical composition of a wrought iron is
C = 0.02%
Si = 0.12%
S = 0.108%
P = 0.02%
Slag = 0.07%
The wrough tiron is produced from pig iron by remelting it in the puddling furnace of reverberatory type. The molten metal free from impurities is removed from the furnace as a pasty mass of iron and slag. The balls of this pasty mass each about 45 to 65 kg in weight, are formed. These balls are then mechanically worked both to squeeze out the slag and to form it into some commercial shape.
The wrought iron is a tough, malleable and ductile material. It cannot stand sudden and excessive shocks.
It can be easily forged or welded. It is used for chains, crane hooks,
railway couplings, water and steam pipes.
It is an alloy of iron and carbon, with carbon content upto a maximum of 1.5%. The carbon occurs in the form of iron carbide, because of its ability to increase the hardness and strength of the steel. Most of the steel produced nowadays is plain carbon steel or simply carbon steel.
A carbon steel is defined as a steel which has it properties mainly due to its carbon content and does not contain more than 0.5% of silicon and 1.5% of manganese. The plain carbon steels varying from 0.06% carbon to 1.5% carbon are divided into the following types depending upon the carbon content.
1) Dead mild steel
upto 0.15% C
2) Low carbon or mild steel 0.15% to 0.45% C
3) Medium carbon steel 0.45% to 0.8% C
4) High carbon steel 0.8% to 1.5% C
Effects of Impurities on Steel:
The following are the effects of impurities on steel.
The amount of silicon in the finished steel usually ranges from 0.05% to 0.30%. Silicon is added in low carbon steels to prevent them from becoming porous. It removes the gases and oxides, prevent blow holes and thereby makes the steel tougher and harder.
It occurs in steel either as iron sulphide or manganes sulphide. Iron sulphide because of its low melting point produces red shortness whereas manganese sulphiede does not effect so much. Thererfore, manganese sulphide is less objectionable in steel than iron sulphide.
It serves as a valuable deoxidising and purfying agent, in steel. Manganes also combines with sulphur and thereby decreases the harmful effect of this element remaining in the steel. When used in ordinary low carbon steels, manganese makes the metal ductile and of good bending qualities. In high speed steels, it is used to tougher the metal and to increase its critical temperature.
It makes the steel brittle. It also produces cold shortness in steel. In low carbon steels, it raises the yield point and improves the resistance to atmospheric corrosion. The sum of carbon and phosphorus usually does not exceed 0.25%.
FREE CUTTING STEELS:
The free cutting steels contain sulphur and phosphorus. These steels have higher sulphur contents than other carbon steels. In general, the carbon content of such steels vary from 0.1% to 0.45% and sulphur from 0.08% to 0.3%. These steels are used where rapid machining is the prime requirement.
HEAT RESISTING STEELS:
The steels which can resist creep and oxidation at high temperatures and retain sufficient strength are called heat resisting steels. A number of heat resisting steels have been developed as discussed below:
1. Low Alloy steels:
These steels contain 0.5% mlybdenum. The main application of these steels are for super heater tubes and pipes in steam plants, where temperatures are in range of 400 to 500 degree Centigrade.
2. Valve steels:
They are used for automobile valves. They posses good resistance to scaling at dull red heat, although their strength at elevated temperatures is relatively low.
3. Plain Chromium steel:
These steels are very good for oxidation resistance at high temperatures as compared to their strength which is not high at such conditions.
4. Austenitic Chromium nickel steels:
These steels have good mechanical properties at high temperature with good scalig resistance. They are used in gas turbine disc and blades.
HIGH SPEED TOOL STEELS:
These steels are used for cutting metals at a much higher cutting speed than ordinary carbon tool steels. The carbon steel cutting tools do not retain their sharp cutting edges under heavier loads and higher speeds. Following are the different types of high spped tool steels:
1. High speed tool 18-4-1:
This steel, on an average contains 18% tungsten, 4% Cr and 1% vanadium. It is considered to be one of the best of all purpose tool steels. It is widely used for drills, lathe, planer and shaper tools, milling cutters, reamers, broaches, threading dies, punches etc.
2. Molybdenum high speed steel:
This steel contain 6% tungsten, 6% molybdenum, 4% Cr and 2% vanadium. It has excellent tough and cutting ability. These types of steels are better and cheaper than other type of steels. It is particularly used for drilling and tapping operations.
3. Super high speed steel:
This steel contains 20% tungsten, 4% Cr, 2% vanadium and 12% Cobalt. It is used for heav cutting which impose high pressure and temperature on the tool.
It is defined as that steel which when correctly heat treated and finished, resists oxidation and corrosive attack from most corrosive media. The different types of stainless are discussed below:
1. Martensitic Stainless Steel:
The chromium steels containing 12 to 14% Cr and 0.12 to 0.35% C are the first stainless steel developed. These steels can be easily welded and machined. These steels may be used where the corrosion conditions are not too sever, such as for hydraulic, steam and oil pumps, valves and other engineering components.
2. Ferrite Stainless Steel:
These steel containing greater amount of Chromium (from 16 to 18%) and about 0.12% carbon. These steels have better corrosion resistant property than martensitic stainless steels.
3. Austenitic Stainless Steel:
This steel containing high content of both Chromium and nickel. The most widely used chemical composition is 18% CR and 8% nickel. Such steel is commonly known as 18/8 steel. These steels are used in the manufacture of pump shafts, rail road car frames and sheathing screws, nuts and bolts and small springs. This steel provide excellent resistance to attack by many chemicals.