Page 32 - CITS - Fitter Trade Practical - Volume -2

FITTER - CITS

This can also be defined as:
The controlled heating of steel to a particular temperature, (soaking maintaining) for sufficient at that temperature,
and cooling at a controlled rate in order to obtain certain properties or conditions.
Some of the common industrial heat treatment operations are as follows:
a Annealing

b Normalising
c Hardening
d Tempering
Ferrous Metals

Iron and steel, which are termed as ferrous metals from bulk of the metals used in Engineering industry. Steel
I different conditions is the important raw material for toolmaker and the tools developed are mostly meant for
processing steel. Hence a clear concept of steel is essential for the Toolmaker.
Iron and steel used in engineering industry is either as pig iron , cast iron , wrought iron and steel of which there
is straight carbon or alloy steel. Carbon steels are low carbon, mild steel, medium carbon and high carbon variety.
• Pig iron is the production from blast furnace where iron ore, coke and lime heated to about 1700° C produce
pig iron and slag. Approximate composition of pig iron is 3 to 4% carbon including free graphite 1-3% silicon,
1% phosphorous and 1-2% manganese. Its main use is as raw material for casting and steel making.

• Cast iron is the product got by melting pig iron with suitable ingredients. It has good comprehensive strength.,
low tensile strength, less warpening and high co-efficient of expansion. Largely used for machine bodies. The
cast iron with #% silicon is grey cast iron, while that with 1% chilled) is white cast iron. In order to improve the
quality of cast iron it is converted into malleable castings.
• Wrought iron: It is the purest form of iron having a composition of C 0.06% Mn ).05% P 0.068%. S 0.009%
and Si 0.1%. However, it has very little strength and finds only limited practical application. Due to the high
corrosion resistance it is used for pipe lines in corrosive conditions.
• Steel is an alloy of iron and carbon. By removal or excess carbon and impurities in pig iron steel is obtained.
Steel is produced by Bessemer converter. Electric furnace and open hearth process. A recent methods are
L.D. and Kalda Processes. Steel produced are in the shape of ingots, blooms (forging 150 mm and above)
billets (for bars etc. 30 mm to 150 mm) slabs for sheet rolling and smaller rolled products.
Steels with less than 0.15% C is mild steel
0.15 to 0.25 is low carbon steel
0.25 to 0.6 is medium carbon steel and
0.6 to 1.4 is high carbon steel
Carbon content of steel changes its tensile strength upto 1% C, Strength increases and ductility reduces, From
1-1.5% C strength reduces and hardness increases.
• Alloy steel : Steels that contain in addition to carbon, some alloying elements purposely added to produce
a desired effect other than oxidation.Even though carbon still plays a predominate part in determining the
characteristics of steels. The importance of alloy steels lies in the fact that they permit a much wider range of
physical and mechanical properties then is possible in plain carbon steels. The increase of tensile strength
and hardness must necessarily be accompanied by a corresponding loss in toughness and ductility . For
e.g. austenity chrome – nickel steels and head filed’s manganese steel may show a combination of tensile
strength of about 115 Kg/mm2 with elongation of 70% . Some alloy steels possesses a tensile strength of even
350 Kg/mm2 which is obviously impossible with plain carbon steels
The important alloying elements are Ni, Cr, Mn, W, V, Mo, Si, Co
The purpose for which alloys are added to steel are one or a combination of the following:
• To obtain high tensile strength or hardness without brittleness
• To obtain more uniform hardness throughout a state

• To obtain resistance to creep at high temperatures or to retain strength and other properties at very high or low
temperatures.

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CITS : C G & M - Fitter - Exercise 61