HEAT TREATMENT OF STEEL

The physical properties of steel are to a large extent affected by the grainy

structure of the metal. Properties such as ductility, malleability, hardness, and the

ability to be welded, forged or machined can be imparted to the metal by a variety

of processes which basically involve heat treatment.

Heat treatment is defined as a process or a number of processes involving the

heating and cooling of a metal or alloy in the solid state. Annealing, normalising,

hardening and tempering are some of the processes. They involve the following:

• Heating

• Holding for heat saturation and completion of structural changes

• Cooling at the requisite rate

The rate of heating and cooling depends on the size and shape of the work,

thechemical composition of the metal and the property intended to be imparted to

themetal.

Effect of Heat on the structure of steel

When steel is heated it expands, as is normal. However if the heating is continued,

it is found that, at two certain temperatures, the expansion gets arrested and instead

of expanding, the steel actually starts contracting. These temperatures are known as

the ‘critical points’ of the metal and are around 6000 - 700° C and 720° - 900° C

depending on the composition of steel. The phenomenon continues for a short

while till structural changes take place in the metal after which normal expansion

resumes with rise in temperature. This phenomenon also occurs during cooling but

it is not so pronounced. The expansion is due to the increased vibration of the

molecules due to the supply of energy in the form of heat while the contraction is

due to the realignment of molecules, dissolution of and reaction between the

constituents of the steel. The rate of cooling determines the characteristic grainy

structure of the metal.

These changes in volume are important factors in the heat treatment of steel. If the

metal can be caught and its structure frozen while it is in the transition stage,

special properties can be imparted to the metal. While heating, the work piece must

be allowed to soak in the heat and get saturated uniformly otherwise one part of the

work piece will be at a temperature different from the other. This will cause

internal stresses to be set up leading to warping, distortion and cracking.

Annealing

Annealing is done to impart softness, elasticity and ductility to the metal and to

relieve any internal stresses which may be present in the metal.

The metal is heated to about 20° - 30° C above the critical point and after being

held for a while at this temperature for the transformation to be complete, it is

cooled slowly at a rate of around 50° C per hour. The cooling may be done in the

furnace or in some medium e.g. a sand bath, which reduces the rate of cooling.

Annealing can be ofdifferent types e.g. full annealing, process annealing,

isothermal annealing, etc.

Normalising

Normalising is used to restore the original properties to a metal. It corrects the

structure of overheated steel, relieves stresses and improves the machinability of

carbon and low alloy steels by making them less hard. Normalising is also used to

harden and strengthen steel that has softened.

The metal is heated to 300 - 500 C above the critical temperature and allowed to

cool in still air.

Hardening

Steel used in certain applications e.g. metal chisels, drill bits, etc. need to be hard

and resistant to indentation. In order to harden it, it is heated to 200 - 30° C above

the critical point and then quenched. Quenching’ is the process of cooling the steel

rapidly from the temperature to which it has been heated. This is done by

immersing it in a quenching bath. These baths may be air, water, various kinds of

oil, brine, molten salts, etc. depending on the chemical composition, shape and size

of the metal.

While hardness is an attractive property, it is usually accompanied by brittleness.

Tempering

Tempering consists of heating the metal to about 250° C, retaining this temperature

for a period depending upon the mass and degree of toughness required and then

quenching or cooling in air.

Tempering relieves stress and brittleness in hardened steel and makes it ductile

without loss of hardness i.e. it makes the metal tough.