Heat treatment
Definition : Properties of metals and alloys may be changed by changing their microstructure. This may be done by heating and cooling the metals. Thus heat treatment is the process of obtaining the desired properties by changing the microstructure of metals.
These microstructures are obtained by heating and cooling the metals in its solid state under controlled conditions. Heat treatment process may be carried out by –
(i) heating the metal, to a predetermined (required) temperature in solid state
(ii) soaking (holding) the metal at that temperature for a required time so that whole of the metal attain the required temperature,
(iii) Cooling the metal at a required rate to obtain the desired microstructure and hence the desired properties.
Classification: Various heat treatment processes may be classified (or enumerated) as follows :
(1) Hardening
(2) Tempering
(3) Annealing
(4) Normalizing
(5) Surface Hardening
(a) Carburizing
(i) Solid/pack carburizing
(ii) Liquid Carburising
(iii) Gas Carburising
(b) Cyniding
(c) Nitriding
(d) Flame hardening
(e) Induction hardening
Purpose of Heat Treatment:
Heat treatment process serve one or more of the following purposes:
(i) Improve mechanical properties such as hardness, strength, toughness and ductility.
(ii) Improve machinability.
(iii) Improve resistance to corrosion, wear, abrasion & heat.
(iv) Improve /modify electrical and magnetic properties.
(v) Relieve internal stresses produced during cold working.
(vi) Prepare the metal for further operations.
(vii) Refine grain size.
(viii) Change chemical composition of the surface.
(ix) Remove gases.
(x) Remove cracks and distortions.
(xi) Produce hard surface and tough core.