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Principle of heat treatment of metal

Short answer

Heat treatment involves controlled heating, soaking, and cooling of metal to change its microstructure and mechanical properties. The principle is that heating steel above its critical temperature forms austenite, and the cooling rate dictates the resulting structure—slow cooling yields soft pearlite, while rapid quenching produces hard martensite—thereby controlling hardness, strength, and ductility.

What is heat treatment of metal? State its main principle.

Heat treatment is a controlled sequence of heating a metal (steel) in the solid state to a definite temperature, holding (soaking) it there for a set time and then cooling at a chosen rate, in order to change its internal structure (grain size, phases) and so obtain the desired mechanical properties without changing its shape.

Principle

The properties of steel depend on its microstructure. When steel is heated above the critical temperature (about 723-910 C, depending on carbon), its structure changes to austenite (a solid solution of carbon in gamma-iron). On cooling, austenite changes into different structures according to the cooling rate:

Cooling rateStructureProperties
Very slow (in furnace)Coarse pearlite + ferriteSoft, ductile
Moderate (in air)Fine pearliteStronger, tough
Very fast (quench)MartensiteVery hard, brittle

Thus by controlling the heating temperature, soaking time and cooling rate, the structure and so the properties (hardness, strength, ductility, toughness) can be adjusted. The main processes are annealing, normalizing, hardening (quenching), tempering and surface hardening.

✓ Answer checked by Claude Opus 5.5 (high effort)

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