18-12-2012, 02:26 PM
Basic principles of heat treatment
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Heat treatment of a metal or alloy is a technological procedure, including controlled heating and cooling
operations, conducted for the purpose of changing the alloy microstructure and resulting in achieving
required properties.
There are two general objectives of heat treatment:
Hardening is a process of increasing the metal hardness, strength, toughness, fatigue resistance.
Strain hardening (work hardening) – strengthening by cold-work (cold plastic deformation).
Cold plastic deformation causes increase of concentration of dislocations, which mutually entangle one
another, making further dislocation motion difficult and therefore resisting the deformation or increasing the
metal strength.
Grain size strengthening (hardening) – strengthening by grain refining.
Grain boundaries serve as barriers to dislocations, raising the stress required to cause plastic deformation.
Solid solution hardening – strengthening by dissolving an alloying element.
Atoms of solute element distort the crystal lattice, resisting the dislocations motion. Interstitial elements
are more effective in solid solution hardening, than substitution elements.
Annealing
Annealing is a heat treatment procedure involving heating the alloy and holding it at a certain temperature
(annealing temperature), followed by controlled cooling.
Annealing results in relief of internal stresses, softening, chemical homogenizing and transformation of the
grain structure into more stable state.
Annealing stages:
Stress relief (recovery) – a relatively low temperature process of reducing internal mechanical
stresses, caused by cold-work, casting or welding.