chapter  3
44 Pages

Fundamental Concepts in Steel Heat Treatment

References .......................................................................................................................... 163

Further Reading .................................................................................................................163

Thepurposeofheat treatment is to causedesired changes in themetallurgical structure and thus in

the properties of metal parts. Heat treatment can affect the properties of most metals and alloys,

but ferrous alloys, principally steels, undergo the most dramatic increases in properties, and

therefore structural changes in iron-carbon alloys are considered in this chapter. In general, the

most stable steel structures are producedwhen a steel is heated to the high-temperature austenitic

state (to be defined later) and slowly cooled under near-equilibrium conditions. This type of

treatment, often referred to as annealing or normalizing, produces a structure that has a low level

of residual stresses lockedwithin the part, and the structures canbepredicted fromanequilibrium

diagram. However, the properties that interest heat treaters the most are those exhibiting high

strength and hardness, usually accompanied by high levels of residual stresses. These are meta-

stable structures produced by nonequilibrium cooling or quenching from the austenitic state.