6. Heat Treatment

Heat treatment is the process where requisite properties of formed and finished products are attained using predetermined heating and cooling rates. The treatment is most significant for ferrous materials because the phase transformation from austenite to ferrite can be controlled as the process involves nucleation and growth, which is reflected in time temperature transformation (TTT) curves of different alloys. The time being an important factor, rate of cooling (continuous cooling transformation, CCT curves) also plays an important role. Thus by heat treatment, depending on alloy content, it is possible to get different transformed products like martensite, bainite and pearlite. The major heat-treating processes for carbon and low alloy steels are annealing, normalizing and quenching where the distribution of ferrite and carbide are controlled by alloying and rate of cooling. Quenching which forms hard brittle martensite when tempered also improves the toughness of steel. Quenching is used also for surface hardening as well as for age hardening of some ferrous and non-ferrous alloys. For non-ferrous alloys, annealing is used. In case of austenitic stainless steels annealing requires quenching from high temperature. The chapter presents the mechanism of different heat treatment processes and discusses how the formation of different phases/transformation products can be explained. The role of alloying elements and cooling rates in controlling transformation and the importance of hardenability of ferrous alloys have been discussed. Austenitic stainless steels require solution and stabilizing annealing which involves rapid cooling unlike annealing of ferritic material, which involves slow cooling. The chapter also includes techniques of surface hardening by procedures other than heat treatment.