The principal types of heat treatment of steel The principal type The principal types of heat treatment of steel-作业1.docx s of heat treatment can be described in the constitutional diagram in the following manner. Annealing is a structural recrystallization of heating above Ac3 and subsequent slow cooling. With heating above Ac1 but below Ac3, full recrystallization will not occur and the procedure is called partial annealing. The state of annealed steel is close to the state of structual equilibrium and its structure is pearlite + ferrite, pearlite, or pearlite + cementite. If a steal is heated above Ac3 and then cooled in the air, this will be first step to change it to a state farther from the structural equilibrium. This type of heat treatment is called normalizing and is an intermediate stage between the second-group procedure (annealing) and the third-group procedure (hardening). Hardening is heating above the critical point, Ac3, followed by quick cooling. With slow cooling, austenite decomposes into ferrite + cementite at Ar1. With increasing cooling rate, the transformation occurs at lower temperatures. As the Ar1 point is lowered, the ferrite-cementite mixture becomes more and more fine-disperse and hard. If the cooling rate is so high and the undercooling so substantial that the precipitation of cementite and ferrite dose not take place, there will be no decomposition of the solid solution and austenite (r-solid solution) will transform into martensite (supersaturated solid solution of carbon in a-iron). Incomplete hardening is heat treatment procedure with heating the metal above Ac1 but below Ac3 (Accm), after which the steel structure retains hypoeutectoid ferrite (hypoeutectoid cementite). Tempering is a procedure of heating a harden steel to a temperature below Ac1 and then cooling to room temperature at certain cooling rate. Steels can be subjected to various types of chemical heat treatment, depending on the element that diffuses in steel. The saturation of steel in carbon is called carburizing, in nitrogen, nitriding, in aluminium, alitizing, in chromium, chromizing, etc. Thermomechanical treatment of steal consists of heating the metal to austenitic state, deforming in that state (above Ac3 in stable state or in unstable undercooled state) and final cooling which is associated with the transformation of the strain hardened austenite.