Performance heat treatment, namely quenching and tempering heat treatment, is the key process to control the mechanical properties of large and medium-sized forgings. Different typical structures and corresponding properties can be obtained by performance heat treatment. If the martensite structure is obtained after quenching and the tempered soxite can be obtained after high temperature tempering, the better the strength, plasticity and toughness of forged special-shaped parts can be matched to obtain higher comprehensive mechanical properties. If the lower bainite structure is obtained after quenching, the mechanical properties after high temperature tempering are similar to those after martensite tempering, and have higher impact toughness. After quenching, upper bainite, granular bainite or pearlite are obtained. After tempering at high temperature, the strength and toughness are low, and the comprehensive mechanical properties are poor. If there is ferrite in the quenched tissue, the comprehensive mechanical properties after high temperature tempering will be significantly reduced, especially the impact toughness will be significantly reduced. According to the characteristics of high strength and toughness of large and medium-sized cylindrical forgings, martensite or mixed microstructure of martensite and lower bainite is more likely to appear after quenching.
High performance heat treatment processes are usually used to obtain good strength and toughness. Austenitizing temperature, tempering temperature, holding time and quenching cooling rate are important process parameters of heat treatment. In order to obtain high strength and good low temperature toughness, it is necessary to cool the forged abnormity parts with large cylindrical section quickly in order to obtain as much lower bainite as possible after quenching, and to obtain even tempered bainite and fine carbide particles after tempering.