Microstructure and mechanical property formation of heat treated low-carbon chromium-nickel-molybdenum steels
摘要
Low-carbon Cr-Ni-Mo steels widely used in mechanical engineering are studied: 18Kh2N4MA, 25Kh2N4MA, 25KhN3MA. Thermokinetic diagrams are plotted by a dilatometric method. Temperature-time ranges for microstructure constituent formation are established. It is shown that bainite within the steels studied may be formed both above and below the Ms temperature. Features of isothermal bainite transformation are investigated, kinetics of bainite transformation are determined, as well as the quantitative ratio of microstructure constituents formed as a result of austempering. It is established that the largest amount of bainite within the steel structures studied (80–95%) is achieved at a temperature near Ms. Mechanical properties (strength, ductility, impact strength) of the steels being studied are analyzed after various heat treatment methods: various cooling intensities, upper and lower bainite austempering. It is shown that formation of upper bainite has an ambiguous effect on steel impact strength.