Microstructure and properties of low alloy CrMo steel processed with varied isothermal temperatures

Abstract

The microstructure and properties of low alloy CrMo steel after isothermal treatment at different temperatures were studied. The experiment process is to rapidly cool the samples that have been held at 940 ℃ for 2 h to a salt bath furnace at different temperatures (300–500 ℃) for isothermal treatment, and then air cooling to ambient temperature. The microstructure after isothermal treatment is granular bainite, which is a mixed microstructure composed of bainitic ferrite (BF) and martensite/austenite (M/A) island. The morphology, size and volume fraction of M/A islands change with the change of isothermal temperature. The mechanical properties are also closely related to isothermal temperature. With the increase of isothermal temperature, the impact toughness and yield strength (YS) decreased. By studying the effects of different strengthening mechanisms and multiphase on YS, the results show that YS mainly depends on BF. According to the SEM and EBSD characterization of the fracture, it is found that the stress in the sample is mainly concentrated near the M/A, which makes the crack easy to initiate on the M/A island or its interface. The brittle fracture tendency of materials is related to M/A island. The larger the size of the M/A island, the more likely brittle fracture will occur and the less ductile it will be.