Microstructure Evolution and Precipitation Behavior of 0Cr16Ni5Mo Martensitic Stainless Steel during Tempering Process

The microstructure, morphology of precipitates and retained austenite and the volume fraction of retained austenite in 0Cr16Ni5Mo stainless steel during the tempering process were analyzed using optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD) and scanning transmission electron microscope (STEM). The results show that the microstructure of the tempered steel is mainly composed of tempered martensite, retained austenite, and delta ferrite. In the case of samples tempered from 500 to 700 °C, the precipitates are mainly M23 C6, which precipitate along the lath martensite boundaries. The precipitate content increases with the tempering temperature. During the tempering process, the content of retained austenite initially increases and then decreases, the maximum content of retained austenite being 29 vol. % upon tempering at 600 °C. TEM analysis of the tested steel reveals two morphology types of retained austenite. One is thin film-like retained austenite that exists along the martensite lath boundary. The other is blocky austenite located on packet at the boundary and the original austenite grain boundary. To further understand the stability of reversed austenite, the Ni content in reversed austenite was measured using STEM. Results show a significant difference in nickel concentrations between reversed austenite and martensite.