Effect of tempering on microstructure and tensile properties of niobium modified martensitic 9Cr heat resistant steel

The effect of tempering on the microstructure of niobium modified 8.8 wt% chromium steel has been evaluated. Steel has been prepared using the conventional melting and casting route. Homogenization and forging is done at 1100 °C. Dilatometric study shows that the Ac1, Ac3 and Ms temperatures are 800, 855, and 131 °C, respectively. Initial cast and forged microstructures consist of martensite/ferrite. The samples are subsequently tempered at 500-800 °C for various intervals of time (1-5 h). The microstructure of the tempered sample is analyzed using optical microscopy, scanning electron microscopy, and X-ray diffraction. High Resolution Transmission Electron Microscopy (HRTEM) is used to identify the precipitate. Nanometer-sized precipitates (50-200 nm) are observed after tempering at 700 °C for 1 h. Niobium rich MC type carbide precipitates and chromium rich M23C6 type precipitates are observed after tempering at 700 °C. Tensile strength decreases with increasing tempering temperature. Maximum tensile strength of 920 MPa is observed after tempering at 700 °C and maximum elongation of ~11% is observed after tempering at 750 °C.