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 degrees C. Dilatometric study shows that the Ac-1, Ac-3 and M-s temperatures are 800, 855, and 131 degrees C, respectively. Initial cast and forged microstructures consist of martensite/ferrite. The samples are subsequently tempered at 500-800 degrees 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 degrees C for 1 h. Niobium rich MC type carbide precipitates and chromium rich M23C6 type precipitates are observed after tempering at 700 degrees C. Tensile strength decreases with increasing tempering temperature. Maximum tensile strength of 920 MPa is observed after tempering at 700 degrees C and maximum elongation of similar to 11% is observed after tempering at 750 degrees C. (C) 2014 Elsevier B.V. All rights reserved.