Role of Heat Treatment on Grain Refinement and Microhardness of 9Cr–1Mo–V–Nb Steel

Effect of ‘double austenitizing’ (DN) on microstructure evolution and mechanical properties of martensitic 9Cr–1Mo–V–Nb (P91) steel were studied and compared with the ‘conventional normalizing’ (CN) process. In CN treatment, P91 steel is normalized at 1050 °C for 1 h, finally air cooled. In DN treatment, the first stage of normalizing was performed at 1050 °C/1 h/air cool. The second stage of normalizing was performed in the temperature range of 950–1150 °C for 1 h (950 °C-DN1, 1050 °C-DN2, 1150 °C-DN3), followed by water quenching. The grain size was measured 42 and 35 µm for CN and DN1 treatment, respectively. The double normalizing (DN) produced complete martensitic microstructure as a result of complete dissolution of precipitates. In DN-based heat treatment, optimized microstructure and mechanical properties were obtained for the sample that normalized at 950 °C, followed by water quenching.