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 degrees C for 1 h, finally air cooled. In DN treatment, the first stage of normalizing was performed at 1050 degrees C/1 h/air cool. The second stage of normalizing was performed in the temperature range of 950-1150 degrees C for 1 h (950 degrees C-DN1, 1050 degrees C-DN2, 1150 degrees C-DN3), followed by water quenching. The grain size was measured 42 and 35 mu m 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 degrees C, followed by water quenching.