Effect of tempering on microstructure evolution and mechanical properties of X12CrMoWVNbN10-1-1 steel

The investigation of X12CrMoWVNbN10-1-1 steel, an European steel developed for ultra-super critical power rotor, has been carried out for studying the influence of tempering on the microstructure evolution and the mechanical properties, so that the optimal tempering temperature can be determined. The microstructure was characterized using optical and scanning electron microscope, as well as transmission electron microscope. The phase identification and microchemistry of precipitates were done by physicochemical phase analysis method. After austenitization at 1080 °C for 16 h, the steel exhibited a martensite structure with some retained austenite and Nb-rich MN, Fe-rich M3C particles. Additional carbides and nitrides precipitation, changes in dislocation structure and the formation of subgrains occurred during the process of tempering. The precipitation of Cr-rich M2N, Cr-rich M7C3 and Nb-rich MN was detected in the samples tempered below 550 °C for 18 h. The formation of Cr-rich M23C6 was identified after tempering at 570 °C for 10 h. The Cr-rich M7C3 carbides were replaced gradually by Cr-rich M23C6 during the tempering over the range of 570 °C and 800 °C. The precipitation behavior was discussed in detail from the microchemical perspective. Hardness and impact energy were also determined. The results showed that hardness was found to be nearly stabilized up to 550 °C and then decreased with increasing tempering temperature. In contrast, impact energy was found almost stable up to 570 °C and then increased. A correlation between microstructure and mechanical properties is established. Finally, a careful conducted analysis of fractography of impact samples was done using SEM and EDS.