Relationship between microstructure and mechanical properties of M50 ultra-high strength steel via quenching-partitioning-tempering process

Novel quenching-partitioning-tempering (Q-P-T) and traditional quenching-tempering (Q-T) processes are conducted for M50 ultra-high strength steel. An excellent combination of strength and toughness is obtained via the Q-P-T process. The highest impact absorption energy of the Q-P-T specimen increases by 93% compared to the Q-T specimen. However, the yield and ultimate tensile strength of Q-P-T specimens are almost the same as those of Q-T specimens. Microstructure analysis indicates that the maximum fraction of retained austenite in the Q-P-T specimen is 7.5%, which is much higher than that in the Q-T (2.0%) specimen. Meanwhile, more fine cracks appear in the fracture Q-P-T specimen. The precipitation of fine and diffuse omega phase, Fe3C and Cr23C6 is observed during the partitioning process. The present work also reveals that the Q-P-T/Q-P process can be applied to high-carbon steel with little Si or Al.