The Effect of Nb on the Microstructures and Hardness of Quenching-Partitioning-Tempering Medium Carbon Steels

Nb-free and Nb-bearing medium carbon steels treated by quenching-partitioning-tempering process were investigated. The microstructural observations and hardness were analyzed by optical microscope, transmission electron microscope, X-ray diffraction and hardness test. Results show that when quenched at 210oC and partitioned at 450oC ultra fine-grained microstructures of martensite, retained austenite and carbides are obtained. The Nb-bearing steel has finer martensite lath, more volume fraction of retained austenite and higher hardness compared with that of Nb-free steel. The addition of Nb is responsible for not only the precipitation strengthening but also the more volume fraction of retained austenite during quenching-partitioning-tempering process.