Influence of the decomposition behavior of retained austenite during tempering on the mechanical properties of 2.25Cr-1Mo-0.25 V steel

The as-quenched microstructure of 2.25Cr-1Mo-0.25 V steel heavy forgings is granular bainite, which is composed of bainitic ferrite and blocky islands of martensite and retained austenite (RA). In this study, the characteristics of RA decomposition and its effects on the mechanical properties of the steel are investigated. The results show that RA decomposes into a cluster of coarse M23C6 carbides and ferrite during standard tempering at 700 degrees C. These coarse carbides decorate the boundary of the cluster, thus deteriorating the impact toughness of the steel. Accordingly, the size and distribution of these carbides are tentatively modified by introducing pre-tempering at different temperatures ranging from 180 degrees to 650 degrees C before the standard tempering at 700 degrees C. This is because during pre-tempering, RA first decomposes into various transitional microstructures such as martensite, bainite or pearlite, which further transform into M23C6 carbide clusters during the subsequent 700 degrees C tempering. The experimental results show that 455 degrees C is the optimal pre-tempering temperature to improve the impact toughness of the steel after the 700 degrees C tempering. Microstructural observations reveal that during the 455 degrees C pretempering step, the RA completely decomposes into bainite consisting of fine bainitic packets and a high density of M3C carbides, which provide additional nucleation sites for M23C6 carbides inside the carbide clusters during the subsequent 700 degrees C tempering, and thus avoid the formation of coarse M23C6 distributed along carbide cluster boundaries.