Phase Transformation of a Cold Work Tool Steel during Tempering

The hardness and microstructure evolution of a 8% Cr cold work tool steel during tempering for 40 h were investigated. Transmission electron microscope examinations showed that M3C carbides precipitated from supersaturated martensite after tempering at 350 °C. When the tempering temperature was higher than 520 °C, the M23C6 carbides precipitated to substitute for M3C carbides. After ageing at the temperature of 520 °C for 40 h, it was observed that very fine and dense secondary Mo2C precipitates were precipitated. Thus, it can be concluded that the early stage of Mo2 C-carbidc precipitation is like to be Gunier–Preston (G–P) zone formed by [Mo-C] segregation group which is responsible for the secondary hardening peak at 520 °C. Overageing at 700 °C resulted in recovery of martensitic microstructure and precipitation of M23C6 carbides. When ageing at 700 °C for more than 20 h, recrystallization occurred, which resulted in a change of the matrix morphology from martensitic plates to equiaxed ferrite. It was noticed that the size of recrystallized grain/subgrain was very fine, which was attributed to the pinning effect of M23C6 precipitates.