Abstract
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 Mo2C-carbide 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 micro-structure 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.
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Foundation Item: Item Sponsored by National Key Technologies Research and Development Program of China (2007BAE510B04)
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Chi, Hx., Ma, Ds., Xu, Hx. et al. Phase transformation of a cold work tool steel during tempering. J. Iron Steel Res. Int. 23, 484–488 (2016). https://doi.org/10.1016/S1006-706X(16)30076-0
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DOI: https://doi.org/10.1016/S1006-706X(16)30076-0