Phase Transformation of a Cold Work Tool Steel during Tempering

• Published in: Journal of iron and steel research, international international, 2016-05, Vol.23 (5), p.484-488
• Main Authors: CHI, Hong-xiao, MA, Dang-shen, XU, Hui-xia, ZHU, Wang-long, JIANG, Jian-qing
• Format: Article
• Language: English
• Subjects: Applied and Technical Physics; carbide; cold work tool steel; Engineering; Machines; Manufacturing; Material; Materials Engineering; Materials Science; Metallic Materials; Physical Chemistry; Processes; secondary hardening; tempering; 冷作工具钢; 回火温度; 回火过程; 回火马氏体; 显微镜检查; 晶粒尺寸; 相变; 碳化物
• ISSN: 1006-706X; 2210-3988
• DOI: 10.1016/S1006-706X(16)30076-0

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 M_3C carbides precipitated from supersaturated martensite after tempering at 350 ℃. When the tempering temperature was higher than 520 ℃,the M_（23）C_6 carbides precipitated to substitute for M_3C carbides. After ageing at the temperature of 520 ℃ for 40 h,it was observed that very fine and dense secondary Mo_2C precipitates were precipitated. Thus,it can be concluded that the early stage of Mo_2C-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 ℃. Overageing at 700 ℃ resulted in recovery of martensitic microstructure and precipitation of M_（23）C_6 carbides.When ageing at 700 ℃ 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 M_（23）C_6 precipitates.