STUDY ON SECONDARY HARDENING MECHANISM OF Cr8Mo2SiV STEEL

Cr8-type cold work die steels, such as Cr8Mo2SiV steel have been widely used in recent years due to high strength and high toughness, These steels have obvious secondary hardening effect. It has been widely reported that the secondary hardening mechanism of these die steels resulted from the combination of retained austenite and carbide precipitation. However, less attention has been paid on the secondary hardening of Cr8Mo2SiV steel. In order to investigate the secondary hardening mechanism of Cr8Mo2SiV steel, the hardness, retained austenite and precipitation of Cr8Mo2SiV steel were investigated in this paper by SEM, EDS, TEM and XRD analysis. Experimental results indicate that the secondary hardening peak of Cr8Mo2SiV steel which was quenched at 1030 degrees C appears at 520 degrees C. Deep cryogenic treatment observably reduces the content of retained austenite, and thus increases the tempering hardness before secondary hardening peak and the secondary hardening peak shifted to low temperature by about 20 degrees C. The tempered hardness of Cr8Mo2SiV steel decreased linearly with the increase of soaking time when tempering at 520 degrees C. The secondary hardening mechanism of Cr8Mo2SiV steel is the combination of the transformation of retained austenite and the early stage of Mo2C precipitation, and the role of transformation of retained austenite is more obvious. The early stage of Mo2C-carbide precipitation is likely to be G.P. zone which formed by [Mo-C] segregation group. As the tempering time extended, Mo2C precipitated, and it was dispersive and uniformly distributed.