Carbide transformation behaviors of a Cr–Mo–V secondary hardening steel during over-ageing

Abstract

The transformation of carbides in a 1.9Cr-1.4Mo-0.3 V secondary hardening steel that was subjected to over-ageing at 600 °C–700 °C has been investigated. The carbides were characterized using scanning electron microscope (SEM), x-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-AES), and transmission electron microscopy (TEM) preformed on carbon replicas. The results indicate that MC, M _2 C, and M _3 C were formed during over-ageing from 600 to 700 °C, whereas M _7 C _3 , and M _23 C _6 started to be formed at 650 and 700 °C, respectively. In addition, the co-existence of hexagonal and orthorhombic M _7 C _3 structures in a carbide particle was firstly observed. M _3 C was transformed to other carbides, and the formation of both M _2 C and M _23 C _6 may follow the ‘separate nucleation’ mechanism, whereas M _7 C _3 was transformed from M _3 C via the ‘ in situ nucleation’ mechanism. The crystallographic orientation relationships between the in situ transformed M _7 C _3 and M _3 C are ${(11\bar{2})}_{{{\rm{M}}}_{{\rm{3}}}{\rm{C}}}\,//\,{(3\bar{3}0\bar{1})}_{{{\rm{M}}}_{{\rm{7}}}{{\rm{C}}}_{{\rm{3}}}}$ ${\rm{and}}\,{[312]}_{{{\rm{M}}}_{{\rm{3}}}{\rm{C}}}\,//\,{[10\bar{1}3]}_{{{\rm{M}}}_{{\rm{7}}}{{\rm{C}}}_{{\rm{3}}}}.$

Keywords

• secondary hardening steel
• carbides
• over-ageing
• in-situ nucleation