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, M2C, and M3C were formed during over-ageing from 600 to 700 °C, whereas M7C3, and M23C6 started to be formed at 650 and 700 °C, respectively. In addition, the co-existence of hexagonal and orthorhombic M7C3 structures in a carbide particle was firstly observed. M3C was transformed to other carbides, and the formation of both M2C and M23C6 may follow the ‘separate nucleation’ mechanism, whereas M7C3 was transformed from M3C via the ‘in situ nucleation’ mechanism. The crystallographic orientation relationships between the in situ transformed M7C3 and M3C are