The Evolution of Complex Carbide Precipitates in a Low Alloy Cr–Mo–V Steel after Long-Term Aging Treatment

摘要

Complex carbide precipitates in a quenched and tempered low alloy Cr–Mo–V steel after long-term aging at 650 °C for 13,000 h and 30,000 h were investigated in this study. The mass fraction and sizes of precipitates were quantified by electrolytical extraction technique. The types of precipitate were further studied by combined X-ray diffraction and transmission electron microscopy with selected area electron diffraction and energy dispersive spectrometry. A series of carbide precipitates, namely MC, M 7 C 3 , M 6 C, and M 2 C, were found existing in the near-equilibrium state. The precipitate sequence of these carbides was identified as MC + M 7 C 3 + M 2 C → MC + M 2 C + M 7 C 3 + M 6 C → MC + M 7 C 3 + M 6 C. It was clarified that the stable phases for the investigated steel aged at 650 °C were composed of MC, M 7 C 3 , and M 6 C. For the first time, the in-situ transformations of M 2 C to M 6 C and M 7 C 3 to M 6 C were directly observed. It was also observed that the nucleation site of the M 6 C was located at the interface of M 7 C 3 carbides and the matrix. The orientation relationships between the secondary phases of the in-situ transforming carbides aged for 13,000 h and 30,000 h at 650 °C were established. The coherent interfaces between these secondary phases became incoherent with prolonged aging treatment due to the exerted strain field of the growing carbides.