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

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, M₇C₃, M₆C, and M₂C, were found existing in the near-equilibrium state. The precipitate sequence of these carbides was identified as MC + M₇C₃ + M₂C → MC + M₂C + M₇C₃ + M₆C → MC + M₇C₃ + M₆C. It was clarified that the stable phases for the investigated steel aged at 650 °C were composed of MC, M₇C₃, and M₆C. For the first time, the in-situ transformations of M₂C to M₆C and M₇C₃ to M₆C were directly observed. It was also observed that the nucleation site of the M₆C was located at the interface of M₇C₃ 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.