| Summary: | 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<sub>7</sub>C<sub>3</sub>, M<sub>6</sub>C, and M<sub>2</sub>C, were found existing in the near-equilibrium state. The precipitate sequence of these carbides was identified as MC + M<sub>7</sub>C<sub>3</sub> + M<sub>2</sub>C → MC + M<sub>2</sub>C + M<sub>7</sub>C<sub>3</sub> + M<sub>6</sub>C → MC + M<sub>7</sub>C<sub>3</sub> + M<sub>6</sub>C. It was clarified that the stable phases for the investigated steel aged at 650 °C were composed of MC, M<sub>7</sub>C<sub>3</sub>, and M<sub>6</sub>C. For the first time, the in-situ transformations of M<sub>2</sub>C to M<sub>6</sub>C and M<sub>7</sub>C<sub>3</sub> to M<sub>6</sub>C were directly observed. It was also observed that the nucleation site of the M<sub>6</sub>C was located at the interface of M<sub>7</sub>C<sub>3</sub> 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.
|
|---|
