Characterization of precipitates in X12CrMoWVNbN10-1-1 steel during heat treatment

Highlights: • We characterize precipitates by physicochemical phase analysis, XRD, TEM, and etc. • Precipitation sequence of secondary phase during heat treatment has been revealed. • Carbide precipitation behavior explains the rationality of heat treatment process. - Abstract: The characterization of precipitates in X12CrMoWVNbN10-1-1 steel during the heat treatment was carried out for revealing the evolution of the precipitates. In addition to other microstructural parameters (such as dislocation and subgrains), the precipitate also plays an important role for microstructural stability which is a prerequisite for long term creep strength. In this paper, the precipitates during the heat treatment for this steel were characterized using physicochemical phase analyses and transmission electron microscopy. It was found that the Fe-rich M₃C carbides and Nb-rich MX particles were detected in the samples cooled in furnace from austenitization at 1080 °C for 16 h. However, after water cooling, only Nb-rich MX particles existed. During tempering at 570 °C for 18 h, the formation of Cr-rich M₇C₃ was detected but was replaced partially by Cr-rich M₂₃C₆. Additional Cr-rich M₂N nitride was also found. After two successive tempering (570 °C + 690 °C) for 24 h, Cr-rich M₇C₃ was completely replaced. The microchemical analyses of the extracted residues during heat treatment were also discussed. The results gave rise to an indication that the precipitation of precipitates nearly completed in first tempering and the transformation from Cr-rich M₇C₃ to Cr-rich M₂₃C₆ mainly occurred in the second tempering