A Study of High Temperature Carburization and its Correlation with the Creep-Rupture Properties of a 30 Cr/30 Ni Austenitic Steel

The interaction of environment with alloy microstructure can significantly alter the mechanical properties of the alloy. In this study, a 30 Cr-30 Ni austenitic steel was carburized under two different thermodynamic conditions whereby two distinct microstructures were developed at the same level of carburization. The response of the two microstructures to deformation by creep was investigated at 1000 and 1050 C. Upon carburization at 950 C, the resulting microstructure consisted of thick grain boundary carbides and fine M₂₃C₆ carbides in the vicinity of the grain boundaries, while a nonprotective MnCr₂O₄ layer developed on the alloy surface. Carburization at 1150 C, on the other hand, developed an M₇C₃ surface layer, also unprotective, while uniformly distributed coarse M₂₃C₆ carbides formed in the alloy interior. Creep tests showed that the former microstructure led to an enhancement of creep strength and time to rupture at the expense of rupture ductility. In the latter case, carburization drastically reduced both creep strength and time to rupture while improving ductility markedly. Mechanisms are suggested to explain the carburization process and the observed creep behavior.