Precipitation and growth of Laves phase and NbC during aging and its effect on tensile properties of a novel 15Cr–22Ni–1Nb austenitic heat-resistant steel

The microstructure evolution and the mechanical properties of 15Cr–22Ni–1Nb austenitic heat-resistant steel were investigated during the aging up to 24 h at 780 °C, 820 °C and 860 °C. A large amount of Laves phase and nano-sized NbC were precipitated during the aging. The NbC particles exhibit excellent coarsening resistance, and the coarsening rate constant of nano-sized NbC particles is four orders of magnitude smaller than that of Laves phase during the aging at different temperatures. The room temperature tensile strength of the heat-resistant steel after aging at 820 °C for 4 h is highest among different aging treatments. The volume fraction of Laves phase increase with the increase in aging temperature and holding time, resulting in a decrease in the ductility of the heat-resistant steel. Ostwald ripening was observed during the aging for 8 h at different aging temperatures. Consequently, the high temperature ultimate tensile strength of heat-resistant steel after the aging for 8 h is lower than that for 4 h and 24 h. The precipitation strengthening contributed by Laves phase and nano-sized NbC was estimated. The yield strength contributed by submicron Laves phase particles is smaller than that provided by nano-sized NbC particles.