Effect of High-Temperature Ageing on Microstructure and Creep Properties of S31042 Heat-Resistant Steel

S31042 steel is a typical 25Cr-20Ni-type austenitic heat-resistant steel with excellent oxidation and corrosion resistance, and its creep rupture strength can be improved by the addition of Nb and N. This austenitic steel is widely used in superheater and reheater in ultrasupercritical power plants. At high temperatures, its performance is associated with the formation and evolution of Z, MX, and M23C6 phases. Till date, few studies have addressed the precipitation behavior of the Z phase in austenitic steel and the reinforcing mechanism of different M23C6 phases remains unclear. To clarify this, the ageing treatment of S31042 steel was performed at 1050 degrees C, and the evolution behavior, thermal stability, and strengthening mechanism of the precipitates during creep tests were investigated. Furthermore, the relation between precipitate evolution and high-temperature performance was elucidated via OM, SEM, TEM, and creep tests. The supersaturation degree of the alloying components in solution-treated S31042 steel decreased after ageing at 1050 degrees C and the driving force for M23C6 phase formation became smaller, resulting in a discontinuous distribution of the rod-like M23C6 phase along the austenite grain boundaries during the creep tests. At high stress levels, this discontinuous distribution of the rod-like M23C6 phase along the austenite grain boundaries increased the resistance to grain boundary sliding without changing the ductility, thus improving the rupture ductility of the steel. At low stress levels, the strengthening effects of the M23C6 phase discontinuously distributed along the austenite grain boundaries in aged steel were not as strong as those in solution-treated steel.