Precipitation behavior in boundaries and its influence on impact toughness in 22Cr25Ni3W3CuCoNbN steel during short-term ageing

Abstract

Abstract The 22Cr25Ni3W3CuCoNbN steel is a new kind of heat-resistant steel and is developed as super-heater and reheater materials used in the A-USC boilers at 700\xa0°C. Its impact toughness directly affects the safety of boilers in the process of startup and shutdown. The microstructure evolution and second phase precipitation behavior are accounted for the change of mechanical properties. In the present study, the 22Cr25Ni3W3CuCoNbN steel was aged at 700\xa0°C for up to 1000\xa0h. Using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD), the twin structure evolution and the precipitation of M23C6 carbides on grain boundaries and twin boundaries were analyzed, and their influence on the toughness was discussed. The grain size of the investigated steel was stable and numerous coherent and incoherent twins which were formed during solution heat-treatment still retained after ageing. The M23C6 carbides precipitated on both grain boundaries and twin boundaries. The amount of M23C6 carbides on grain boundaries was higher than that on incoherent boundaries, which in turn was higher than that on coherent boundaries. On grain boundaries, the critical interfacial nucleation energy was the lowest compare to the coherent and incoherent boundaries, thus the nucleation of M23C6 carbides on grain boundaries was the easiest. The M23C6 carbides formed a network on grain boundaries, but precipitated as parallel plates on incoherent boundaries and kept as dispersed particles on coherent boundaries. The impact toughness decreased obviously at the early ageing stage, which is the result of rapid precipitation of M23C6 carbides at grain boundary. The twin structure is beneficial to increase the impact toughness, because the coherent twin can disperse the impact force, and the incoherent twin can reduce the precipitation of M23C6 carbides at grain boundary by promoting the precipitation of M23C6 carbides at the incoherent twin boundary.