Enhancement of intergranular corrosion resistance of Super304H by a two-step gradient cooling strategy

•A two-step gradient cooling strategy promotes the re-precipitation of Nb(C, N).•The enrichment of Fe, Cr, and Ni elements surrounding Nb(C, N) hinders the formation of M23C6.•Significant intergranular corrosion resistance enhancement of Super304H. Niobium (Nb) addition could enhance the thermal stability of Super304H through the formation of Nb(C, N) precipitates. This study proposes an innovative two-step gradient cooling strategy that promotes dispersed Fe, Cr, Ni-round Nb(C, N) re-precipitation in the matrix. Through the strategy, the deleterious precipitation of M23C6 at grain boundaries (GBs) is significantly inhibited compared to the solution-treated Super304H during 650 °C isothermal aging. The distribution and element optimized Nb(C, N) through the strategy is of great significance for improving the thermal stability and intergranular corrosion (IGC) resistance of Super304H during service.