Effect of microstructure evolution on mechanical properties and oxidation behavior of Super304H and HR3C after long-term service

The oxidation structure, microstructure and mechanical properties of Super304H and HR3C are investigated after running at 630 °C/3 MPa for 40,000 h. After service, a duplex oxide is formed on the steam side of heat-resistant steel, and it is possible that the surface treatment has caused surface deformation of Super304H, leading to an increase in corrosion resistance. For Super304H, a chain-like distribution of M₂₃C₆ has been formed at the grain boundaries, causing creep cavities, NbCrN and ɛ-Cu particles have precipitated inside the grains. For HR3C, there is a significant aggregation of Cr at the grain boundaries, with only a small amount of M₂₃C₆ and σ phase, and a large number of nanoscale NbCrN particles appear within the grains. The stable and dispersed precipitates inside the grains hinder the movement of dislocations, allowing heat-resistant steel to maintain higher strength than its as-received condition after long-term service. The microstructure evolution at grain boundaries leads to a decrease in toughness and plasticity of heat-resistant steel, and the continuous evaporation of Cr in high-temperature steam will cause a gradual decrease in Cr concentration and a deterioration in oxidation resistance of heat-resistant steel.