Evolution of Σ3 grain boundaries and σ-phase precipitation in advanced austenitic heat-resistant steel Super304H during long-term service

• The evolution of Σ3 grain boundary was analyzed by five-parameter analysis method. • M 23 C 6 and σ-phase exhibit transformation and competitive coexistence relationship. • The growth of σ-phase is unidirectional and depends on the diffusion of matrix Cr. • σ-phase causes Cr-depleted zones, reducing corrosion resistance of material. The samples of Super304H steel boiler reheater tubes that had been in service for 3 years and 10 years respectively at 650 ℃/5 MPa were obtained. In this study, the evolution of Σ3 grain boundaries and σ-phase was characterized, the precipitation kinetics of the σ-phase were calculated, and the corrosion resistance of the material was tested. This study revealed the nucleation-growth mechanisms of σ-phase under thermomechanical coupling and its interaction with grain boundary evolution. The results indicated that the fraction of coherent Σ3 grain boundaries decreased with increasing service time, showing a reduction 15.99 % after 10 years of service, due to the coupling effect of temperature and stress. During service exposure, M 23 C 6 carbides were preferentially nucleated at random grain boundaries and incoherent Σ3 boundaries. The metastable M 23 C 6 particles formed at triple junctions served as nucleation sites for σ-phase and gradually transformed into the σ-phase. The σ-phase growth predominantly depended on Cr diffusion from the matrix and the growth mode was unidirectional. After 10 years of service exposure, the micron-size σ-phase co-existed with the stable M 23 C 6 precipitated in a competitive precipitation relationship. The Cr-depleted zones and precipitation free zones adjacent to σ-phase particles deteriorated both corrosion and creep resistance.