The Transformation of Residual δ Ferrite in Nuclear-Grade 316H Stainless Steel under Aging and Its Influence on Properties

To investigate the transformation behavior of residual δ-ferrite in nuclear-grade 316H stainless steel during aging and its effects on mechanical properties and intergranular corrosion resistance, high-temperature aging experiments are conducted at 750 °C for 2000 h on three heats of steel with varying δ-ferrite contents. Microstructural changes are analyzed using scanning electron microscopy–energy-dispersive X-ray spectroscopy, X-ray diffraction, and mechanical and intergranular corrosion tests. The results show significant transformations of residual δ-ferrite during aging. Initially, δ-ferrite transforms into M₂₃C₆, which grows within ferrite islands. With extended aging, M₂₃C₆ evolves into coarser rod-shaped σ phases, accompanied by chain-like M₂₃C₆ precipitates at grain boundaries. Further aging results in M₂₃C₆ and σ phases transforming into secondary austenite. Higher δ-ferrite content suppresses grain boundary M₂₃C₆ precipitation, leading to σ phase dominance, whereas lower δ-ferrite content results in M₂₃C₆ as the primary residual phase. Mechanical properties show an initial decrease, then increase, followed by stabilization in yield and tensile strength, while plasticity and impact toughness initially decrease and later improve. Higher δ-ferrite content enhances strength but reduces plasticity and toughness. Lower δ-ferrite content increases susceptibility to intergranular corrosion during aging, but long-term aging effectively mitigates this corrosion.