Insights on microstructure evolution of austenite welded joints subjected to creep-fatigue loading

The crack growth behavior and microstructural evolution of 316H austenitic stainless steel welded joints subjected to load-controlled creep-fatigue crack growth at 550 °C was investigated in this study. A comprehensive analysis of the fracture mechanisms in both the base metal (BM) and weld metal (WM) was conducted employing advanced characterization. The results revealed that cracks in both BM and WM preferentially propagate along paths of high energy and low resistance, characterized by substructured and deformed grains. In the BM, twin boundaries demonstrated high fracture resistance; however, with increased dwell time, the interaction between planar slip bands and twins induced a detwinning effect, thereby reducing the fracture resistance. In contrast, the WM exhibited interdendritic cracking primarily due to the precipitation of carbide and σ phase at the δ/γ interface, causing cracks to propagate along the ferrite network during extended dwell times. Furthermore, the formation of subgrains in austenitic dendrites and the transition of ferrite in the WM significantly reduced the texture strength of the original columnar crystals, impacting the overall creep-fatigue crack growth behavior. These results underscore the critical role of microstructural evolution in determining fracture resistance and crack propagation behavior in 316H austenitic stainless steel welded joints under creep-fatigue loading. The insights gained from this study are essential for improving the design and durability of high-temperature components in advanced reactors.