Creep rupture behavior of Super304H austenitic steel welded joints

• Grain coarsening, reduced NTBs and Nb carbides dissolution in HAZ promote continuous intergranular M 23 C 6 precipitation. • Loss of M 23 C 6 /matrix coherency in HAZ accelerates creep damage. • Preventive measures such as post-weld local pre-strain + solution treatment are proposed to suppress M 23 C 6 network. Super304H steel is a key material for high-temperature tubing in ultra-supercritical (USC) boilers, yet its welded joints are critical failure zones susceptible to creep damage. This study investigates the creep rupture behavior and microstructural evolution of Super304H welded joints at 923 K under stresses of 140–240 MPa. The joints exhibited approximately 20 % lower creep rupture strength than the base metal (BM), with fractures consistently occurring in the heat-affected zone (HAZ). Microstructural analysis revealed that grain coarsening, reduced density of twin boundaries, and dissolution of Nb carbides in the HAZ promoted continuous intergranular precipitation of M 23 C 6 carbides during creep. Electron backscatter diffraction (EBSD) analysis indicated a transition in the orientation relationship between M 23 C 6 and the matrix from coherent in the BM to non-coherent in the HAZ. Void coalescence, facilitated by grain boundary sliding and stress concentration around these carbides, led to intergranular crack initiation and propagation. It is concluded that the continuous network of non-coherent M 23 C 6 carbides is the dominant mechanism responsible for the creep failure of the HAZ. To mitigate this, preventive measures such as low welding heat input, post-weld local pre-strain combined with solution treatment, and alloy composition optimization were proposed to suppress detrimental M 23 C 6 network.