Assessing residual stress and high-temperature mechanical performance of laser-welded P91 steel for fusion power plant components

Residual stress poses significant risks to the life management of key engineering components in nuclear fusion reactors. It is often induced within the narrow heat-affected zone (HAZ) during remote laser welding, essential for maintaining and assembling in-vessel components. Residual stress interacts with high temperatures, degrading mechanical properties and altering fracture mechanisms. Establishing a correlative methodology with microstructures to investigate these stresses and understand their effects on mechanical properties within the narrow HAZ is crucial for extending the reactor's lifetime. In this study, advanced residual stress measurements, including plasma-focused ion beam and digital image correlation, were employed to reveal heterogeneous stress distribution, discovering the peak tensile residual stress of 150 MPa at the interface of the fusion zone (FZ) and HAZ, and peak compressive residual stress of 550 MPa within HAZ. The residual stress effects on micro-hardness are quantitatively evaluated, showing a 25% hardening effect at HAZ/BM interface and a 10% softening effects at fusion line. The deformation mechanism is further analysed using tensile testing, where the residual stress effects were prominent in the low-stress field, reducing 6.5% strain evolution, while microstructures reinforced the material in the high-stress field. At elevated temperatures, the joint exhibited a significant reduction from 532 MPa to 345 MPa for yield strength and from 12 % to 8 % for elongation, with distinct fracture mechanisms observed through fractography compared to its behaviour at room temperature. These findings provide critical insights into enhancing laser welding processing and maintaining the structural integrity of in-service nuclear fusion reactor components.