Effect of post-weld heat treatment on high temperature tensile strength of laser-welded K439B joint

K439B is a γ′-strengthened Ni–Co–Cr-based cast superalloy developed for hot-end components operating at 800 °C. In this work, the effect of post-weld secondary aging duration at 845 °C on the microstructures and 800 °C tensile strength of laser-welded K439B joints were experimentally investigated, and the contributions of strengthening mechanisms to the yield strength (YS) were evaluated using a microstructure-based model. The as-welded joint exhibited pronounced microstructural heterogeneity, and the absence of γ′ phase in fusion zone (FZ) resulted in relative low strength. The post-weld heat treatment (PWHT) promoted γ′ precipitation throughout the joint, together with M 23 C 6 precipitation along grain boundaries (GBs), thereby reducing regional microstructural heterogeneity. With increasing secondary aging duration, the mean γ′ diameter increased from approximately 31 to 62 nm, while its fraction increased slightly from approximately 21% to 23%. The YS and ultimate tensile strength (UTS) first increased and then decreased, reaching peak values of 763.8 MPa and 781.3 MPa, respectively, after 10 h of aging. Meanwhile, the fracture location shifted from the FZ to base metal (BM) or heat-affected zone (HAZ). The predicted YS evolution agreed well with the experimental results, providing a quantitative basis for optimizing PWHT of precipitation-strengthened nickel-based superalloy weldments.