Electrically assisted post-weld rapid heat treatment of laser beam welded DP 590 steels: Microstructural and mechanical properties

This study investigates the impact of electrically assisted post-weld heat treatment (EA-PWHT) on the microstructure and mechanical properties of laser beam-welded (LBWed) DP 590 steel. EA-PWHT utilizes electro-pulsing to minimize microstructural inhomogeneity throughout the weld. Two EA-PWHT strategies, single-pulse (SP) and multiple-pulse (MP) electro-pulsing, are considered. The secondary current density in MP heat treatment delays cooling between Ac 3 and Ac 1 temperatures. Optical microscopy reveals that heat-affected zones (HAZs) near the weld disappear after EA-PWHT. Electron microscopy of the LBW-only weld zone (WZ) shows fine columnar grains with martensitic needles, which turn into coarse columnar grains after SP heat treatment, and a bi-modal structure with martensitic plates after MP heat treatment. Martensite in the LBW-only WZ (57 %) decreases to 41 % and 36 % after the SP and MP heat treatments, respectively. After EA-PWHTs, the epitaxial grains of the HAZs with tempered martensite transform into coarse equiaxed grains with fresh martensite. While the fresh martensite increases microhardness around the weld, the microhardness within the WZ decreases due to reduced martensite after EA-PWHT. The MP heat treatment reduces the residual stress within the WZ, transforming its nature from tension (LBW-only) to compression through microstructural changes. Overall, EA-PWHTs effectively control the joint microstructure without altering base material properties. • EA-PWHT improved LBWed DP 590 steel properties without altering BM. • EA-PWHT reduced martensite inside WZ, causing microstructural stabilization. • Electro-pulsing and Joule heating eliminated microstructural anisotropy between HAZ and BM. • MP treatment shifts tensile residual stress to compressive, enhancing weld macro behavior.