Significantly improving mechanical properties of ultra-high strength steel joints via optimal in-situ post-weld heat treatment

By applying optimal in-situ post-weld heat treatment (PWHT), in-situ austenitization was achieved in the fusion line (FL) of ultra-high strength steel (2 GPa steel, tensile strength exceeding 2000 MPa) resistance spot welding (RSW) joint, and mechanical properties of the joint were significantly improved. In the fusion line of 7 kA weldment without in-situ post-weld heat treatment, the presence of a C-poor zone led to the formation of fine-lath γ-austenite and a decrease in dislocation density, resulting in the “hard-soft-hard” sandwich structure. The cracks of the 7 kA weldment propagated along the fusion line. The maximum force of 7 kA weldment was only 12.8 kN. The optimal in-situ post-weld heat treatment parameter (6 kA + 2 s) caused the re-partitioning of the C element and the formation of full martensite in the original fusion line, eliminating the “hard-soft-hard” sandwich structure. Consequently, the crack in 7 kA-(6 kA + 2 s) weldment no longer propagated along the fusion line. The maximum force of 7 kA-(6 kA + 2 s) weldment reached 22.2 kN. The efficient, accurate and optimal post-weld heat treatment process can eliminate element segregation and change the microstructure in the fusion line of ultra-high strength steels resistance spot welding joints, thereby altering the crack propagation path and significantly enhancing the mechanical properties of the joints.