Microstructures and Charpy Toughness of DP980 Steel Heat-Affected Zone Simulated Via 2.5-D Rosenthal Thermal Cycles

This study examines the microstructural evolution and mechanical properties of medium-thickness DP980 advanced high-strength steel subjected to simulated welding thermal cycles. Gleeble simulations reproduced the thermal histories of coarse-grained (CG), fine-grained (FG), intercritical (IC), and subcritical heat-affected zone (SCHAZ) subregions. Microstructures were analyzed by scanning electron microscopy (SEM), and X-ray diffraction (XRD), while Vickers microhardness and Charpy impact tests were performed at room temperature and -40 ℃. Results showed that CGHAZ and FGHAZ developed polygonal prior-austenite grains and lath martensite, yielding higher hardness and toughness than the base metal. In contrast, ICHAZ contained a heterogeneous mixture of ferrite, martensite, retained austenite, and martensite-austenite (M-A) constituents, which led to significant toughness loss at both temperatures examined. The SCHAZ contained tempered martensite within ferrite and showed only moderate changes in impact energy. Although overall trends were similar, impact toughness decreased further at -40 ℃. These findings highlight the critical role of intercritical microstructures in governing toughness and emphasize the need to control welding parameters to suppress detrimental M-A formation in DP980 steel.