Effect of Post-Weld Intercritical Annealing on the Microstructure and Tensile Properties of a Gas Tungsten Arc Welded DP700 Steel

The aim of this study was to investigate the effect of post-weld intercritical annealing (IA) on the microstructure and tensile properties of a gas tungsten arc-welded DP700 steel sheet. In this regard, DP700 steel sheets were first joined using gas-tungsten arc welding. The welded sheet was then annealed at intercritical temperature of 720 °C for 120 s followed by water quenching. Microstructure and microhardness of the as-welded and post-weld heat-treated samples were characterized, as well as tensile properties. Results showed that the microstructure varied from ferrite-tempered martensite in the subcritical heat affected zone to a mixture of bainite, Widmanestatten ferrite, ferrite-carbide aggregate and grain boundary ferrite in the fusion zone. This heterogeneity in the microstructure led to the formation of hardened and softened zones across the welded joint. Strain localization in the softened zone during tensile testing resulted in a joint efficiency of ~ 81% and early fracture. After post weld IA, dual phase microstructures comprising of ferrite and martensite, with different sizes and morphologies, were formed across the welded joint. This new microstructure resulted in the elimination of hardened and softened zones, which in turn led to high joint efficiency of ~ 98%.