Investigations of Microstructure and Mechanical Properties of Post-Weld Heat-Treated DP780 Steel TIG Welds

This study investigates the microstructure and mechanical properties of DP780 steel that has been tungsten inert gas welded and post weld heat treated. Microscopy studies revealed that the weldment’s microstructure varied from martensite in the fusion zone to a mixture of martensite and ferrite in the heat affected zone (HAZ). This heterogeneity in the microstructure resulted in the formation of hardened and softened zones in the cross section of the weldment. The DP780 as-welded joint exhibited lower strength and ductility [yield strength (YS): 492 ± 5 MPa, ultimate tensile strength (UTS): 668 ± 8 MPa, and percent elongation (%El): 8 ± 1] compared to the base metal (BM) (YS: 538 ± 2 MPa, UTS: 794 ± 5 MPa, and %El: 27 ± 2) due to strain localization in the subcritical HAZ. The weldments subjected to post weld heat treatment (PWHT) at 500°C exhibited lower strength and higher ductility (YS: 471 ± 3 MPa, UTS: 624 ± 5 MPa, and %El: 13 ± 1) than the weldments subjected to PWHT at other conditions: 300°C (YS: 501 ± 7MPa, UTS: 658 ± 6 MPa, and %El: 9 ± 1) and 400°C (YS: 492 ± 3 MPa, UTS: 649 ± 5 MPa, and %El: 11 ± 1). The decrease in strength and ductility after PWHT can be attributed to the tempering of martensite present in the weldment. Erichsen cupping tests indicated a reduction in the formability of the as-welded joint due to the presence of a softened zone. While a significant increase in formability is observed in the weldments subjected to PWHT with an increase in temperature, the formability is still inferior to that of the BM due to the inhomogeneity in the microstructures across the weldment.