Regional Pre-Heat Treatment System Development and Applications for HAZ in RSW Joining of High-Strength Steels

Abstract

STRENX and DP steels are advanced high-strength dual-phase steel types used in the automotive industry for producing lightweight vehicles that have low cost, high fuel efficiency, and can carry more load volume. In this study, STRENX 700 CR and DP 800 steels were joined by resistance spot welding (RSW) using medium frequency direct current (MFDC). Some parts were joined under atmospheric conditions, while others were joined by applying regional pre-heating (RPH) to their heat-affected zones (HAZ) before welding. The design, manufacturing, and adaptation of the RPH system to the RSW machine were carried out as part of this study, considering it as a new system. After the joining process, non-destructive (visual inspection, magnetic particle) and destructive (microhardness, tensile-shear, cross-tension, fatigue) tests were performed on RSW connections between STRENX 700 CR and DP 800 steels. Hardness tests revealed that the RPH applied to the HAZ before welding reduced the hardness of the weld metal by approximately 8%, while the microstructure analysis showed that the RPH increased both the weld metal and the HAZ width. The tensile-shear strength increased by about 7%, and the cross-tension test strength increased by about 5% with the RPH applied to the HAZ before welding. In both the tensile-shear and cross-tension tests, all failures occurred in the form of button shearing from the HAZ. The RPH treated specimens showed the highest fatigue life, with an average of 947,632 cycles.