Enhancing weld strength in high-strength steels: the role of regional preheating in RSW

Abstract

In this study, automotive in sector use STRENX 700 CR and DP 800 steels were joined by resistance spot welding using medium frequency direct current. Some parts were joined under atmospheric conditions, while others were joined by applying regional preheating to their heat-affected zones before welding. The design, manufacturing, and adaptation of the preheating system to the welding machine were carried out as part of this study, considering it as a new system. After the joining process, nondestructive and destructive tests were performed on resistance spot welding connections between STRENX 700 CR and DP 800 steels. Hardness tests revealed that the preheating applied to the heat-affected zones before welding reduced the hardness of the weld metal by approximately 8 %, while the microstructure analysis showed that the regional preheating increased both the weld metal and the heat-affected zones width. The tensile-shear strength increased by about 7 %, and the cross-tension test strength increased by about 5 % with the preheating applied to the heat-affected zones before welding. In both the tensile-shear and cross-tension tests, all failures occurred in the form of button shearing from the heat-affected zones. The regional preheating treated specimens showed the highest fatigue life, with an average of 947,632 cycles.

Formation of Oxides and Sulfides during the Welding Process of S700MC Steel by Using New Electrodes Wires

“…After the bend test, the minor cracks from the root side in the Y1 sample were observed. The result is consistent with the literature data, emphasizing the role of preheating in the bonding process of high-strength steels and its impact on the quality of the obtained joints [ 25 , 37 ]. In the remaining tested samples, no cracks were found in both the face and root bending tests.…”