Effect of post-weld tempering treatment on performance of DP600 spot welded joint

Abstract

In order to meet the increasing requirements of the automotive industry for material strength grades and performance, the application of advanced high-strength steel automotive panels, represented by dual-phase steel, in lightweight vehicle bodies is increasing. Resistance spot welding is the main joining process for vehicle body manufacturing. How to improve the tensile strength and fatigue performance of dual-phase steel resistance spot welding joints is of great significance to the welding structure of the body. In this paper, resistance spot welding of cold-rolled DP600 dual-phase steel in the welding current range of 9000 A to 12000 A was carried out. The effects of postweld tempering treatment on microstructure, microhardness, shearing, and fatigue of joints were compared. The results show that the microstructure transformation occurs in the weld nugget area after postweld tempering, and the tempered martensite appears to reduce the microhardness of the weld nugget area; the nugget diameters, tensile shear strength, and failure energy are all higher than those without tempering treatment; tempering improves the fatigue strength of the spot joints, and regardless of whether the welded joints are tempered, cracks are all generated near the heat-affected zone of the joint after the fatigue test.