Spot Weldability of Dual-Phase Steel

Abstract

The spot welding characteristics of VAN-QN (80), a dual-phase HSLA (high strength, low alloy) steel, were examined with regard to the strength and ductility of spot welds under static, impact and cyclic loading and compared to a conventional V-N HSLA steel and SAE (Society of Automotive Engineers) 1008 low carbon steel. The maximum tensile shear strength of VAN-QN (80) spot welds was found to be proportional to the thickness and ultimate tensile strength of the base metal. VAN-QN (80) spot welds, therefore, were only slightly lower in strength than those in a conventional 80 ksi yield strength HSLA steel but up to twice the strength of welds in low carbon steel. Weld ductility was lower for VAN-QN (80) than for low carbon steel but greater than for conventional HSLA steel and increased with material thickness. The impact and fatigue properties of VAN-QN (80) welds were similar to those in conventional HSLA and low carbon steels. Weldability, as determined by the range of current over which acceptable welds could be produced, decreased with increasing thickness and was the same for VAN-QN (80) as for low carbon steel. The excellent weldability of VAN-QN (80) steel is attributed to a combination of the following properties: moderate hardenability, insensitivity of the dual-phase microstructure to softening by the heat inputs typical of spot welding, and a low yield strength to ultimate tensile strength ratio.