Abstract
Fatigue strength of spot welded hot rolled high-strength sheet steel joints in long-life testing does not vary with tensile strength of the sheets. By extensive experiments using solid solution hardened steel, precipitation hardened steel and dual phase steel, two effective methods for improving the fatigue strength have been found.
One method is to generate moderate expulsion by welding with a current just above the upper lobe curve; endurance limits at 107 cycles increase by about twice as compared with those of the normal welding method. In this case, the decrease in tensile shear strength is less than 15%. This improvement in fatigue strength is due to the increase in effective area of corona bond, and due to the increase in residual compressive stress of nugget.
The other is the controlled tempering treatmet during the welding schedule; endurance limits at 107 cycles increase by about twice as compared with those of the normal welding method. This improvement is due to the residual compressive stress at the weld generated by the tempering treatment.
