Factors of improving fatigue strength in resistance spot welded high-strength steel sheets with tempering treatment

Abstract

Fatigue strength of resistance spot welded high-strength steels does not improve with increasing base metal strength. Therefore, cold-working method and tempering treatment have been proposed as methods for improving fatigue strength. These methods control residual stress, hardness, and both of them to increase fatigue strength. However, it is difficult to say that the effects of hardness and residual stress on the fatigue strength of resistance spot welded joints have been clarified. In this study, to clarify the effects of hardness and residual stress on fatigue strength, these factors were controlled independently by using cold-working method and tempering treatment. As a result, the fatigue life was improved in the joints to which compressive residual stress was applied by cold-working. On the other hand, the fatigue life decreased with the joint where the press load increased and the hardness increased. From these results, it was clarified that the compressive residual stress is a factor for improving the fatigue strength and the increase in hardness is a factor for decreasing the fatigue strength in the resistance spot welded joint.