腐食ピットを有する摩擦圧接継手の疲労破壊特性

Abstract

Studies on fatigue behavior of friction welded butt joints composed of similar carbon steel in ordinary atmospheric environment have revealed that the fatigue strength of the joint is higher than that of the base metal, but the joint shows the larger scatter of fatigue lives. On the other hand, results of corrosion fatigue tests on the joint carried out in artificial seawater indicated that, (1) the corrosion fatigue strength of the joint was almost the same as that of the base metal in long life region, (2) scatter of the corrosion fatigue lives of the joint bacame smaller with the increase in fatigue life in comparison with the case of base matal specimen, and (3) above mentioned characteristics were governed by corrosion pits preferentially generated near the weld interface.The main aim of the present study is to clarify the cause of the fatigue strength reduction by corrosion pits. For this purpose the joint and base metal specimens were immersed in artificial seawater at stress free condition to generate corrosion pits, and rotating bending fatigue tests were carried out on the specimens with corrosion pits near weld interface in ordinary atmospheric environment. Reference data were also obtaind by using specimens without corrosion pits. Results of the study clearly indicated that the corrosion pits near the weld interface decreased the fatigue strength of the joint. Fatigue strength reduction due to the corrosion pits was well correlated with the hardness at fracture site. Furthermore, the fatigue strength reduction factor took considerablely high value for the specimen immersed in artificial seawater during long time, which suggests that the other detrimental factor such as grain boundary corrosion gives the effect to decrease the fatigue strength of the joint with corrosion pits. Additionally, effect of corrosion pits on the fatigue strength reduction was discussed by replacing tentatively with the effect of circumferential notch.