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Summary
Studies on the fatigue behaviour of friction welded butt joints between similar carbon steels in ordinary atmospheric environments have revealed that the fatigue strength of the joint is higher than that of the base metal, but the joint shows the larger scatter in fatigue life. On the other hand, the 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 the long life region; (2) the scatter in corrosion fatigue life in the joint became smaller with an increase in fatigue life than that of the base metal specimen; and (3) the above mentioned characteristics were governed by corrosion pits preferentially generated near the weld interface.
The main aim of this 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 in a stress free condition to generate corrosion pits, and rotating bending fatigue tests were carried out on the specimens with corrosion pits near the weld interface in an ordinary atmospheric environment. Reference data were also obtained 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 the fracture site. Furthermore, the fatigue strength reduction factor had a considerably higher value for the specimen immersed in artificial seawater for a long time, which suggests that the other detrimental factors such as grain boundary corrosion have the effect of decreasing the fatigue strength of the joint with corrosion pits. Additionally, the effect of corrosion pits on the fatigue strength reduction was examined with the added effect of a double circumferential notch.