By using S35C/S35C friction welded tubular butt joints fabricated under five different friction welding processes, a series of tests were conducted to determine the effect of welding conditions on joint performance. At first, the hardness measurement revealed that, though the extent of heat affected zone was different from each other depending on the welding condition, no noticeable difference was observed in the maximum hardness value at the weld interface of joint. Then, the results of monotonic tensile tests indicated that the joint efficiencies of four joints were sufficiently high excepting only one joint fabricated under the lowest friction pressure condition. But the above-mentioned results did not necessarily reflect on the fatigue strength of the joints, and their fatigue strength behaviors could be subdivided into two groups as follows: Two types of joints showed lower fatigue strength than that of the base material and appeared to have no fatigue limit in the range of this experiment; and in contrast, the fatigue strength of the other three types of joints well exceeded that of the base material and showed a tendency to have a fatigue limit. Such fatigue strength behaviors seemed to be correlated closely to the magnitude of forging pressure in the friction welding process.
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.
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