Research On High-cycle Fatigue Failure And Prolonging Life Technology Of Small Bore Piping Socket Weld

The small bore piping socket weld is mainly used to the connection of the leaderpipe and the branch pipe in the nuclear power plants. In the socket welding pipe thecorrosive liquid flow and induce easily cavitations which result in the harmonicvibration of pipe. In the conditions of vibration and corrosion, the socket welds jointsfrequently present high-cycle fatigue failure; especially in the root of socket weld.Due to their factors, the304L stainless steel which has a better performance incorrosion resistance was selected as examples of vibration fatigue testing. Themechanism and factors of high-cycle fatigue failure of socket weld are studied deeply,and propose the prolonging life technology. The research work and results are listed asfollow:The specimens of socket weld were tested in the vibration table which canimitate the actual vibration environment. However, the vibration fatigue testing usingshaker table exist two problems. One is that the shaker table cannot impose stress likethe hydraulic servo testing machine; the other is how to monitor whether thespecimens run normally. The formula of stress calculation is induced by the theory ofmechanics and machine vibration, the fatigue testing was conducted by the changingof parameters which can influence stress.The socket weld joint of initial design was conducted to investigate thehigh-cycle fatigue property, and the test results show that failures tend to originate toeat higher stress while for the case of lower stress failures tend to occur at the root. Theresearch results present that the radial gap, the size of fillet weld and the flank angleof weld toe can affect the fatigue property and the higher penetration depth canimprove the fatigue property.The groove was used to increase the penetration depth, and the specimens with3mm and5mm grooves were conducted to investigate the effect on the fatigueproperty. The research results show that the groove can increase the damping, whichcan improve the property of vibration resistance; the stress amplitude of rootdecreases a little with increasing penetration depth under the vibration fatigue load;however, the groove can decrease obviously the plastic strain due to the stressamplitude of root exceeding the yield stress, it means that the plastic damage of rootdecrease gradually with increasing the penetration depth which can improve theresistance performance of fatigue crack initiation. Hence, the groove can improve markedly the property of high-cycle fatigue, and prolong obviously the fatigue life.In addition, the groove can improve the fatigue life of root on the conditions oflower stress while can decrease the fatigue property of toe at higher stress owing toincreasing the residual stress of toe. The socket weld subjects usually to the conditionsof lower stress and high-cycle fatigue, so the specimen with groove can improveobviously the high-cycle fatigue life of weld joint.