Residual Stress Development due to High-Frequency Post Weld Impact Treatments for High-Strength Steels

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Allseas Engineering bv is an engineering center of the Allseas group, a major player in the offshore pipe lay market and recently expanding the activities to the offshore heavy lift sector. Because of the necessity to design and build lifting structures and equipment of a tremendously high capacity, the demand for high-strength steels usage has been arisen. These steels are to be welded and used in a harsh offshore environment. Localized heating and melting of a workpiece during welding lead to the build-up of residual stresses. When distortion is prevented due to constraints in structures or due to clamping, stress levels will be high and may exceed the yield strength. Post weld treatments are often carried out to mitigate or re-distribute the residual stresses. Ultrasonic impact techniques make use of the combined effects of the high frequency impacts and ultrasonic oscillations in the treated material. Ultrasonic impact treatment (UIT) has been applied successfully to increase the fatigue life of parts and welded elements, to eliminate distortion caused by welding and other technological processes, to relieve residual stresses, and to increase the hardness of materials. Although reported results on fatigue life are very promising, the detailed changes induced in the treated material and the mechanisms by which such changes occur are still poorly understood. In this research, the main objectives are to understand the stress build-up during multi-pass welding, and the mechanisms involved in post weld ultrasonic impact treatments, how these affect the residual stress state in the welded area.