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Offshore structures are widely used in the oil and gas industry. Since construction and maintenance costs in these structures are very expensive, therefore, it seems necessary to find cost-effective methods to repair and strengthen these structures, especially in the offshore industry. Ultrasonic peening is one of the mechanical stress relief methods used to increase fatigue strength and corrosion resistance in welded joints. In this paper, this method was implemented in both air and underwater atmosphere and the treatment effect on fatigue strength was investigated, also surface hardness and surface cracks in X52-5L welded steel was investigated. The effect of peening on this type of steel has been compared in both underwater and outdoor conditions. The results of the fatigue test showed that the fatigue life of underwater peened specimens was improved by approximately %25.2 and it changes form 42383 cycle up to 50083 cycle while in welded specimens with the same condition and peened in air condition, there is about %10.5 increase in fatigue life. Surface hardness also increases by %14 in underwater peened samples and by %12 in air-peened samples. SEM microscopy and observation of surface micro cracks also showed reduction of surface cracks after peening. Subsequently and loss of stress concentration was shown. It can be concluded that underwater peening increases the fatigue strength and hardness of X52-5L steel compared to air peening.
 

Keywords: Ultrasonic peening, X52-5L carbon steel, mechanical properties, microstructure., Fatigue Life

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