Sustainability of compressive residual stress on the processing time of water jet peening using ultrasonic power

Water jet peening (WJP) is used as a stress improvement method and a countermeasure against stress corrosion cracking (SCC) in the internal structures of reactors in nuclear power plants. However, when residual stress is converted to compressive stress and applied to the specimen surface as a countermeasure against SCC, voids and cracks can easily form inside the specimen because of the increase in the pressure applied to the surface during WJP processing. Recently, multifunction cavitation (MFC), which is WJP using ultrasonic power, has been developed as an alternative to WJP. In MFC-processed low-alloy steel, when the residual stress is converted to compressive stress as an SCC countermeasure, voids and cracks do not form inside the specimen. In this study, to further improve current MFC techniques, the surface modification of low-alloy steel (Cr–Mo steel) was further investigated using 1200 W ultrasonic power. In MFC using 1200 W ultrasonic power, the corrosion resistance, compressive residual stress, and strength of the specimens were improved when the processing time was 10 min; however, decarburization occurred at longer processing times, causing these characteristics to worsen. The decarburization that occurs at high ultrasonic outputs may be caused by an increase in the water temperature and of the heating of the specimen surface. The evaluation of the surfaces of specimens processed for 30 min at ultrasonic powers of up to 1200 W revealed that decarburization does not occur on the specimen surface as long as the power does not exceed 720 W.