Ijiri Masataka, Yoshimura Toshihiko
Sanyo-Onoda City University, 1-1-1 Daigaku-Dori, Sanyo-Onoda-shi, Yamaguchi, 756-0884, Japan.
Heliyon. 2018 Aug 20;4(8):e00747. doi: 10.1016/j.heliyon.2018.e00747. eCollection 2018 Aug.
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.
水喷射喷丸处理（WJP）被用作一种应力改善方法以及核电站反应堆内部结构中应力腐蚀开裂（SCC）的对策。然而，当将残余应力转化为压应力并施加到试样表面以作为防止应力腐蚀开裂的对策时，由于水喷射喷丸处理过程中施加到表面的压力增加，试样内部很容易形成空洞和裂纹。最近，多功能空化（MFC）作为水喷射喷丸处理的替代方法被开发出来，它是利用超声功率的水喷射喷丸处理。在经过多功能空化处理的低合金钢中，当将残余应力转化为压应力作为防止应力腐蚀开裂的对策时，试样内部不会形成空洞和裂纹。在本研究中，为了进一步改进当前的多功能空化技术，使用1200W超声功率对低合金钢（Cr-Mo钢）的表面改性进行了进一步研究。在使用1200W超声功率的多功能空化处理中，当处理时间为10分钟时，试样的耐腐蚀性、压缩残余应力和强度得到了提高；然而，在较长处理时间下会发生脱碳，导致这些性能变差。在高超声输出下发生的脱碳可能是由水温升高和试样表面受热引起的。对在高达1200W超声功率下处理30分钟的试样表面进行评估发现，只要功率不超过720W，试样表面就不会发生脱碳。
