The introduction of tensile residual stress has led to the induction of damage such as fatigue, corrosion fatigue, and stress corrosion cracking (SCC) in stainless steel in association with the influence of environments, components, surface defects, and corrosive factors during its use. Compressive residual stress can be achieved through various techniques. Among several methods, laser peening can be more attractive as it creates regularity on the surface with a high-quality surface finish. However, there is very little research on heavily peened surface and cross-section of stainless steel with very deep compressive residual stress. This work focused on welding and laser peening and the influence of Al coating on the microstructural changes in 304L stainless steel. The specimen obtained by laser peening had a very deep compressive residual stress of over 1 mm and was evaluated based on microstructural and hardness analysis. Therefore, a model for microstructural change by laser peening on welded 304L stainless steel was proposed.
Dry canisters used in nuclear power plants can be subject to localized corrosion, including stress corrosion cracking. External and residual tensile stress can facilitate the occurrence of stress corrosion cracking. Residual stress can arise from welding and plastic deformation. Mitigation methods of residual stress depend upon the energy used and include laser peening, ultrasonic peening, ultrasonic nanocrystal surface modification, shot peening, or water jet peening. Among these, laser peening technology irradiates a continuous laser beam on the surface of metals and alloys at short intervals to add compressive residual stress as a shock wave is caused. This research studied the effect of laser peening with/without a thin aluminum layer on the corrosion properties of welded 304L stainless steel. The intergranular corrosion rate of the laser-peened specimen was a little faster than the rate of the non-peened specimen. However, laser peening enhanced the polarization properties of the cross-section of 304L stainless steel, while the properties of the surface were reduced by laser peening. This behavior was discussed on the basis of the microstructure and residual stress.
Storage canisters used in nuclear power plants operating in seaside areas—where the salt content in the atmosphere is high—may be susceptible to chloride-induced stress corrosion cracking (CISCC). Chloride-induced stress corrosion cracking is one of the ways in which dry storage canisters made of stainless steel can degrade. Stress corrosion cracking depends on the microstructure and residual stress, and it is therefore very important to improve the surface properties of materials. Laser shock peening both greatly deforms the material surface and refines grains, and it generates compressive residual stress in the deep part from the surface of the material. This study focused on the effect of laser shock peening on the stress corrosion cracking of 304L stainless steel. The laser shock peening was found to induce compressive residual stress from the surface to a 1 mm depth, and the SCC properties were evaluated by a U-bend test. The results showed that the SCC resistance of laser-peened 304L stainless steel in a chloride environment was enhanced, and that it was closely related to grain size, the pitting potential of the cross section, and residual stress.
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