Search

One of the traditional methods used to improve the fatigue properties of metallic materials is shot peening. More recently, cavitation peening, in which the surface is treated using cavitation impact, has been developed, and the improvements this makes to the fatigue life and the strength of metallic materials have been reported. In order to clarify the difference between these two methods, stainless steel SUS316 L samples were treated by shot peening and cavitation peening, and the fatigue properties of the samples were evaluated utilizing a displacement controlled plane bending fatigue test. The residual stress and hardness before and after the fatigue test were measured, and the surface roughness of each specimen was also measured. It was concluded that the fatigue life of shot peened specimens at bending stress σa > 450 MPa was longer than that of cavitation peened specimens; however, the fatigue strength of the cavitation peened specimens was slightly larger than that of the shot peened specimens. The compressive residual stress introduced by both peening methods decreased during the fatigue test. The reduction in the compressive residual stress in the shot peened specimens was greater than in the cavitation peened specimens, and after the fatigue test, the compressive residual stress in the shot peened specimens was greater than that in the cavitation peened specimens. It was found that the fatigue strength corresponded well with the yield stress estimated from the Vickers hardness corrected by the residual stress obtained after the fatigue test.

This paper's aims are to compare the effects of advanced mechanical surface treatments on the surface characteristics of AISI 304 austenitic stainless steel. The laser shock peening (LSP), combined with the water jet cavitation peening (WjCP), water jet shot peening (WjSP), and multi-pin ultrasonic impact peening (UIP), is applied to improve the surface quality and increase the hardness, hardening depth, and the compressive residual stress values in the near-surface layers. The laser shock processing is implemented using a submerged laser peening system with a wavelength of 1064 nm. The outcomes of the LSP treatment combined with other peening techniques in different sequences (applied prior or post it) are studied. The experimental results show as compared to the combined LSP + WjCP and LSP + WjSP techniques, the combined LSP + UIP technique results in a lower surface roughness (Ra ∼ 0.15 μm) and higher surface macrohardness (∼ 39.0 HRC5) supported by nanoscale grain structure with grain size of 15-100 nm, which is observed by XRD and TEM analysis. The surface macrohardness is respectively increased by about 48%, 68%, and 80% after the combined WjCP + LSP, WjSP + LSP, and UIP + LSP techniques in comparison with the original sample (22.1 HRC5). All combined peening techniques lead to an increase in the residual stresses values as compared to the single LSP process, providing the hardening depth of about 1 mm. [ABSTRACT FROM AUTHOR]

In an attempt to strengthen the surface of materials, the potential of using a cavitating jet to form compressive residual stress has been investigated. Introducing compressive residual stress to a material surface provides improvement of the fatigue strength and resistance to stress corrosion cracking. In general cavitation causes damage to hydraulic machinery. However, cavitation impact can be used to form compressive residual stress in the same way as shot peening. In the initial stage, when cavitation erosion progresses, only plastic deformation, without mass loss, takes place on the material surface. Thus, it is possible to form compressive residual stress without any damage by considering the intensity and exposure time of the cavitation attack. Cavitation is also induced by ultrasonic, high-speed water tunnel and high-speed submerged water jet, i.e., a cavitating jet. The great advantage of a cavitating jet is that the jet causes the cavitation wherever the cavitation impact is required. To obtain the optimum condition for the formation of compressive residual stress by using a cavitating jet, the residual stresses on stainless steel (JIS SUS304 and SUS316) and also copper (JIS CllO0) have been examined by changing the exposure time of the cavitating jet. The in-plane normal stresses were measured in three different directions on the surface plane using the X-ray diffraction method, allowing for the principal stresses to be calculated. Both of the principal stresses are found changing from tension to compression within a 10 s exposure to the cavitating jet. The compressive residual stress as a result of the cavitating jet was found to be saturated after a certain time, but it starts decreasing, and finally, it approaches zero asymptotically. It could be verified in the present study that it was possible to form compressive residual stress by using a cavitating jet, and the optimum processing time could also be realized. The great difference between the water jet in water and air has also been shown in this regard.