Improvement in fatigue strength of silicon manganese steel SUP7 by using a cavitating jet

It is possible to strengthen metallic materials by using a cavitating jet to introduce compressive residual stress in the material surface, since the impact of collapsing cavitation bubbles peen the surface in the same way as shot peening. In order to demonstrate the improvement in the fatigue strength of a material by using a cavitating jet, an experimental study was carried out. Silicon manganese steel JIS SUP7 was chosen as a test material, since JIS SUP7 is used as a spring material after shot peening. The specimens were exposed to the cavitating jet with upstream pressure p(1) = 20 MPa, downstream pressure p(2) = 0.28 MPa, the cavitation number sigma congruent to p(2)/p(1) = 0.014, the nozzle throat diameter d = 0.842 mm and the standoff distance s = 31 mm. The scanning speed v at which the compressive residual stress took the most significant value was 0.25 mm/s. The compressive residual stress was introduced in the entire surface peened by the cavitating jet under the above conditions. The fatigue strength of the specimen was investigated by a four point bending test. The minimum bending stress sigma(min) was fixed at 123 MPa and the amplitude of the load was varied. The fatigue strength of material peened by the cavitating jet is shown to be about 440 MPa, which is about 10% stronger than the strength without peening.