Improvement of the fatigue strength of SUS304 austenite stainless steel using ultrasonic nanocrystal surface modification

An ultrasonic nanocrystal surfacemodification (UNSM) treatmentwas applied to improve the fatigue strength of typical SUS304 austenite stainless steel. A gradient nanocrystalline plastic deformation layer with increased hardness was created at the specimen surface after UNSM treatment. The average grain size at the top surface was refined to a few nanometers, and the whole thickness of the plastic deformation layer was ~30 to 200 μm, depending on the various treatment conditions. The hardened surface layer was assumed to be the main reason for the improvement in the fatigue strength. The increase in the fatigue strengthwas related to theUNSMapplied static loads. In this work, it was found that a static load of 90 N was optimum to obtain a large cycle fatigue strength limit. Plastic deformation (strain)-induced martensitic transformations were also observed in the surface layer. The mechanism for further grain refinement based on martensitic transformations is discussed.