Abstract: Gradient nanostructure was constructed on the surface of S30408 austenitic stainless steel by using ultrasonic impact treatment (UIT), and the effect of different ultrasonic impact time on surface nanostructure and corrosion resistance of the steel was studied by means of XRD, hardness test, optical microscope, scanning electron microscope and electrochemical test. The results show that after ultrasonic impact treatment, a certain depth of hardened layer is produced on the surface of the material, and the residual compressive stress is introduced. Moreover, both hardness and residual compressive stress increase with the increase of ultrasonic impact time, and reach the maximum 740.12 MPa and 82.22 HRB respectively when the ultrasonic impact time is 300 s. Gradient microstructure is observed on the surface of the steel after ultrasonic impact treatment, which can be divided into nanocrstalline layer, severe plastic deformation layer and matrix. The ultrasonic impact process refines the surface grains and induces martensite transformation, as the grain size is the smallest and the martensite transformation is the largest when the ultrasonic impact time is 300 s, which are 14.82 nm and 39.80%, respectively. The effect of ultrasonic impact treatment on corrosion resistance of the S30408 stainless steel is the result of the joint action of grain size, residual stress, martensitic transformation content, surface defects and so on. When the ultrasonic impact time is 180 s, the corrosion resistance of the specimen is the best, and the self-corrosion current density is 1.22 μA/cm².

Key words: ultrasonic impact treatment, gradient nanostructure, martensite, electrochemistry, corrosion