Study On Surface Integrity And Electrochemical Properties Of Austenitic Stainless Steel By Ultrasonic Impact Treatment

The typical failure forms of key equipment in petroleum and chemical industry,such as fatigue and corrosion,often occur on the material surface,and these forms of failure are very sensitive to the surface microstructure and performance.Improving the surface quality of the material can effectively improve its overall mechanical properties and prolong the service life of the equipment.As a kind of surface strengthening treatment,ultrasonic impact treatment can produce severe plastic deformation on material surface and introduce gradient nanostructures to obtain ideal surface state.In this paper,the effect of different impact parameters on the surface integrity and corrosion resistance of specimens is studied by experimental research and numerical analysis,and the intrinsic mechanism of the improvement of the surface integrity of S30408 stainless steel to enhance the corrosion resistance of the material is analyzed.The main contents of this paper are as follows:(1)The effects of different ultrasonic impact process parameters on the surface integrity(tissue structure,residual stress,hardness,etc.)of the specimens are investigated by metallographic experiments,SEM experiments,XRD,and hardness tests.The results show that the ultrasonic impact treatment forms a gradient nanostructure in the surface area of the specimen from top to bottom as nanolayer zone,severe plastic deformation layer zone and matrix zone,and the thickness of the deformation layer shows a linear growth trend with the increase of impact time and impact speed.The grain size of the surface layer of the specimen is refined with the increase of impact time and impact velocity,and the maximum surface grain size after impact treatment is 43.58 nm and the minimum is 14.82 nm.The ultrasonic impact treatment induces the transformation from austenite to martensite,with the percentage of martensitic phase ranging from 18% to 40% within the impact parameters.After the impact treatment,the surface hardness of the material increases significantly due to dislocation buildup,grain refinement and martensitic phase transformation,with a maximum surface hardness of82.41 HRB and a gradient distribution of hardness from top to bottom.The main mechanism of grain refinement during ultrasonic impact is deformation twinning,while dislocation slip and martensitic phase transformation also play an important role in the grain refinement process.(2)Numerical analysis of the ultrasonic impact process of S30408 stainless steel is carried out using the finite element method to obtain information on the residual stress field and surface morphology of the material under different impact parameters.The martensitic phase transformation mechanism during the impact process is investigated by a modified OlsonCohen martensitic phase transformation model.The results show that the surface homogeneity,residual stress field,surface crater size and equivalent plastic strain of the specimen increase with increasing impact velocity.The residual tensile stress area used to maintain the equilibrium state gradually shifts downward after multiple impacts,and the residual compressive stress field in the surface area is more uniformly distributed,and the residual compressive stress tends to saturate along the depth direction after multiple impacts,and the surface plastic deformation continues to increase.The mapping relationship between impact parameters and martensitic phase transition content is established by a modified Olson-Cohen martensitic phase transition kinetic model.(3)The influence of different impact parameters on the corrosion resistance of S30408 stainless steel is studied by electrochemical corrosion experiments,and it is found that from the surface of the specimen to a certain depth due to the weakening of impact strength,grain refinement decreases,the formation of passivation film surface defects increase,corrosion resistance gradually weaken.The effect of ultrasonic impact treatment on the corrosion performance of stainless steel is the result of the combined effect of grain size,residual stress,martensite content,etc.A mapping relationship between grain size,residual compressive stress,martensite content and corrosion current density is established.Within the studied impact parameters,a more uniform,thicker and more stable passivation film with the best corrosion resistance is formed on the surface of the specimen at an impact time of 180 s and an impact speed of 5 m/s.