Study On Surface Modification Of 304 Stainless Steel Induced By Laser Shock Peening Without Coating

304 austenitic stainless steel has excellent mechanical properties and resistance to uniform corrosion,and is widely used in chemical,pharmaceutical and nuclear power industries.However,it is susceptible to localized corrosion such as pitting corrosion and stress corrosion cracking(SCC),especially stress corrosion cracking(in chloride ion-containing solutions,which can cause sudden cracking and explosion accidents in stainless steel equipment.Harm to people and life and property.Therefore,studying the effect of laser shock treatment on the surface properties of 304 stainless steel and improving the surface properties of the material is of great significance and value for preventing the occurrence of stress corrosion cracking accidents and improving the safe use performance of stainless steel equipment.In this paper,the effects of laser peening without coating treatment on surface properties of 304 austenitic stainless steel were investigated.304 stainless steel samples were processed by laser power density:3.185 GW/cm~2,6.37 GW/cm~2,9.555GW/cm~2,12.74 GW/cm~2,and 15.925 GW/cm~2.The surface morphology,microhardness,grain size,residual stress and metallographic structure of the samples after laser shock treatment were studied and analyzed,and the effects of different laser shock energy densities on the surface properties of the samples were compared.Slow strain rate tests(SSRT)in 3.5 wt.%NaCl solution with the tensile strain rate of1.0×10~-66 s~-11 were carried out in order to investigate the effects of LPwC treatment on the stress corrosion cracking susceptibility and corrosion resistance of 304 austenitic stainless steel.The main work and results of this paper are summarized as follows:(1)The residual stress of the 304 stainless steel surface was measured by X-ray method.The test results showed that the surface of the 304 stainless steel sample after laser shock treatment was severely plastically deformed,resulting in residual compressive stress.As the laser power density increases,the residual compressive stress also increases.When the laser power density is increased to 15.925 GW/cm~2,the surface residual compressive stress reaches-363 MPa.(2)The surface of the sample after laser shock treatment has grain refinement and work hardening,and the macroscopic performance is the improvement of the strength and hardness of the surface material.As the laser power density increases,the microhardness of the surface layer of the sample also increases.The surface hardening of the surface layer mainly occurs within a range of 300?m from the surface of the sample.As the distance from the surface increases,the Vickers hardness value of the surface layer gradually decreases.(3)The X-ray phase analysis of the sample showed that the martensite transformation occurred in the sample after laser shock treatment under the condition of no coating,that is,the martensitic transformation induced by the laser shock process.As the laser power density increases,the martensite content also increases.When the laser power density was 15.925 GW/cm~2,the martensite content in the sample was 3.6%,which was three times that of the untreated sample.(4)The stress corrosion sensitivity index of 304 stainless steel under different laser power densities was analyzed by slow strain rate tensile test.The results show that the stress corrosion sensitivity index of 304 stainless steel specimens is lower than that of unsprayed specimens after uncoated laser shock treatment,and the laser power density increases from 3.185 GW/cm~2 to 12.74 GW/cm~2.The stress corrosion sensitivity index is gradually reduced.When the laser power density is 15.925GW/cm~2,the stress sensitivity index is much larger than that of other power density samples,and the values are similar to those without laser shock treatment.