Experimental Research On The Effect Of Laser Peening On Stress Corrosion Cracking Of 304 Austenitic Stainless Steel

Stress corrosion cracking is the common failure form of austenitic stainless steel. Stress corrosion cracking usually happened in less than the yield limit of the common action of stress and corrosion medium, which often happen suddenly without macroscopic signs. It will cause serious damage to staff and property. Laser peening technology is a new type of surface strengthening technology. It has high energy, high efficiency, environmental protection and easy to control etc which has the incomparable advantages of conventional processing methods. In recent years, both domestic and foreign researchs about the surface laser peening treatment to improve the mechanical properties, fatigue performance and resistance to uniform corrosion performance of materials have achieved great results, but the effect of laser peening process parameters on performance of stress corrosion cracking of 304 stainless steels, yet lack of systematic research.304 stainless steels were treated by laser peening treatment with seven different energy densities and two different overlap rates in this article. The effect of laser peening process parameters on surface performance of 304 stainless steels was studied by the analysis of XRD, scanning electron microscopy(SEM) and micro-hardness. Through the stress corrosion test in 3.5%NaCl solution, the influence of laser peening parameters on 304 stainless steel stress corrosion sensitivity was studied, and the results were compared with the traditional mechanical peening treatment results. Main research results are summarized as follows:(1) After laser peening, the presence of small pits on surface of 304 stainless steels which mean the material produced microscale plastic deformation. The distortion energy accumulated inside was not enough to make the change of austenite, and induced no obvious martensite transformation.(2) Laser peening had refining effect on sample surface grain. Along with the laser energy density and overlap rate increased, samples of 304 stainless steel surface grain size decreased. After laser shot peening, the surface microhardness increased significantly. With the laser peening energy density or overlap rate increased, the surface of the specimen microhardness increased.(3) Through the laser peening, the stress corrosion sensitivity of 304 stainless steels decreased obviously in 3.5% NaCl solution. With the laser peening energy density or overlap rate increased, the stress corrosion sensitivity of specimen decreased. According to the fracture surface morphologies, after laser shot peened, specimens in 3.5%NaCl solution of the fracture surface was flat, no shear lips, showing the characteristics of brittle fracture.(4) The influence mechanism of Laser shot peening on stress corrosion sensitivity of 304 stainless steel could be attributed to reinforcement of mechanical properties and grain refinement. Austenitic stainless steel in NaCl solution stress corrosion cracking would be used "slip-dissolution-fracture" mechanism to explain. Laser peening treatment could enhance the mechanical properties of the material which effectively alleviate the slip step formation, and grain refinement reduces the crack propagation rate. So after laser shot peening, the stress corrosion sensitivity of sample decreased.(5) The influence mechanism of Laser shot peening and shot peening on stress corrosion sensitivity of 304 stainless steel existed a big difference:Shot peening could produce severe plastic deformation that induced martensite transformation, while laser peening treatment would not produce obvious martensitic transformation. Two shot peening treatment were both able to make the sample surface grain refinement, the grain refinement could reduce the stress corrosion sensitivity, while the martensite could improve the stress corrosion sensitivity. So as the laser energy density was increased, the stress corrosion sensitivity gradually decreased; But with the shot peening pressure increases, the stress corrosion sensitivity decreased first and then increased.