Investigation Of Shot Peening Treatment On S30432Austenitic Stainless Steel And Its Characterization

S30432austenitic stainless steel, which was developed on the TP304H stainless steel bythe alloying strengthen and dispersion strengthen theories, is widely used in ultra supercritical power plants due to its superior properties. Shot peening is an important surfacetreatment approach to improve the surface properties of metallic materials. In order tooptimize the properties of S30432austenitic stainless steel, in this dissertation the steelwas treated by the optimized shot peening process. The residual stress distribution andmicrostructure change were investigated, the residual stress relaxation and thermalstability of microstructure were also analyzed, furthermore, the shot peening strengthenmechanism was discussed through the characterization of surface mechanical properties ofshot peened surface of S30432austenitic stainless steel.The results of shot peening experiments on S30432austenitic stainless steel revealedthat the residual stress distribution in the deformed layer was isotropic. The compressiveresidual stress on the surface and the maximum compressive residual stress aftermulti-step shot peening method was increased by adding the step of shot peening. Whenthe sample was treated by triple shot peening process, the compressive residual stress was-778MPa, the maximum value was-865MPa at the depth of10μm. besides, themulti-step shot peening method can not only increase the residual stress value but alsomake the stress distribution more uniform on the surface. The residual stress values wereincreased as increasing the pre-stress in stress peening. A maximum residual stress of-993MPa was observed with the optimized stress peening parameters.The residual stress relaxation of S30432austenitic stainless steel after shot peeningunder the static loading showed that the larger the applied loading was, the more obviousthe relaxation was, and when the applied loading reached the yield strength of the steel, the rate of relaxation was the largest, and it was larger in the direction of applied loadingthan that in the direction vertical to the applied loading. Under cyclic loading conditions,with the cycling number increased, it began to relax and the fast rate of compressiveresidual stress relaxation took place in the initial stage, gradually, the residual stressbecome stable. The larger of the applied loading, the lower of the stable stress value.When under the applied cyclic loading of200,250and300MPa, the drastic relaxationtook place during the first5times,2times,2times cycling. Then the residual stress wererelaxed about30%,40%and65%, respectively.The results of thermal relaxation of residual stress showed that the relaxation took placein the initial period of annealing. the higher temperature resulted faster relaxation rate. Theeffect of annealing time and temperature on the relaxation was described byZener-Wert-Avrami function, and the activation energy of the residual stress relaxationwas159kJ/mol.X-ray diffraction line profile analysis methods were used for the microstructure analysis,and the results were discussed. The Rietveld method was introduced to analyze of themicrostructure after shot peening. the PoPa anisotropic model can not only characterizethe change of microstructure in different crystalline direction but also the lognormaldistribution of domain size. the results of S30432austenitic stainless steel after three stepshot peening showed the domain size on the surface was trend to isotropic. In <111>direction, it was16.3nm which was the largest. The average domain size was increased asthe depth increasing. For stress peening, with the applied stress increased, themicrostructure refinement was more obvious.In the isothermal annealing process, the domain size increased, while micro-strain anddislocation density reduced. The activation energy of grain boundary migration of S30432austenitic stainless steel is259kJ/mol, the activation energy of microstrain is171kJ/mol.The surface yield strength of S30432austenitic stainless steel without shotpeening,which was characterized by X-ray stress analyzing method, is268MPa thesame as the tensile testing experiment. The yield strength σ0.2of S30432austenitic stainless steel after traditional and dual shot peening were about830MPa,940MPa,respectively. Shot peening treatment can greatly improve the yield strength of S30432austenitic stainless steel. The microstructure refinement was the main reason for theimproved surface yield strength.The strength mechanism of shot peening can ascribe to compressive residual stressstrength and microstructure strength. The compressive residual stress can weaken thetensile stress of the surface. The local fatigue limit model was applied to analyze theinfluence of compressive residual stress to fatigue limit of S30432shot peening sample.The result revealed that the compressive residual stress can enhance the fatigue limit. themicrostructure strength mechanism was that the refined domain size and highermicro-strain and dislocation density after shot peening improved the surface yieldstrength.