Investigation On Shot-peening Strengthening And Microstructure Of Duplex Stainless Steel S32205 Utilizing X-ray Diffraction

Duplex stainless steel(DSS) S32205 is superior to single phase austenitic or ferritic grades because of its better corrosion resistance and higher strength. For this reason, DSS is considered as an excellent choice for various industrial applications. Shot peening can considerably improve the fatigue strength and the fatigue life of cyclically loaded metallic components by introducing fine domain, high density dislocations and compressive residual stress in the near surface region. The residual stresses and microstructures induced by shot peening were investigated, and the relaxation behaviors of residual stresses as well as the thermal recovery and dynamic recrystallization of microstructures were studied in this work. Furthermore, the surface mechanical properties and mechanism of peened DSS were also investigated.After dual shot peening and stress shot peening on the DSS S32205,the results revealed that the compressive residual stress of these treatments were higher than those of conventional shot peening. For example, the compressive residual stress of ferrite and austenite was-822 MPa and-890 MPa, respectively, under 0.30 mmA shot peening intensity, however, the values increased to-835 MPa in ferrite and-967 MPa in austenite as the shot peening intensities were 0.30 mmA + 0.15 mmA. Besides, the depth of compressive residual stress distribution of ferrite was about 300 μm, and that of austenite was about 200 μm. It was also observed that the surface compressive residual stress both in ferrite(-1010 MPa) and austenite(-1189 MPa) under the stress of 450 MPa were larger than those of ferrite(-719 Mpa) and austenite(-849 Mpa) under loading with 0 MPa. With the increasing of depth from the top surface, compressive residual stress in austenite increased to the peak value below the surface and then decreased after shot peening, while, it kept decreasing in ferrite. The effect of stress peening was more obvious compared to the conventional shot peening.The residual stress relaxation behaviors of DSS S32205 induced by shot peening both under applied loading and high temperature were investigated. The results showed that the higher the applied loading or temperature was, the more relaxations were. At high temperature, the thermal relaxation process of residual stresses was analyzed by applying Zener-Wert-Avrami function. The activation energy of austenite(116 KJ/mol) was lower than that of ferrite(178 KJ/mol), which demonstrated the stability of residual stress of austenite was lower than that of ferrite. While under cyclic loading, a higher applied tensile stress resulted in more relaxation of residual stress. The linear relationship between residual stress and the logarithm of cycle number demonstrated that the relaxation rate in austenite was higher than that in ferrite.With the method of X-ray diffraction line profile analysis, the surface layer characteristics of shot-peened DSS S32205 were investigated. The variations of domain size, microstrain, dislocation density and compound fault probability both in ferrite and austenite demonstrated that shot peening process could change microstructure in the surface layer. Under the same shot peening intensity, the lognormal distribution of domain size obtained from the Rietveld’s analysis indicated that the microstructure of austenite was easier to be refined. Weight fraction of austenite in DSS decreased hardly as austenite was strongly influenced by the increased content of alloying elements. For stress peening, with the increasing of applied stress, the microstructure refinement became more obvious.The recovery and recrystallization behaviors during isothermal annealing of shot-peened DSS were obtained by in-situ x-ray diffraction analysis and the isotropy model in Rietveld fitting method. The domain growth activation energy of austenite was 242 KJ/mol, and the microstrain relaxation activation energy of austenite was 203 KJ/mol, both of which were higher than those of ferrite(216 KJ/mol and 189 KJ/mol, respectively). It was concluded that the microstructure thermostability was influenced by the annealing temperature, annealing time, diffusion and dislocation density induced by shot peening treatment, and the austenite showed higher microstructure thermostability than ferrite.X-ray stress analysis for determining surface yield strength was used to investigate the influence of shot peening on surface mechanical properties of DSS S32205. The higher proof stress was obtained both in ferrite and austenite after shot peening. The increment gradient of yield strength in the surface of austenite(131%) was larger than that of ferrite(122%). The results showed that the compressive residual stress, refined domain size, and high dislocation density in the surface layer after shot peening lead to the improved yield strength. The strengthening mechanism of shot peening on DSS S32205 can be separated to stress strengthening and microstructure strengthening.