Investigate On Shot Peenig Treatment On SAF2507 Duplex Stainless Steel And Its Characterization

SAF2507 duplex stainless steels（DSS）are extensively applied in the chemical,petrochemical and nuclear industries,due to their excellent corrosion resistance and improved mechanical strength.As a superior surface plastic deformation strengthening method,shot peening（SP）can significantly improve the surface hardness,fatigue life and stress corrosion cracking（SCC）resistant of metallic components by improveing surface microstructure and inducing compressive residual stress（CRS）.In the present study,the SAF2507 DSS were shot peened to improve their surface properties,the resdiaul stress field,microstructure and strength properties of the shot peened material were studied systemiclly.High CRS was introduced into the surface layer after conventional SP.CRS increased at first,then decreased with the increase of depth and finally transformed to tensile stress inside the matrix.For the sample peened at 0.60 mmA intensity,the maximum compressive stresses in austenite and ferrite were not obtained at the impacted surface,but at the subsurface,which were-932 and-790 MPa,respectively.Larger magnitude of compressive stress and deeper compressive region had been developed with the increase of SP intensity.Under the same SP parameters,the CRS in austenie was much larger that in ferrite at the same depth,but the depth of compressive region for ferrite was slightly higher than that for austenite.Multi-step SP could further improve the CRS in near surface layer,but had slight influence on the the depth of compressive region.After peening with0.50+0.20+0.10 mmA intensity,surface CRS of-1070 and-910 MPa were obtained for austenite and ferrite.Stress SP generated a thicker surface region with higher compressive stress than conventional SP in the longitudinal direction,but no notable increases in the compressive stress are observed in transversal direction after stress SP.A 3D finite element model with numerous randomly distributed shots was developed to simulate the SP process.The simulated results indicated that the maximum Mises plastic strain cuased by SP were located at the subsurface,thus the maximum compressive stresses were not obtained at the impacted surface,but at the subsurface.As the corverage increase,the surface CRS and maximum CRS increased,the location of the maximum CRS shifted gradually to the surface.With the increasing of shot velocity,maximum CRS and its corresponding depth as well as the depth of compressive region improved,but the surface CRS slightly increased.The residual stress relaxation behavior of shot peened SAF2507 DSS under high temperature was investigated.The results indicated that most of the stress relaxation took place within the initial 30 min and the relaxation behavior could be described by the Zener-Wert-Avrami function.The higher temperature resuled in faster relaxation rate,austenite exhibited higher stress relaxation rate than ferrite.The derived activation energies of the stress relaxation（ΔH）were about 67 and 62kJ/mol for austentic and ferritic phase,respectively.The residual stress behavior of shot peened SAF2507 DSS under cyclic loadings showed that most of the stress relaxation took place within the first cycle,then residual stress became stable gradually with the increaseing of cycle number.The larger the applied loading,the higher the relaxation rate and the lower the steady stress value.After 30 cycles under the applied loading of 300,400 and 500MPa,the surface residual stress of ferrite were-498,-281 and-16 MPa,which were 31.8%,61.5%and 97.8%reduction,and the surface residual stress of austenite were-728,-555and-225MPa,which were 29.3%,46.1%and 78.2%reduction.The microstructural evolutions of the SAF2507 DSS after SP were investigated by XRD line profile analysis.The calculations according to Voigt method indicated that SP significantly refined the grains,improved dislocation density and microstrain.The minimum domain size and maximum dislocation density were both obtained at the impact surface.For the sample peened at 0.50mmA intensity,the surface domine size of austenite and ferrite was 18 and 25 nm,the discation density was 2.45×10¹⁵ and 1.32×10¹⁵/m²,respectively.The plastic deformation was heterogeneous between the two phases during SP because of the different the lattice structure and the stacking fault energy（SFE）.Under the same SP conditions,austenite suffered more deformation than ferrite,representing by more refined grains,higher microstrain and dislocation density.Rietveld method was also employed to characterize the microstructure changes of the SAF2507DSS after SP.The TEM observations also confirm that the plastic deformation in this two-phase material was inhomogeneous during SP.For the ferrite with bcc structure and high SFE,the plastic deformation mainly depended on the development of dislocation.While for the austenite with fcc structure and low SFE,it had been proved that the grain refinement was also tightly dependent on the mechanical twin and strain-induced martensite transformation.After isothermal annealing under high temperature,a heavy reduction in dislocations and dislocation cells accompanying with the growth of subgrains was observed in the recrystallized microstructure.The recrystallized grains formed in austenite were much finer than that in ferrite.SP could accelerate the precipitation process during annealing.A mass ofσphase precipitated in the shot peened layer,which was considered as a consequence of the higher driving force of precipitating intermetallic compounds and the higher diffusion rate of the elements by plastic deformation.The activation energy of grain boundary migration for austenite and ferrite were 257 and 220 kJ/mol,the activation energy of microstrain were 82.6and 76.9 kJ/mol,respectively.The in-situ XRD method was ultilized to investigate the surface yield strength of the shot peened SAF2507 DSS.The results showed that the surface yield strength of austenite and ferrite were 780 and 1100 MPa after SP,which were59%and 104%,respectively,greater than that of the unpeened sampel.The microhardness results indicated that shot peening improved the surface mechanical strength of the austenitic and ferritic steel.The hardness of austenite was found significantly higher than that of ferrite in the near surface region.The enhancement of the surface strength were mainly ascribed to the CRS and optimized microstructure including grain refinement,dense dislocation configuration and the formation of martensite and mechanical twins.