Investigation On Shot Peening And XRD Characterization Of Nickel Based Alloy Inconel 625

Due to the superior thermal,fine fatigue and creep properties,Inconel 625 has been widely used in aerospace,chemistry and nuclear power industry.Shot peening process was well known to improve micro hardness,stress corrosion and fatigue resistance of materials.During shot peening process,high compressive residual stress(CRS)and cold work was introduced in the deformation layers of the material.As a result,smaller domain size and high micro strain was generated.Higher CRS and finer microstructure can improve the mechanical properties of material.The Inconel 625 components are normally working at very high temperatures and high applied loadings,such as turbine engines,nuclear reactor and pumps,etc.,CRS and microstructure could be relaxed at elevated temperatures and high applied loadings.Fatigue strength and yield strength also could be huge decreased.In this paper,Inconel 625 was variously shot peened,then CRS distributions in the deformation layers were studied.The relaxation behaviors were also investigated after various isothermal treatments and applied loadings.Microstructures includes domain size and micro strain were characterized by X-Ray diffraction line analysis method.The recovery and recrystallization behavior were studied,and the mechanism of shot peened Inconel 625 was also discussed.CRS distributions of shot peened Inconel 625 show that CRS,the maximum compressive residual stress(MCRS)and the depth of the compressive residual stress(DCRS)in the deformation layers can be significantly increased by increasing intensity of traditional shot peening.CRS and MCRS of traditional shot peened Inconel 625 is-751 MPa and-1086 MPa with peening intensity 0.65mmA,steel balls.Dual shot peening optimized the CRS distribution.CRS and MCRS of traditional shot peened Inconel 625 is-878 MPa and-1114 MPa.Pre-stress shot peening can further improve CRS with a lower intensity of traditional shot peening.The accuracy of the X-Ray diffraction residual stress analysis method was verified by hole drilling method.Thermal relaxation behavior of residual stress shows that the relaxation rate is related to both temperature and annealing time.The higher temperature resulted to the faster relaxation.The relaxation of CRS in the surface layer was mainly occurred in the first 15 minutes.CRS was decreased from-837 MPa to-487 MPa,-332 MPa and-178 MPa at 500?,600? and 700?,respectively.However,there was still high CRS in the subsurface layers after 60 minutes isotheraml treatments.The activation parameter m and the activationenthalpy AH of dual shot peened Inconel 625 was calculated by Zener-wert-Avrami equation,which was 0.4934 and 1.59eV.The residual stress relaxation behaviors of Inconel 625 under applied loadings showed that the relaxation rate was related to applied loadings and cycle numbers.Higher applied loadings resulted to lower CRS.CRS relaxation was mainly occurred at the beginning of the cycle.Under the cyclic loads of 150,250 and 300 MPa,the CRS decreased more drastically after first 5 cycles,and after 30 cycles the corresponding CRS finally decreased by 20%,45%and 75%,respectively.Voigt linear analysis method and Rietveld method were used to calculate the microstructure in the deformation layers includes domain size,micro strain and dislocation density.The result showed that domain size was significantly refined after various shot peening treatments.Domain size on the top surface was about 50 nm.In the depth of 0?150?m,which were less than 100 nm.Domain size was changed slowest in the depth after dual shot peening treatment.Micro strain has been continuously reduced from the surface layer,and its influence layer depth is about 350 ?m.Shot peening intensity is the main factor affecting the domain size and the micro strain.Pre-stress shot peening further refined domain size and improved the micro strain with the same intensity of traditional shot peening.The higher applied prestress,the more obvious effect of domain size refinement and micro strain generation in the deformation layers.Microstructure changes in the deformation layers also showed that the recovery and recrystallization was occoured during the isothermal treatments.With the temperature and annealing time increasement,domain size was increased.However,micro strain and dislocation density was decreased.After various isothermal treatments,domain size was less than 100 nm in the depth range of 0?200 ?m,which was not obviously changed.There was also high micro strain and dislocation density in the deformation layers.The recrystallization activation energy and the relaxation activation energy of Inconel 625 was calculated to be 268 kJ/mol and 198 kJ/mol,respectively.Microhardness and yield strength were effectively improved after various shot peening treatments.The surface microhardness was increased from 198 HV to 580 HV after dual shot peening.After different isothermal treatments,micro hardness of dual shot peened Inconel 625 were slightly decreased from 580 HV to 550 HV,514 HV and 457 HV,respectively.The yield strength was 985 MPa by utilizing in-situ XRD method,which was twice improved after shot peening.The analysis of shot peening mechanism shows that higher compressive residual stresses,domain size refinement,increase of micro strain and dislocation density in deformation layer are important reasons for the increase of microhardness and yield strength.