Effect Of Shot Peening On The Oxidation Properties Of GH3535 Superalloy At Elevated Temperature

HastelloyN alloy is a kind of nickel-based superalloy specially designed for molten salt reactor by Oak Ridge National Laboratory（ORNL）in the United States.It has excellent corrosion resistance and neutron radiation.GH3535 alloy is a new domestic nickel-based superalloy which is developed based on HastelloyN alloy and has excellent corrosion resistance and high strength.However,the oxide film of the alloy is prone to exfoliation at high temperature,resulting in the intensified oxidation,which indicates that the oxidation resistance of GH3535 alloy needs to be further improved.In this paper,GH3535 alloy was shot-blasted to improve oxidation resistance.The discontinuous weight gain method is used to study the isothermal oxidation behavior of GH3535 alloy at 900℃.The oxidation kinetic curves were obtained by recording the data of GH3535 alloy.The physical properties and stress state of GH3535 alloy before and after shot peening were measured by X-ray stress tester and micro hardness tester.Scanning electron microscope（SEM）,energy spectrum analyzer（EDS）,X-ray diffractometer（XRD）and electronic probe microanalysis（EPMA）were used to study the surface and cross-section morphology and phase composition of the oxidized samples.The results indicate that shot peening can change the surface state of GH3535superalloy.After shot peening,the coarse grains on the surface of the alloy transform into sub-crystals and equiaxed grains.Meanwhile the high-speed projectile continued to hit the alloy surface,which caused the increasing surface strain,the dislocation density increasing,and the surface residual stress increasing.Under the condition of 0.45Nmm peening strength,its surface hardness can reach 521HV,and the depth of the hardened layer is about 160μm.After shot peening,the oxidation rate of GH3535 superalloy decreased gradually in the process of constant temperature oxidation at 900℃.In addition,with the shot peening strength increasing,the oxidation resistance of the alloy is enhanced.Compared with the non-shot peening alloy,its oxidation weight gain was 79.7%lower.After shot peening,the oxide film of GH3535 alloy appeared to be delaminated.The outermost oxidation layer is Ni O,Ni Fe₂O₄,Ni Cr₂O₄ and other composite oxides,and the innermost is continuous Cr₂O₃。However,the outermost layer of non-shot peening alloy is Ni Cr₂O₄,Ni Mo O₄ and other composite oxides,and the middle is the Mo O₂ inner oxide layer.EPMA element distribution results show that the high-density dislocation and"fine-grained layer"introduction have changed the element diffusion mechanism.Grain boundary and dislocation can be used as fast channels for Cr elements to accelerate the diffusion of Cr elements,so Cr rich oxides can be rapidly formed.Because of the shot peening,the continuous densest Cr₂O₃ layer will be occurred between the oxide layer and the matrix,which effectively improves the oxidation resistance of GH3535 superalloy.With the increase of shot-peening strength,the integrity of the alloy surface is destroyed.Scale-like protrusions and small pits appear on the surface of alloy with greater shot peening strength,and voids and cracks appear on the alloy substrate,resulting in an adding of oxidation increase with the increase of peening strength.Under the condition of 0.45Nmm peening strength,alloy oxidation gain is the least.Converly,under the condition of 0.9 Nmm peening strength,alloy oxidation gain is the most among all samples.The oxide film of the non-shot peening GH3535 peeled off severely,but the oxide film on the GH3535 surface with different shot peening strength showed no obvious peeling trace.Without the shot peening,the composition of the oxide film is mainly composed of Ni O,Ni Cr₂O₄,Ni Mo O₄.Moreover,the composition of the peeled film is mainly mixture of Ni O,Ni Cr₂O₄ and Mo.The alloy oxide films with different shot peening strength are mainly composed of Ni O,Ni Fe₂O₄ and Ni Cr₂O₄,the innermost layer of which is Cr₂O₃ layer.The above results indicate that shot peening process has a great influence on the oxidation performance of GH3535 superalloy.