Effect Of Strain Amplitude On Surface Integrity Evolution Of Gradient Structure 316 Stainless Steel

316 austenitic stainless steel,due to its excellent mechanical properties and corrosion resistance,is widely used in nuclear energy,chemical,and food industries.However,its disadvantages,such as low hardness and low fatigue strength,has seriously limited its wide industrial usage.Thereby,surface strengthening technology is very often applied to produce austenitic stainless steel with a gradient structured surface layer.The gradient structured stainless steel,unlike its untreated counterpart,exhibits improved strength-ductility combination.It can also reveal enhanced fatigue property,which is attributed to the grain refinement,compressive residual stress and work hardening produced by surface strengthening methods in the surface layer of gradient structure,due to their inhibition effects on crack initiation and propagation.Thus,the present work focuses on the investigation of surface integrity evolution,cyclic behavior and failure mechanism of 316 stainless steel with a gradient structured surface layer produced by conventional shot peening under strain-controlled tension-compression low-cycle fatigue experiments.The surface integrity parameters,such as microstructure,hardness,residual stress and surface roughness,are investigated by optical microscopy,electron-backscattered diffraction,hardness tester,residual stress analyzer and 3D optical profilometry respectively.The following results are obtained:(1)To investigate the fatigue evolution process under total strain amplitudes of0.5%and 1.1%,the fatigue testing is interrupted at various predetermined number of cycles:N₀?N₂₀?N_f/2 and N_f.The fatigue test results show that both the untreated and peened specimens exhibit a three-stage cyclic deformation behavior:initial cyclic hardening,near cyclic stabilization and secondary cyclic hardening regardless of the imposed total strain amplitude.However,the presence of gradient structured surface layer is found to accelerate the processes of initial cyclic hardening and near cyclic stabilization,which in return are observed to play a very important role in fatigue life decrement of peened specimens as compared to untreated specimens.The surface integrity analyses reveal that the percentage of austenite transformed to martensite,the level of color gradation in grains,the value of high plastic strain,the fractions of low-angle grain boundaries and of geometrically necessary dislocations are increased with the increment of number of cycles for both untreated and peened specimens.Yet,fatigue-induced deformations in the bulk interior of the peened specimens are revealed to be more intense than that of the untreated one.This explains why the initial cyclic hardening and near cyclic stabilization stages are shortened for peened specimens in comparison to the untreated ones.(2)To investigate the fatigue failure process,the fatigue tests are performed under a range of different total strain amplitudes:0.5%,0.7%,0.9%and 1.1%.For both the untreated and peened specimens,with the increase of total strain amplitude:the fractions of fatigue-induced martensites and slip bands increase;the magnitude of compressive residual stress decreases,for which is found to drop sharply to a quasi-zero value at high total strain amplitudes of 0.9%and 1.1%;the values of hardness in the near surface region decrease,but that in the bulk increase;and the number of crack initiation sites and the average spacing of fatigue striation increase,while the mean size of dimples decreases.The increasing of total strain amplitude also causes the increase of surface roughness value S_afor untreated specimens but the decrease of surface roughness value S_afor peened specimens.All the obtained results provide the reasons to the decrement in fatigue life when the total strain amplitude is increased from 0.5%to 1.1%.Besides,at failure,a linear relationship is found between the surface hardness and surface compressive residual stress when they are both normalized with a surface roughness parameter known as maximum peak height(S_p).In brief,the present work,dealing with the fatigue behavior and failure analysis of gradient structured 316 stainless steel under strain-controlled low-cycle fatigue testing,hereby shows that the decrease of fatigue life for peened specimens is not only related to the shortened initial cyclic hardening and near cyclic stabilization stages,but also to the fatigue-induced changes in surface integrity parameters,it lays a theoretical foundation for the selection of gradient structure surface layer of 316stainless steel under different working conditions.