Microstructural Evolution Leading To Crack Initiation Of Gradient Structured 316 Stainless Steel

AISI 316 stainless steel having an austenitic crystalline structure,is a face-centered cubic material.AISI 316 austenitic stainless steel,due to its superior properties,has been widely applied in many industrial fields.But owing to the shortage of fatigue resistance,As the reliability of machine constructions under complicated operating conditions has nowadays becoming a matter of great importance to engineering industries,this has thereby motivated the fabrication of a new type of material called the gradient structure.This gradient structure,which possesses dual property of high strength and comparable ductility,can not only inhibit crack initiation but also retard crack growth and thus,improving the reliability and fatigue life of structural components.The present work focuses on the investigation of fatigue behavior and microstructural evolution of shot peened AISI 316 austenitic stainless steel subjected to interrupted fatigue tests in the low-cycle fatigue regime until the occurrence of fatigue crack initiation.The fatigue behavior and fatigue crack initiation life of untreated and shot peened specimens are examined by interrupting tensile-tensile fatigue testing at different number of cycles,e.g.0 cycle,100 cycles,500 cycles and N_iwhich represents the number of cycles required for crack initiation,under two distinct strain amplitudes(0.9%and 1.1%).The detailed assessment on the surface morphology evolution and microstructure evolution of untreated and shot peened specimens during interrupted fatigue experiments are carried out by scanning electron microscope(SEM)-electron backscatter diffraction(EBSD),three dimensional optical profilometer and atomic force microscopy.The main findings of the present work are as follows:1)The untreated and peened specimens tend to show a similar cyclic deformation behavior at low strain amplitude,i.e.first cyclic hardening then followed by cyclic softening,but the secondary hardening tend to occur at high strain amplitude for peened specimens.Besides,the peened specimens are found to have higher fatigue crack initiation life than the untreated specimens at low strain amplitude,for which is found to be decreased with increasing strain amplitude.2)At a constant strain amplitude,the magnitude of surface roughness,extrusion height,plastic strain value and the fraction of low angle grain boundaries and geometrically necessary dislocations are found to be increased with the increment of number of cycles for both the untreated and peened specimens.However,the fatigue-induced deformation is found to be more intense in the outer deformed surface layer than in the bulk interior of peened specimens.It is also found that the fatigue-induced deformation rate of peened specimens at higher strain amplitude is rather globally slower than that at lower strain amplitude.3)When the number of cycles is equal to the fatigue crack initiation life,crack initiation is found to be more likely to occur along the slip band in the bulk interior rather than in the outer deformed surface layer of peened specimens.This happens due to the presence of a higher extrusion height which acts as stress concentrator sites and thus favoring crack initiation process.Through the present work,the surface morphology and microstructure evolution of AISI 316 stainless steel during low-cycle fatigue testing has been elucidated,and the effect of shot peening coverage rate and strain amplitude on microstructure the surface morphology and microstructure evolution is further revealed.These elements obtained for AISI 316 stainless steel can further lay a theoretical foundation for engineering applications.