Micromechanism Of Pre-deformation On Deformation Induced Martensitic Transformation Of Austenitic Stainless Steels

Due to the low yield strength of austenitic stainless steel,it is prone to plastic deformation during manufacturing and service,causing the austenite phase to transform into martensite.The martensitic transformation process will change part of the properties of the material and affect its use.At present,the influencing factors of deformation-induced martensitic transformation mainly include chemical composition,deformation temperature,strain rate and deformation mode,and the stress state has a significant impact on the internal grain changes of the material or the properties of the overall material.Therefore,it is particularly important to study the influence of stress state on deformation-induced martensitic transformation;ultrasonic shock is a common method in the strengthening process of austenitic stainless steel,and the mechanism of deformation-induced martensitic transformation during impact needs to be further clarified and perfect.This article takes 304 austenitic stainless steel as the research object,and uses ultrasonic shock to induce martensite on its surface.With the help of residual stress analysis,X-ray diffraction analysis(XRD),microhardness,optical microscope(OM),electron backscatter diffraction(EBSD),in-situ EBSD,transmission electron microscope(TEM)and other means,through pre-deformation and subsequent deformation in this way,the influence of the surface stress state on the deformation-induced martensite transformation is studied,and the dynamics and crystallographic laws of the stress state on the deformation-induced martensite transformation are discussed.At the same time,the phase transformation mechanism of martensite transformation induced by ultrasonic shock is further improved.Through experimental analysis.the conclusion is as follows:(1)After ultrasonic impact of 304 austenitic stainless steel with different pre-tension strains,a certain thickness of strengthening layer and nanocrystalline layer will be formed on the surface.As the amount of pre-tensioning strain(surface tensile stress)increases,the thickness of the strengthening layer produced by ultrasonic impact gradually decreases,and the surface microhardness also gradually increases with the increase of the amount of pre-tensioning strain,up to 420.5 HV.However,the grain size of the surface layer did not change significantly with the increase of the amount of pre-stretching strain.(2)Ultrasonic shock-induced martensite transformation has obvious preferred orientation;after pre-stretching deformation(the stress state becomes complex),the preferred orientation of deformation induced martensite will be sharply weakened,the texture strength is significantly reduced,and ultrasonic There are different types of martensite transformation in the impact process,that is,strain-induced martensitic transformation occurs on the outermost surface,stress-induced transformation occurs in the subsurface layer,and the reversal of martensite transformation occurs when the stress is released.(3)Through EBSD and TEM analysis,it can be seen that during the phase transformation,the nucleation and growth positions of martensite are mainly located near the grain boundaries,twin boundaries and the intersection of shear bands.The morphology of martensite is mainly based on It is mainly lath,and the transformation from austenite to martensite in the phase transformation process conforms to the Xishan relationship.(4)Under different stress states,the martensite transformation induced by deformation of 304 stainless steel is mainly the direct transformation of austenite into ?-martensite,and there is a small amount of mesophase ?-martensite.Among them,the stress state is different,the martensite transformation variable and the transformation rate are different;combined with the in-situ EBSD analysis,the martensite change caused by deformation has the same trend as the dislocation density;at the same time,it is found that the preferred location for dislocation storage is the original austenite Martensite induced by bulk grain boundaries,shear bands and deformation.