Effect Of SMAT On Microstructure And Mechanical Properties Of AISI301 Stainless Steel After Cryogenic Treatment

In the present work, in order to improve the mechanical properties of the AISI301 stainless prepared by common commercial rolling, surface mechanical attrition treatment(SMAT) with previous cryogenic treatment(CT) has been carried out on the AISI301 stainless. Microstructure and mechanical properties of AISI301 stainless with or without SMAT after CT were investigated by means of optical microscopy(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), microhardness test and uniaxial tensile test. Besides, effects of different CT time on the mechanical properties of AISI301 stainless were analyzed, mechanism of formation of gradient nanostructure after SMAT were specific investigated.The main conclusions are as follows:(1)After CT processing, the residue martensite of AISI301 stainless steel transform to martensite, at the same time, mechanical properties including tensile properties, strain hardening properties, power of fracture and other performance were improved with various degrees. After CT for 24 hours, austenite and martensite with shape of needle and lath couldn't be found in the sample. The microstructure exhibits a uniform structure of austenite and less carbide comparing to the untreated one, which make every martensite XRD diffraction peak were strengthened. Simultaneously, some diffraction peaks were enhanced and others were weakened. After CT for 7 days, the microstructure sbecome more uniform and exhibits more serious slip of martensite. Uniform microstructure and martensite result in the improvement of tensile properties, the strain hardening behavior andpower of fracture, the difference of CT 24 hours and CT 7 days is very small. The average microhardness increased from 267 HV to 321HV(120%) and 343HV(128%) respectively, the yield strength increased from 251 MPa to 475.7MPa(189%), tensile strength increased from 1166 MPa to 1332.5MPa(114%), power of fracture upgrade from 34315KJ/m3 to 39651KJ/m3, but the elongation have decreased to 43% from 35.6%.(2) Gradient nanostructure and martensite gradient were formated on AISI301 stainless steel treated by SMAT after CT 24 hours due to severe plastic deformation, and the overall mechanical performance increase with the increasing of SMAT duration. The martensite in the surface layer was shred most seriously and the degree decreases gradually along the thickness direction, thickness of bilateral deformable layer was about 400?m. Hardness distribution after SMAT process exhibits a "V" shape, hardness of the top surface and the vore increased from 270 HV to 571 HV and 410 HV with an amplitude of 111.4% and 51.8% respectively. After SMAT 30 min, the yield strength increased from 251 MPa to 704MPa(280%) and tensile strength improved from 1166 MPa to 1426MPa(36%), but the elongation of AISI301 stainless steel declined. Strain hardening rate improved with the increasing of SMAT duration in stage III. Poisson's ratio of the core region is lower, this makes coarse grain layer whcih is not compatible with uniformly plastic deformation occur actually in this state, resulted in the reducement of strain hardening rate at stage IV. Fracture mechanism changes from the ductile fracture to quasi-cleavage fracture then to cleavage fracture.(3) 24 hours was determined as the time parameter of CT before SMAT in order to reduce the loss of plastic caused by increase of strength, then SMAT process was used to AISI301 stainless steel to forming gradient nanostructure. Coarse-grained layer and gradient nano-layer are mutual restraint, to obtain a better strength-ductility ratio. The main reason is: First, microstructure of sample treated by CT for 24 hours became more uniform, and main objective of this study is to obtain a gradient state of martensite volume or size along the thickness direction. Second, SMAT process makes the grains and martensite slip bands of AISI301 stainless steel refined. The surface shear stress and strain rate gradually decreased with the increasing of depth from the top surface, and make the formation of the surface gradient nanostructure by plastic deformation eventually. Due to the lower strain, the core of sample hardly changed, so gradient of martensitizing and shredding degree was formed.