| Austenitic stainless steel has been widely used in industrial circle attributed to the excellent corrosion resistance and good formability.However,the low hardness and strength of the traditional coarse-grained austenitic stainless steel affects its performance and service life to some extent.So it is particularly essential to improve the strength and hardness,consequently the wear-resistant performance,of austenitic stainless steel on the basis of maintaining good corrosion resistance and good formability.As a result,304 austenitic stainless steel was used as the researching object in this paper,the surface nanocrystalline stainless steel(grain size is 36nm)was obtained by the surface shot peening(SP)process.Then a bi-modal microstructure(BMS)in the surface nanocrystalline stainless steel was achieved by annealing at different temperature.Next,Optical microscopy(OM)and X-ray diffraction(XRD)were applied to analyise the microstructure and phase compositions of fine grained surface on stainless steel,respectively.The BMS was characterized quantitatively by electron backscattered diffraction(EBSD).The hardness measurement,wearing tests were used to characterize the wearing resistance of stainless steel with different state.Forming limit diagram(FLD)measurement and tensile tests were applied to analyize the formability of stainless steel.Such electrochemical tests as open circuit potential(OCP),potentiodynamic polarisation and electrochemical impendence spectroscopy(EIS)were carried out to compare the corrosion behaviour of SP samples with the as-received one.(1)The surface nanocrystillized shot speeing processes.The results of OM suggest that 304 austenitic stainless steels were treated with solid solution(1050?-60min)to obtain homogeneous austenitic stainless steels,the average grain size of which is about 20 ?m.Then,a series of surface nanocrystalline stainless steels are obtained through shot peening,where the pressure and time of shot peening is 0.25-0.4MPa and 6-10 min,respectively.The sample subjected by shot peeing with 0.3 MPa-6min was choiced as the object of the subsequent tests according to the smaller grain size(36 nm)and lower content(50.2%)of formation-induced martensite(DIM)Here,the thickness of nanocrystalline layer on stainless steel is about 50 ?m from the result of OM observation.(2)The annealing processes of BMS.Three different BMS can be obtained by annealing the surface nanocrystalline stainless steel at 600 ?,650 ?,700 ? for 30 min,respectively.The grain size distribution and volume content of the three annealed samples were analyzed by EBSD.The results show that the grain size and volume content of the coarser grains in BMS increases with increasing annealing temperature.The average grain size and volume of the coarse grains is about 3 ?m and 66%,respectively,and the corresponding values of finer grains in BMS are about 58 nm and 6.4%.(3)The wearing resistance.The hardness test results show that the surface microhardness of the as-received stainless steel is about 175 HV.After shot peening at 0.3MPa-6min,the surface hardness is increased by 1.5 times,up to 430 HV.As the distance from the surface increases,the hardness gradually decreases.However,the hardness of the annealed samples after shot peening at different temperatures of 600°C,650°C,and 700°C was lower than that of the as-shot peening specimens,but still higer than that of the as-received specimens.Wear experiments were performed on the wear resistance of three different surface structure specimens.By comparing to the wear scar width,friction coefficient,wear amount loss,and wear rate of different samples under the same conditions,the wear resistance of the as-shot peening sample is the best,and that of the as-received sample is the worst,and those of the samples annealed after the shot peening are middle between the two sample mentioned above.(4)The formability.The results of tensile tests and deep drawing tests show that the strength of 304 stainless steel after shot peening is improved significantly,but the ductility and consequently the forming performance is reduced.However,the ductility of the sample annealed at different temperatures after shot peening was gradually recovered although the strength was decreased in a certain extent.The formabilities of three samples including the asreceived one,as-shot peeing one and the annealed on at 650 for 30 min were studied by comparing to the positions and shapes of FLD.The results show that the formability of the three kinds of samples are different,in which that of the as-received sample is the best,that of the asshot peening one decreases,and that of the specimen was obviously recovered,however lower than that of the as-received sample after the annealing at 650° C,(5)The corrosion resistance.Finally,the corrosion resistances of five groups of stainless steel specimens(as-received sample,as-shot peeing one,and three annealed samples at different temperatures)were evaluated by means of OCP,potentiodynamic polarization curve,and EIS.The results show that the corrosion resistances of the samples annealed at 650°C and 700°C are better than that of the as-received sample.Based on the above experimental results,the BMS in the surface layer on stainless steel can be obtained by post-shot peening annealing at 650°C for 30 min.This surface BMSed stainless steel has simultaneously the excellent wear resistance and good formabiltiy as well as the better corrosion resistance. |
PDF Full Text Request