| 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.Especially in corrosive environments,its wear resistance is even worse.Therefore,this paper will study how to improve the tribocorrosion resistance of austenitic stainless steel.Firstly,the surface nanocrystalline stainless steel 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 calculate grain size and 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.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.Finally,the friction corrosion resistance of each sample was compared by tribocorrosion test.(1)The results of OM suggest that 304 austenitic stainless steels were firstly treated with solid solution(1150?-30 min)to obtain homogeneous austenitic stainless steels,the average grain size of which is about 40 ?m.Then,a series of surface nanocrystalline stainless steels are obtained through shot peening,where the pressure and time of shot peening is 0.4-0.6 MPa and 4-10 min,respectively.The sample subjected by shot peeing with 0.5 MPa-6 min was choiced as costing less peening time and meeting the thickness of the refining layer of at least 70 ?m.XRD analysis shows that the average grain size of the surface refining layer of the sample is about 18 nm,and the deformed martensite content is 59.1%,and the thickness of the refining layer is about 83 ?m.(2)Three different BMS can be obtained by annealing the surface nanocrystalline stainless steel at 650?,700?,750? 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.After annealing at 650 °C,the average grain size of nanocrystalline stainless steel is lower than 1 ?m.There is no abnormal growth and the BMS features are not obvious.After annealing at 700°C,the average grain size of stainless steel is still lower than 1 ?m,but some of the grain size has been larger than 1 ?m,thus,the BMS features become relatively obvious;when the annealing temperature is increased to 750 °C,the grain has grown significantly.(3)The hardness test results show that the surface microhardness of the as-received stainless steel is about 183 HV.After shot peening at 0.5 MPa-6 min,the surface hardness is increased by 1.57 times,up to 471 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 650°C,700°C,and 750°C is 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 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)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 resistance of shot peening nanometer samples is worse than that of the original coarse-grained samples,because the high content of deformation-induced martensite;the corrosion resistance of shot peened samples after annealing treatment is improved to some extent,and with annealing temperature increasing,the improvement is gradually reduced,but the corrosion resistance of the annealed sample after shot peening is better than that of the original coarse-grained sample.The corrosion resistance of the BMS samples(annealing at 650°C,700°C after shot peening)is significantly better than the original coarse-grained one.(5)The results of tribocorrosion test show that the tribocorrosion resistance of the original coarse-grained samples is the worst under the load of 10 N.The tribocorrosion resistance of the three annealed samples decreases with the increase of annealing temperature.The tribocorrosion resistance of the as-shot peening sample is worse than that of shot peening +650°C,indicating that the effect of hardness on the tribocorrosion resistance under low load is reduced.The effect of corrosion performance is more obvious.In the case of 20 N load,the law of tribocorrosion resistance changed,and the performance of the original coarse crystal sample was still the worst.The performance of the three annealed samples still deteriorated with the increase of annealing temperature and the friction corrosion resistance.However,the highest hardness of the shot peened specimens has the best tribocorrosion resistance,indicating that the hardness is a major factor affecting the friction and corrosion resistance under high load.And the wear resistance law is consistent with the wear resistance law of Chapter 4.Among them,the BMS samples(shot peening +650°C,700°C annealing 30 min),regardless of loading load and surface state changes,their tribocorrosion resistance is significantly better than the original coarse crystal 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?700°C for 30 min.This surface BMSed stainless steel has simultaneously the excellent wear resistance and good corrosion resistance as well as the better tribocorrosion resistance. |
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