| A set of surface ultrasonic impact/rolling system was developed by ourselves.The system was used to process 45 steel by ultrasonic surface rolling processing(USRP).The effect of USRP on the fatigue properties of 45 steel was studied,and the effect of USRP on the fatigue properties of 45 steel was explored.The experiment is divided into three steps:the first step is to design an USRP device and install it on the lathe to achieve the effect of automatic rolling;the second step is to use the designed USRP system to treat 45 steel cylinder samples,and to study the surface hardness,roughness,surface microhardness,surface residual stress and surface group of 45 steel with different processing parameters.In the third step,the fatigue specimens of 45 steel were processed by USRP,and the effect of this technology on the fatigue properties of 45 steel was studied.The test results show that:(1)USRP can improve the microhardness of the sample surface.When the other processing parameters remain unchanged,the surface microhardness reaches 414 HV(227HV)when the rolling passes are 4 passes,which is 82%higher than that of the non-rolling ones,and the surface microhardness decreases when the rolling passes are too large.When the rolling static pressure is 300 N,the surface microhardness reaches 416 HV(234 HV),which is 78%higher than that of the non-rolling ones,and when the rolling static pressure is 300 N,the surface microhardness reaches 416 HV(234 HV).After 300 N,the pressure continues to increase,and the surface microhardness decreases.When the amplitude is 10 microns,the surface microhardness reaches 409 HV(224 HV for non-rolled specimens),which is 82.6%higher than that of non-rolled specimens.When the amplitude is more than 10 microns,the surface microhardness decreases.(2)USRP can improve Ra value of sample surface roughness.Other processing parameters remain unchanged,the Ra value of sample surface roughness is the lowest at 6rolling passes,which is 0.144 micron(Ra=0.986 micron in non-rolling state),excessive passes will destroy the surface of parts and increase the roughness value;at 250N,the Ra value of roughness is the lowest,which is 0.079 micron(Ra=1.041 micron in non-rolling state),and the static pressure is more than 250N,the roughness value increases;at 8 amplitude,the Ra is 8.When the rolling sphere is micron,the surface roughness Ra value is the lowest,which is 0.191 micron(Ra=1.134 micron for non-rolling specimens).With the increase of the amplitude,the roughness Ra value increases.(3)USRP can make the surface structure of metal rheological,make the cementite aggregate,even form"tumorous"structure,increase the hardness of the surface metal of the sample,but at this time the surface metal will become very brittle due to the increase of hardness,easy to form fine cracks.USRP can improve the grain size of surface metal and achieve the effect of grain refinement.With the increase of static pressure and rolling pass,the grain size decreases.(4)Fitting the fatigue test data,the S-N curve is obtained,and the equation of each curve is obtained.The S-N curve equation of non-rolling specimens is:lgN(10)11.247lg(35)?(28)34.513(1.6×10~5?N?5.2×10~6);the S-N curve equation of 200 N/6specimens is:l gN(10)9lg(35)?(28)29.221(2.4×10~5?N?5.3×10~6);the S-N curve equation of 400N/6 specimens is:l gN(10).6857lg(35)?(28)24.178(1.3×10~6?N?5.3×10~6);and the S-N curve equation of 400 N/10 specimens is:l gN(10).7705lg(35)?(28)26.145(6.5×10~5?N?5.2×10~6);(5)Under the same fatigue cycle test conditions,the fatigue strength of rolling specimens is higher than that of non-rolling specimens.The fatigue strength is 380.287 MPa for 200 N/6 channel and 342.898 MPa for non-rolled specimens at 1.0×10~6cycle times.The fatigue strength is increased by 10.90%.The fatigue strength of the rolled specimens with 400N/6 processing parameters was 447.727 MPa,and the fatigue strength was increased by30.57%.The fatigue strength of the rolled specimens with 400 N/10 processing parameters is411.657 MPa,and the fatigue strength is increased by 20.05%.The fatigue strength of rolling specimens with processing parameters of 200N/6 is 294.44MPa,and that of non-rolled specimens is 279.417MPa,which increased by 5.37%under 1.0×10~7 cycles.The fatigue strength of the rolling specimens with 400 N/6 processing parameters is 320.020MPa,and the fatigue strength is increased by 14.51%.The fatigue strength of the rolling specimens with processing parameters of 400 N/10 is 305.316MPa,and the fatigue strength is increased by9.27%.(6)Under the loading stress of 360MPa,the fatigue life of the non-rolled specimens is5.784×10~5times,and the maximum fatigue life of the non-rolled specimens is 4.61×10~6times after USRP,which is 7.71 times of that of the non-rolled specimens.Even the second group of specimens with the least increase is 2.83 times of that of the non-rolled specimens.By comparing the second and third groups of specimens,the fatigue properties of the specimens increased by 172.34%when the rolling static pressure increased from 200 N to 400N.Compared with the third group and the fourth group,the fatigue properties of the specimens decreased by 37.01%when the number of rolling passes increased from 6 to 10.Compared with the second group,the fatigue performance of the fourth group increased by71.55%.USRP technology can improve the fatigue life of specimens.(7)Cracks in 45 steel base metal originate on the surface of the sample.The main reason is that the"wave crest"on the surface of the sample is not smoothed and the"wave trough"is filled.The stress concentration on the surface of the sample is large and the cracks are easy to initiate.In the crack propagation region,the river pattern is similar to cleavage,and there are secondary cracks and tearing edges.The morphology of dimples is the main feature of the instantaneous fracture zone.The main fracture mode of base metal is quasi-cleavage fatigue fracture.(8)The crack source of the USRP specimens mainly originates in the inclusions of the subsurface layer,and there are more than one crack source in some specimens.The crack initiation positions of specimens with different rolling parameters are different,mainly because the surface properties of specimens are different with different rolling parameters.Ultrasound surface rolling processing does not change the propagation behavior of cracks.Secondary cracks and similar cleavage river patterns can still be found in the propagation zone of cracks.In the rapid fracture region,the specimens show dimple morphology,and there are spherical inclusions and second phase particles in individual dimples.In summary,the fatigue fracture of 45 steel under ultrasonic rolling is quasi cleavage fracture,and USRP can only change the location of crack initiation,but can not change the form of crack propagation and fatigue failure. |
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