Research On Cleanliness Evaluation And High Cycle Fatigue Performance Of Axle Steel For High Speed Trains

With the rapid development of modern railway vehicles in the direction of high speed and heavy load in recent years,there have been increasing demands for the service reliability of various vehicles parts.Among these parts,axle is a key component related to the long-term safe operation of vehicles,and damage and failure of axle caused by fatigue is highly prone to occur during high-speed running.Therefore,it is requested that the axle must have a sufficiently high fatigue safety factor.Owing to its high productivity efficiency,high yield of steel and low cost compared with mould casting process,continuous casting process has gained increasing application.However,axle steel is still primarily produced by mould casting process at present in China.To ensure that the continuous casting axle steel meets the metallurgical quality and performance level of the mould casting axle steel has become an urgent research topic.Therefore,in this dissertation,the metallurgical quality and fatigue performance of both the mould casting and continuous casting high-speed axle steel bars of industrial trial production were investigated mainly using rotating-bending fatigue test and fatigue crack growth(FCG)test.The main contents and conclusions are as follows:Both metallographic method and hydrogen embrittlement(HE)-tensile method were used to evaluate the non-metallic inclusion levels of axle steels produced by mould casting and continuous casting processes.The experimental results show that the average size of the largest inclusions in the mould casting and continuous casting axle steels obtained by the metallographic method are 12.1 μm and 16.4 μm,respectively.The maximum inclusion sizes of mould casting and continuous casting axle steels obtained by the HE-tensile method were 22.9 μ and 25.3 μm,respectively.These results show that the size of the inclusions in the mould casting axle steel is smaller than that in the continuous casting axle steel.The results of energy dispersive X-ray spectrometer(EDS)analyses of these inclusions indicate that the inclusions in the mould casting axle steel are mostly strip-shaped oxide-sulfide composite inclusions,while the inclusions in the continuous casting axle steel are mostly spherical composite oxide inclusions mainly containing Al2O3,CaO and MgO.Moreover,the extreme value statistics method was to predict the maximum inclusion size in different volume steels.It is found that the size of the maximum inclusions increases with increasing the volume of the steel,and the predicted maximum size of the inclusions in the continuous casting steel is significantly larger than that in the mould casting steel.The high-cycle fatigue properties of both tested mould casting and continuous casting axle steels were evaluated by rotating-bending fatig·ue test.The results show that the S-N curves of the tested two steels exhibit obvious horizontal lines,i.e.,fatigue limits exist.The fatigue limits determined by the staircase method for the two steels are 449 MPa and 426 MPa,respectively,and the fatigue limit ratios,i.e.,the ratio of fatigue limit to ultimate tensile strength,are 0.54 and 0.53,respectively.That is to say,the fatigue limit of continuous casting axle steel is 23 MPa,i.e.,a reduction by～5.4%,lower than that of the mould cast steel.SEM analysis of all the fatigue fractured specimens revealed that most of them initiated from specimen surface matrix except a few specimens were originated from coarse surface inclusions.The fatigue crack growth rates of mould casting axle steel and continuous casting axle steel were measured by standard C(T)specimens.It is found that the fatigue crack growth rate of the latter is slightly higher than that of the former in the lower stress intensity factor range △K.The obtained Paris equations for the tested two steels are:da/dN=1.10×10-13(△K)3.02(mould casting),da/dN=7.73×10-14(△K)3.09(continuous casting).It is concluded from the above analyses that the better fatigue properties of the mould cast steel than the continuous cast steel are attributed to the higher metallurgical quality,finer microstructure and higher strength of the former.