| High-speed and heavy-haul transportation will be the main trend of future railway development all over the world.The complex interaction between wheel and rail inevitably can cause damage exacerbation with a characteristic of structural failure of vehicle and track,which leads to a reduction of wheel/rail system's service-life.Vigorous development of high-speed railway technology has played a role in boosting China's economic and social take-off.However,failure of components in high-speed railway system is becoming increasingly serious,and the most typical phenomenon is still contact damage and wear of wheel/rail.As a branch of laser processing,laser cladding is a kind of advanced material surface modification technologies,which can prepare superior coatings without defects of pores and cracks on the surface of wheel/rail material so as to improve wear resistance and prolong its life-time.The cladding material of Fe-Co matrix composite patent powder applied in all tests,four categories of alloy coatings were prepared with addition of different contents of yttrium oxide,yttria stabilized zirconia and molybdenum to original alloy powder.Properties of rolling contact fatigue and wear resistance of laser clad strengthened coating with ultrasonic vibration was conducted by using GPM-30 microcomputer control rolling contact fatigue tester under dry sliding condition,the phase analysis,worn surface morphologies,plastic deformation of sectioned discs,composition of micro spraying area,micro-hardness of raw material and coatings were studied by means of X-ray diffraction(XRD),scanning Electron Microscopy(SEM),energy dispersive X-ray Spectrometer(EDAX)and micro-hardness measurement(HV-1000DT),respectively.The main conclusions of this paper are obtained as follows:1.The wheel damage analysis results of CRH380 AL and CRH2-ER8 selected in the field indicate that tread crack belongs to the category of rolling contact fatigue.Plastic deformation easily appeared in wheel sub-surface metal under complex wheel-rail contact stresses,when plastic deformation has accumulated to its limitation then cracks occurred on wheel tread,fatigue crack propagated along the direction of plastic deformation under the continuing rolling contact stresses of wheel-rail,and eventually developed into macroscopic crack and spalling.In addition,the results show that wheel tread of CRH380 B and CRH2 made in Sumitomo Metal Industries,Ltd,whose damage results from abrasions,and most of them can be worn out during the operation,and the possibility of further propagating into the tread crack or spalling is small,which usually has no obviously influence on operation safety.2.The optimum parameters of ultrasonic vibration assisted laser cladding strengthening treatment are obtained through experiments,and the laser power is 1500 W,the pre-put powder thickness is 1.5mm,scanning velocity is 360mm/min and the defocus distance is 360 mm.As for ultrasonic vibration device,the input power is 600 W and vibrational frequency is 19 KHz.Laser cladding coatings are obtained with excellent quality,compact microstructure,less porosity and clear interface under optimum parameters.Columnar crystals are found in cladding coating,large columnar crystals are observed around binding line,and equiaxed dendrite with fine grain is observed at the top of the coating.The results show that the constituent phases of the laser cladding alloy coating mainly have Fe-Cr martensitic structure,?-(Co,Cr)solid solution,(Fe,Ni)solid solution and dispersedly separated hard phase of carbides,borides,silicides containing Cr,W etc..And also,ultrasonic vibration can increase the uniformity of coating microstructure,refine the grain,and the micro-hardness of the cross-section changes gradually.3.Because of the microstructure with high hardness in the coatings of wheel/rail materials after laser cladding strengthening treatment,such as carbon-containing hard phase,martensite and solid solution,the surface hardness of wheel/rail has improved greatly,the surface hardness of wheel/rail discs at average of 539 HV0.3 and 582 HV0.3,respectively,and with hardness ratio of 1 to 1.08.Compared with wheel/rail substrate,the increased ratios are 88.5% and 83.1%,respectively.Moreover,the wear ratios fell respectively 59.1% and 37.3% for wheel/rail specimens after laser cladding treatment,rolling contact wear resistance,sub-surface plastic flow,the surface plastic deformation resistance and spalling resistance were remarkably improved.The wear mechanism was changed from spalling and severe fatigue damage to mild abrasive damage and light fatigue wear.4.After the addition of yttrium oxide,due to the special chemical properties of yttrium atoms,which are beneficial to eliminate the stress and hinder the crack propagation continuingly in cladding coatings.Yttrium oxide gathers around boundary,which slows the rapid condensation speed and inhibits grains to grow thicker and bigger.After adding 1.5% rare earth yttrium oxide,the coatings of wheel/rail discs compared with coating without adding rare earth yttrium oxide shows the optimal effect.The surface wear rate continued to decline,with 5.6% and 7.3% decline rate,respectively,and the wear situation has been further improved.5.After adding yttrium stabilized zirconia,ZrO2 particles as the second phase in the alloy powder prone to draw dehiscence under thermal stress,which can prevent transverse-section from contracting and play the role of toughening.At the same time,the grain with high elastic modulus has the pinning effect on the cracks,which make the crack deflection bypass and dissipate the energy of cracks,so the toughening effect is achieved.What's more,due to the presence of tetragonal zirconia,it can be transformed into monoclinic zirconia under residual thermal stress,the residual thermal stress can be overcome by the difference of the thermal expansion coefficient between cladding coating and substrate,which can not only improve the cracking of cladding coating,but also participate in the phase transformation toughening and improve the quality of cladding coating.After adding 3.5% rare earth yttrium oxide,the coatings of wheel/rail discs show the optimal effect.Rolling contact fatigue wear test results indicate the average wear rates of the four-period were 1.48×10-9g/mm and 1.31×10-9g/mm,respectively.The surface wear rate continued to decline,with 56.5% and 61.6% decline rate,respectively,and the wear situation has been further improved.6.With the addition of molybdenum powder,the content of Mo powder was increased,which led to the combination of the C element and the nucleation rate of the cladding layer.At the same time,molybdenum carbide crystal is close packed six square lattice,with a high melting point and hardness,it is there,the cladding coating exhibited good wear resistance and anti-fatigue performance,wheel rail ring surface of the cladding layer wear is greatly improved.After adding 4% Mo powder,the coating has the best fatigue resistance and wear resistance. |
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