Study Of Technology And Mechanisms Of Laser Surface Compositing To Enhance Railway Service Life

With the rapid development of high-speed and heavy-haul trains,the damage of wheel/rail friction pair is getting more and more severe,which increases the cost of the railway transportation and threatens the train safety.It is a great challenge to improve the service life of rail,which is determined by the wear resistance and the contact fatigue resistance of the rail.In order to ensure toughness and contact fatigue resistance(CFR)of rail,the present technical standards restrict the hardness of rail to be beneath HB400 and the martensite structure is forbidden in rails,which make the wheel/rail service in low hardness,and the service life of rail is rather short.In this dissertation,the influence of laser all-area quenching to enhance the wear and fatigue of rail samples is studied.It is found that the microhardness of rail is enhanced from HV300 to HV750~850 by laser quenching and the wear resistance is hence significantly raised.However,a leading crack occurred in the hardening layer,which may couse rail fracture.Therefore,the laser all-area hardening should be restricted in rail strengthening,similar to other surface processing technique.When adopting laser glazing technique,the roughness increases greatly due to the surface melting.There are a lot of metallurgical defects such as cracks generated in the melted zone,which indicates that the laser glazing is not suitable for rail surface strengthening.In order to increase wear resistance without noticeably deteriorating their CFR,a novel laser surface compositing(LSC)based on selective laser quenching is proposed to strengthen the rails firstly.The influence of the parameters of LSC to hardening depth is studied.The results of rolling contact fatigue/wear experiments show that LSC can enhance wear resistance without noticeably deteriorating CFR,therefore the cracks propagating into the matrix are able to be avoided,and the service life of rail is extended.This resut has broken through the traditional worldwide restriction that the martensite structure on rail surfaces are forbidden in the past 100 years,and has risen up a new path for enhance rail service life.Based on the above research,a high repeated-scanning laser quenching(RSLQ)is firstly proposed in order to enhnce laser processing efficiency,which is realized by high-speed deflection of galvanometers,in which the quenching process is changed from single scanning to repeated scanning.The influence of processing parameters of RSLQ to hardening depth and surface morphology has been studied systimatically.The temperature fields on the surface and inside of the rails are reproduced by numerical simulation,and the mechanism of hardening layer enhancement is revealed.Finally,the bending fatigue properties of LSC real rails are tested according to railway standard ‘TB/T 1632’.The results indicate that the fatigue properties of rails treated by LSC are not worse than untreated rails,even though the loading increases by 8.5%.After experiment,the macro or micro cracks are not found on rails treated by LSC.There is no obvious change in microhardness of the loading area before and after experiment.The on-site testing results show that the service life of rail has been extended 2.2~3.5 times by LSC.The service life for heavy-load railway can be extended 10.5 times as much as that without laser treatment.In summary,LSC breaks through the standard that the martensite structure has been forbidden on rail surface,and hence can enhance the service life of rail greatly.RSLQ has made the efficiency of LSC step into a new level.This study establishes the foundation for the application of laser surface strengthening in railways.