Research On The Heat Treatment Process Of U75V Rail Based On The Coupling Relationship Between Wheel Rail Wear And Rolling Contact Fatigue

With the rapid development of railways towards high-speed passenger transportation and heavy freight,transportation density of railways continue to increases and so do the speed and axle load of trains.Now steel rails are subjected to serious damage by wear and rolling contact fatigue,which a relationship is detected between these two failure forms.The high strengthening of steel rails is demanded to meet the rapid development of railways and the microstructure of pearlite is usually adapted for the steel rails.Online heat treatment is the most effective and economical process to achieve the high strength of the steel,that is,to refine the pearlite lamellar spacing by accelerated cooling rate after hot-rolling during the steel-making to improve the mechanical properties.It is of great significance to realize the coupling and balanced development between these two failure forms by studying online heat treatment procedure to control the microstructure of the steel.This research work will be benefit to rail performance and the driving safety of trains.In the dessertation,the influence of pearlite lamellar spacing on wear and rolling contact fatigue of rail is systematically studied.The coupling model between wear and rolling contact fatigue is established for U75 V steel rail.Moreover,the optimal online heat treatment is proposed,by which the pearlit lamellar spacing and mechanical properties of the steel can stably be controlled.Finally it is achieved the coupling and balanced development of wear and rolling contact fatigue.The main content and conclusions are as follows.(1)During wear test the morphology of worn surface of the steel sample develops from groove trace to lamellar peeling and then to fatigue crack with the decrease of pearlite lamellar spacing.The mass loss of the worn samples decreases,the depth of the surface plastic deformation layer becomes shallow and the work hardening caused by plastic deformation becomes more serious.The fatigue crack initiated from the plastic deformation layer propagates along the plastic deformation line to the interior of the material.The length of the crack increases from 21.3 μm to 60.2 μm with the decrease of pearlite lamellar spacing,while the angle between crack and surface increases from10.6 ° to 30.7 °.By achieving coupling matching between wear and contact fatigue of U75 V steel,an optimal pearlite lamellar spacing range is determined to be165.0-180.0 nm.(2)Under the cooling rate of 1.5-2.0 °C/s,the pearlite lamellar spacing obtained by thermal simulation test and high temperature confocal test are 166.4-183.6 nm and168.8-179.0 nm,respectively.With the increase of cooling rate,the pearlite lamellar spacing of U75 V steel decreases.Combined with the model of coupling matching between wear and rolling contact fatigue,the cooling rate U75 V steel should be controlled within 1.5-2.0 °C/s.(3)Jetting air is determined to be applied to the heat treatment of U75 V steel.The test platform of the heat treatment is designed and the nozzle arrangement is determined.Based on the controlling of cooling rate range and the calculation results of heat transfer,the key designed parameters are as following: The nozzle diameter is 8mm;the distance is 48 mm between the nozzle and the surface;the longtitudinal spacing of the nozzle is 32 mm and the jetting air pressure is 5.4-8.1 k Pa.Finally,the test platform is constructed indeed.(4)In online heat treatment test,with the increase of initial cooling temperature,the guarantee ability of the microstructure and mechanical properties of U75 V steel in rail head is enhanced.The pearlite lamellar spacing decreases with the increase jetting air pressure,and the mechanical properties of U75 V steel in the rail head are improved.With the extension of jetting time,the depth of the hardened layer of the rail head increases,while the fluctuation of the cross-section hardness gradient decreases in the surface plastic deformation layer.In the heat treatment,the cooling rate of the outer layer of U75 V steel in the rail head is 2.19 °C/s,and the cooling rate of the inner layer is1.92 °C/s.Finally,the optimal heat treatment parameters are determined as following:the initial cooling temperature is 850 °C,the spray pressure is 8 k Pa,and the spray time is 160 s.(5)In industrially production the mechanical properties of U75 V heat-treated steel are as 1261.1 MPa of tensile strength,12.7% of elongation after fracture and 370.2 HB of tread hardness,which meet the standard of China ’s railway industry TB/T2344.1-2020.The pearlite lamellar spacing(165.4-170.6 nm)from the surface of rail head to 10 mm depth is fit to relationship of the coupling between wear and rolling contact fatigue.(6)After 6 months test on the selected typical railway lines in our country rail,the comprehensive evaluation of wear and rolling contact fatigue performance is carried out and a quantitative evaluation method for the coupling degree of rail wear and rolling contact fatigue is established.The coupling quantitative evaluation index of wear and rolling contact fatigue of U75 V heat-treated steel is 0.12,which is much lower than that of hot-rolled rail and heat-treated U75 V rails produced by conventional fabriction.It is verified the couple and matching between the wear and rolling contact fatigue in the actual line.