| Wheel/rail friction plays an key role in guiding,traction and braking in the operation of train vehicles.So it is an extremely important friction pair in railway system.In particular,the rapid development of modern high-speed and heavy haul trains has brought new opportunities and challenges to the research of wheel rail friction.Mechanism of wheel/rail friction is a very complex process,but the change of temperature,load,speed,will directly change the microstructure of wheel/rail material during the process of train,as well as the coefficient of friction.The change could lead to some uncertain risks for the train operation.Therefore,the relationship between working conditions,such as temperature,load,speed etc.,and friction coefficient of wheel/rail steel is a burning question in present.This article first summary and review the research of other scholars,and obtained the change regularity of wheel-rail friction coefficient with temperature,load and speed.In particular,the theoretical formula of friction coefficient varying with temperature was summarized and derived.Then we conducted the friction tests of wheel-rail material on a specially designed friction tester,to verify the reliability and reasonability of the formula.The main conclusions are as follows:(1)Through collection and analysis on the works of other researchers,we found that the friction coefficient changes with temperature may be divided three stages,namely the rising stage within low temperature range(25?~300?),the rapid decline stage within medial temperature range(300?~700?)and the slow decrease stage within high temperature stage(700?~925?).(2)The experimental results of the influence of temperature on the friction coefficient of wheel/rail show good agreement with the theoretical estimates in general,although the experimental results are slightly higher than the theoretical value.When the temperature is below 400 ?,the maximum value of the theoretical estimation appears on 300 ?,while the experimental results show two maximum values.After the temperature is higher than 400 ?,the variation of the theoretical estimation and experimental results is consistent,both show a downward trend.(3)In the temperature range of 100 ?~400 ?,the variation of friction coefficient undergoes a special stage with the increase of temperature.When the temperature up to 200 ?,the plate and the pin began to turn blue.As the temperature increase,the color begins to deepen,and this situation continued until 400 ?.This phenomenon is also called the blue phenomenon of the wheel rail steel(pearlitic steel).It is precisely because of this blue brittle phenomenon makes the change law of friction coefficient is different,the blue brittle phenomenon makes the material hardness and tensile strength has increased,making the brittleness of the material reduced.The variation of the friction coefficient is different from the result of theoretical calculation in this temperature range,which is combined with the impact of the load and velocity.(4)The experimental results of the effect of sliding speed on the friction coefficient of wheel-rail steel show that the friction coefficient of wheel/rail steel is gradually decreasing with the increase of sliding speed.However,due to the small amount of work on the sliding speed,there is no other trend,which is the direction of experimental study in the future.(5)The experimental results of the influence of load on the friction coefficient of wheel/rail steel show the friction coefficient of wheel/rail steel varies slightly with the increase of load at different temperatures,but in most cases,the friction coefficient decreases gradually with the increase of load,especially when the load increases from 250 N(the contact stress is 8.84 MPa)to 300 N(the contact stress is 7.07 MPa),the friction coefficient decreases.There are other cases of different performance,such as at 200 ? the friction coefficient shows the trend of “decrease-rise-reduce” with the increasing load,and reduce the trend,and obtain the minimum value at the load of 200 N(the contact stress is 7.07 MPa).It should be noted that 200 ? is also the beginning of the blue brittleness phenomenon.It can also reflect the influence of the blue brittleness on the friction coefficient.(6)Friction will be accompanied by wear and tear.From the results of several experiments,when the temperature is below 600 ?,the friction and wear loss is not large and the material loss occurred mainly in the abrasive wear stage.However,when the temperature is higher than 600 ?,the carburizing phenomenon will occur.As a result,the friction and wear loss will increase,and the steady state of friction will be shorten. |
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