Crack Propagation And Microstructure Evolution On The Friction Surface Of Brake Disc For High-speed Train

Due to the increasing engineering significance for service reliability of brake disc in braking system from Chinese railway transportation,the crack propagation and microstructure evolution on the friction surface of brake disc material 28Cr Ni Mo and serviced high-speed railway brake disc were studied with the combination of theories and experimental methods in this paper.The studied brake disc material 28Cr Ni Mo is a medium carbon law alloy steel with tempered sorbite microstructure,the tested fracture toughness of 28Cr Ni Mo was 117.9MPa·m^1/2 and the crack propagation threshold was 9.9 MPa·m^1/2.The friction and wear tests of 28Cr Ni Mo with cracks and without cracks on the friction surface showed that the friction coefficients without cracks increased first and then reached to a stable level,and those with crack remained unchanged.Meanwhile the friction coefficient and wear of the specimen with cracks was smaller than that without cracks.It was indicated that the cracks weakened the friction properties.The thermal fatigue tests of 28Cr Ni Mo indicated that the heat checking was caused by the alternate cooling and heating during braking.Compared with the crack caused by mechanical stress,the heat checking formed at lower temperature gradient.The temperature evolution on the disc during braking was investigated by experiments.The results showed that a hot ring formed first and expanded to the whole friction surface,and the hot ring consisted of alternative hot spots and cold zones.Three stages were defined in the variation of the maximum temperature during braking,i.e.,rapid increase,stabilization stage and falling slowly.Correspondingly,with the affection of the temperature the evolutions of frictional coefficient also consisted of three stages,i.e.,rapid increase,falling slowly and increase again.The one-dimension heat conduction equation was used to predict the maximum temperature in the disc during baking.The predicted results were in agreement with the experimental results.The results of the thermal stresses achieved by FEA were lower than the yield stress of the28Cr Ni Mo,which supported that the thermal cycling stress by braking was the main cause to form the heat checking on the brake disc.The propagation of heat checking on the friction surface was investigated.The experimental results indicated that the heat checking was initially formed on the disc friction surface and then the heat checking located in radial direction developed to the radial main crack.The formation mechanism of heat checking was that during braking process the heat formed on the brake disc surface created hot stresses to form the heat checking.The crack initiation and propagation mechanism was that with the occurrence of heat checking caused by heating and cooling processes during braking,the friction surface roughness increased,which in turn caused local stress concentration and then the localized friction stress and thermal stress drove the heat checking to propagate and coalesce with the radial main crack.The crack propagation process showed that the long crack with large dimension in depth direction propagated slowly,while the short crack with small dimension in depth direction propagated fast and coalesced with the radial main crack.The distribution of thermal stress and friction stress was the largest on the surface and decreased in the depth direction.The thermal-mechanical stress which was the crack driving force at the crack tip of heat checking was large,while that at radial main crack was nearly zero.Based on the crack propagation results of the full-scale bench tests,the damage allowance and remaining fatigue life were evaluated,i.e.,the relationship between the increment of the total crack length in a region and the number of braking times could be described byN(28)10^{((35)a(10)42)/15}.With this model,the remained life of the serviced brake disc could be evaluated based on the crack size.The microstructure change on the disc friction surface was investigated.White Etching Layer(WEL)was analyzed by electron back-scattered diffraction and transmission electron microscopy,indicating that the WEL in the friction surface of the disc consisted of nanoscale ferrite grains,which caused a gradient hardness distribution in the depth direction.The occurrence of WEL could improve the mechanical properties of brake discs and might probably reduce the wear of braking pads.Considering the inexistence of an evident boundary between WEL and matrix,the absence of austenite in the WEL and the temperature lower than austenitization,the formation of the WEL was mainly attributed to the surface mechanical plastic deformation caused by the friction between the disc and pad during braking.The results achieved in this paper rich the knowledge and related theories of crack propagation and microstructure evolution on friction surface of high-speed train steel brake disc,which also have important engineering application references to service reliability,optimum of maintain regulations and reuse of served discs for Chinese high-speed train steel brake discs.