Study On Thermal Fatigue Performance And Life Prediction Of Cast Steel Brake Disc Of High Speed Train

Brake disc is one of the important basic braking devices of high speed trains. The safety and reliability ofbrake disc directly affects the train.In the long service process, brake disc is repeatedly subjected to the cyclic temperature andthermal fatigue cracking caused by the cyclic thermal stress is one of the main reasons for brake disc failure.In this paper, the mechanical properties, thermal physical properties and thermal fatigue properties of cast steel materials at different temperatures are studied.The temperature and stress field distribution of brake discs inthe emergency braking and conventional braking conditions are simulated by the method of thermo mechanical coupling.In addition, the influences of surface scratch on the temperature and stress field of brake disc are studied by the submodel technique.At the same time, thermal fatigue test and numerical simulation technology are combined to study the characteristics of thermal fatigue crack initiation, thebehavior of thermal fatigue crack growth and the evolution of the microstructure during thermal fatigue. The stress strain response curve and crack growth rate da/dN～Δ K curve of specimenatthe different circulating temperature areestablished.Finally,the thermal fatigue life of non defective brake disc and scratch brake disc are predicted.The main conclusions of this paper are as follows:(1) The thermal stress exceeds the yield strength of brake disc at the emergency braking, resulting in unrecoverable plastic deformation, and the equivalent residual stress generates on the surfaceis 215MPa.Meanwhile, the circumferential tensile stress is the main stress component of the thermal fatigue failure.There is no residual stress in the friction surface of brake disc during the conventional braking.(2) The influence of scratch defects on the stress distribution of brake disc is significant. The scratch on the brake disc surface cause the local stress concentration, and the equivalent residual stress is 2.32-4.31 times of the non defective disk.The local stress on the surface of brake disc decreases with the increasing of scratch position and notch angle, and the local stress increases with the increase of scratchdepth.Radial stress is the main driving force for crack initiation and propagation of brake disc.(3) The thermal fatigue crack of cast steel is initiated at the notch tip under the combined action of high temperature oxidation corrosion and thermal cycling stress. The crackinitiation mode is transgranular.The propagation mechanism of main crack is the constant passivation-sharpen of crack tip under the action of cyclic stress, making the crack straight forward extension. At the same time, microstructure propagation and high temperature oxidation corrosion and cyclic softening accelerate the crack growth, and the main crackpropagation mode is transgranular.The initiation and propagation mechanism of network crack is that the foldsare producedby the plastic accumulation and the weakening effect of the oxygen element on the grain boundary.Theinitiation and propagation of network cracks mode is intergranular.(4) In the process of thermal fatigue,the hardness of quenching zoneis shownassoftening and hardening cycle.The hardness was periodic oscillation characteristics,but the general trend is cyclic softening.Correspondingly,the tempered sorbite structure which is maintain the martensite position gradually transform into granular bainite structurewhen the cycling maximaltemperatureis 650℃.The structure of cast steel do not change significantly when the cycling maximaltemperatureisbelow 600 ℃.(5) The thermal fatigue crack initiation life of scratch brake disc is shorter than the non defective brake disc by 3.7～5.7 times.At the same time, the thermal fatigue crack initiation life shorten with the decreased of scratch position and notch angle, and shorten with the increase of scratch depth.(6) Scratch brake disc compared with non defective brake disc, the thermal fatigue crack propagation life is reduced by the maximum of 20 times.The crack propagation life is the shortest when the scratching is located at the middle position of the brake discat different scratch position.The crack propagation life is the shortest when the scratch depth is 3mmat different scratch depth.The crack propagation life is the shortest when the scratch notch angle is 60 degrees at different scratch notch angle.