Very High Cycle Fatigue Behavior Of Bainitic Rail Steels

The very high cycle fatigue (VHCF) behaviors of a Mn-Si-Cr alloyed low carbon bainite/martensite multiphase high strength steel were studied using an ultrasonic fatigue testing machine in the present work. The three heat treatments were designed in order to obtain various microstructures and optimized mechanical properties. The effect of microstructure on the VHCF behaviors (e.g. S-N curves, fatigue fracture surface, fatigue crack propagation) was investigated to explore the mechanism of fatigue (especially the non-inclusion crack initiation).Based on the curves of continuous cooling transformation (CCT) and time-temperature transformation (TTT), the effects of cooling rate and holding temperature on microstructure were discussed. The dilatometry simulation results suggested that the cooling rate and isothermal temperature have a significantly influence on the microstructure obtained, which is the foundation of the design of the heat treatment for fatigue samples. The investigation indicated that the experimental steels showed excellent bainite hardenability; and the microstructure changed from granular bainite to acicular bainite/martensite multiphase with increasing of cooling rate.GG1and GG2samples with the microstructure of acicular bainite/martensite multiphase exhibited good combination of strength and toughness. The mechanical properties varied duo to different heat treatment process. GG1and GG2samples with the microstructure of acicular bainite/martensite multiphase reached an excellent trade-off between strength and toughness. The ultrahigh cycle fatigue tests indicated that both of them have a high fatigue strength and the fatigue cracks initiate at the surface of the sample. However, GG3samples is composed of granular bainite and martensite/austenite (M/A) islands with a high tensile strength and ductility. But, it showed a relatively low fatigue strength and the fatigue cracks initiate at the internal matrix microstructure. Most importantly, the research presented that the steels with the microstructure of bainite and martensite have a better compatibility with inclusions than tempering martensite steels.The microstructure of the experiment steels treated by different heat treatments consisted of bainite and martensite which have fining sub-structure of matrix. Besides, the filmy retained austenite located between the martensite/bainite lathes by dramatically improved fatigue threshold value and reduced the fatigue crack growth rate It also suggested that the steels with the microstructure of acicular bainite/martensite showed a higher fatigue threshold value than the steels with the microstructure of granular bainite/martensite.