Study On Methods For Improving Resistance To Wear And Rolling Contact Fatigue Of High-Speed Railway Wheel Steel

Railway wheels are one of the most important components in the railway vehicle. With the rapid development of the high-speed rail, great progress has been made in research and development of the high-speed train in China. In 2016, China standard electric multiple units have been developed and most of the components were produced domestically. However, all of railway wheels have to be imported. Railway wheels are consumable components whose service life is less than 3 years in the high-speed rail. The cost of one imported railway wheel used on high-speed train is about 20,000 RMB. If they can be produced domestically, the cost of railway wheel would be greatly reduced. The wheel damages mainly include wear,mechanically induced rolling contact fatigue (RCF) and thermally induced rolling contact fatigue. In consideration of rail corrugation and wheel out-of-roundness, wheel/rail contact force is significantly increased when increasing the train speed. This increases the risk of the subsurface initiated rolling contact fatigue that is promoted by the material inclusion.Besides, the probability of phase transformation on the wheel tread is increased when increasing the train speed, which indicates that the high-speed operation would increase the thermally induced RCF. Therefore, it is necessary to conduct the research on methods for improving the resistance to wear and RCF for high-speed railway wheel steel. The results can supply important information for material development and fatigue design for railway wheel.This work was supported by Basic Research Association Foundation of High-Speed Rail(U 1134202), Program for the Changjiang Scholar and Innovative Research Teams in Universities (IRT1178), the 2014 Doctoral Innovation Funds of Southwest Jiaotong University and the Fundamental Research Funds for the Central Universities. In this dissertation, rolling/sliding tests were conducted to investigate wear and RCF of an improved wheel steel used on high-speed rail. Meanwhile, the effect of microstructure and surface hardening treatment on the improvement of wear and RCF of railway wheel steel was studied. A novel method was developed to evaluate the effect of micro-inclusion on subsurface initiated RCF and RCF initiated at deep defects. The main contents and conclusions of this dissertation as follows:(1) Research on the combination of strength and toughness of an improved railway wheel steelRecently, an improved wheel steel (HiSi steel) has been optimized from a traditional wheel steel (ER8 steel) by alloy design. In this study, microstructures and mechanical properties of HiSi steel at different depths in the wheel rim were evaluated. The results indicate that HiSi steel has a higher strength without impairing its impact toughness, as compared with ER8 steel. HiSi steel is hardened by solid solution strengthening and refinement of pearlite interlamellar spacing, while the impact absorbed energy is raised by increasing the proeutectoid ferrite fraction. The hardness of materials reduces with an increase in the depth in the wheel rim due to the increasing of proeutectoid ferrite fraction and interlamellar spacing. Materials deeper in the wheel rim exhibit a lower impact absorbed energy because of their larger interlamellar spacing and pearlite colony size.2. Research on wear and RCF of HiSi railway wheel steelCompared with ER8 wheel steel, ER9 wheel steel, HiSi wheel steel and 0.07V wheel steel are strengthened by carbon addition, solid solution strengthening and precipitation strengthening, respectively. In this study, rolling/sliding tests were conducted to investigate effect of different strengthening methods on wear behavior of railway wheel steel. The results show that wear resistance of wheel steel is improved by both carbon addition and solid solution strengthening, whereas it is deteriorated by precipitation strengthening. Wear resistance of wheel steel depends on the worn surface hardness that can be influenced by both bulk hardness and strain hardening. Strengthening methods can increase the bulk hardness of wheel steel to different extents, where the highest and lowest bulk hardness increment is obtained by the solid solution and precipitation strengthening, respectively.Strain hardening is promoted by carbon addition, while it is reduced by solid solution strengthening and precipitation strengthening where precipitation strengthening steel has a greater reduction in strain hardening. Strain hardening of wheel steel is reduced by a high content of proeutectoid ferrite with a low ductility, which is caused by solid solution strengthening and precipitation strengthening.In addition, surface initiated RCF and thermally induced RCF of HiSi steel were investigated. In this study, white etching layer (WEL) was successfully reproduced on the surface of test disc through simulating wheel-rail sliding in the laboratory. The resistance to WEL formation of ER8 and HiSi wheel steels was analyzed by phase transformation kinetic.The results show that silicon addition improves the resistance to WEL formation for HiSi steel through increasing the austenitization temperature. RCF behavior of ER8 and HiSi wheel steels was then evaluated by rolling/sliding tests and finite element analysis. The results show that WEL remarkably reduces the RCF life of wheel steel. Whether containing WEL or not, HiSi steel exhibits a better fatigue strength than ER8 wheel steel.3. Research on influence of laser dispersed treatment on resistance to wear and surface initiated RCF of wheel steelIn this study, laser dispersed treatment was used to improve the resistance to wear and surface initiated RCF of wheel steel. Laser dispersed treatment creates isolated glazed regions on the surface layer of railway wheel steel, which are composed of fine martensite and retained austenite and have an average hardness of 762 HV. The resistance to wear and surface initiated RCF of the laser treated wheel steel was investigated by rolling/sliding tests.The test results show that laser dispersed treatment suppresses the plastic deformation occurring at the surface. This inhibits the treated railway wheel steel from delamination wear and delays the formation of fatigue crack initiation. Wear rate of the laser treated railway wheel steel is about 1/3 that of untreated railway wheel steel and the average rolling contact life of treated railway wheel steel is about double that of the untreated steel.4. Research on effect of micro-inclusion on subsurface initiated RCF and RCF initiated at deep defects for railway wheelSubsurface initiated RCF and RCF initiated at deep defects are the dangerous wheel damages. Both of the RCF failures are associated with the largest inclusion in the railway wheel. Thus, a novel method for evaluating the maximum micro-inclusion size in railway wheel was developed and then the effect of micro-inclusion on subsurface initiated RCF and RCF initiated at deep defects was investigated. In this study, the micro-inclusion size in the railway wheel steel was obtained successfully by ultrasonic fatigue test using pretreated specimens with large risk volumes under the water cooling condition. Evaluation of the inclusion size by the proposed method is more accurate than the traditional approach based on inclusion size characterization from arbitrary two dimensional cross sections. As the method is based on fatigue testing in the ultrasonic frequency regime, it can be conducted in a reasonable amount of time. The maximum micro-inclusion size in the risk volume of the full-scale railway wheel was estimated based on the proposed evaluation method and the statistics of extreme values method. The quantitative effect of micro-inclusion on subsurface initiated RCF and RCF initiated at deep defects for railway wheel was analyzed based on the fracture mechanics.