Fatigue Reliability Behaviors Of High-speed Railway Axle Steel In Very High Cycle Regime And Methods For Fatigue Strength Improvement

The railway axle is one of the most important structures in the railway system and it nearly bears the whole loading of the vehicle. The failure of an axle usually results in a derailment, which may cause further disastrous consequences. The axle damage mainly includes conventional rotary bending fatigue and fretting fatigue at the wheel seat. The design fatigue life of the axle is30years and the axles of high-speed vehicles will experience more than3x109cycles during this time. With the rapid development of high-speed railway in China, the300km/h high speed multiple units has been developed and the indigenization rate of vehicle production is more than70%. However, all the wheels and axles have to be imported. The cost of one pair of wheels/axles is more than600thousand RMB, which will be decreased a lot if they can be produced domestically. In Japan the medium carbon steel with surface hardening treatment is applied for the high-speed railway axles, while the alloy axle steel is used in Europe. Though some experiences of design, manufacture, application and maintenance are obtained in these counties, the fatigue tests on railway axle steel are limited by107cycles and the method preventing the fatigue failure of axle in very high cycle fatigue regime (107-10cycles) is NDT (non destructive testing) technology. Therefore, it is necessary to conduct the research on fatigue reliability behaviors of high-speed railway axle steel in very high cycle regime and methods for fatigue strength improvement. The results can supply important information for material selecting, probability design and safety evaluation for axles.This work was supported by the National Basic Research Program of China (No.2007CB714705), the National Natural Science Foundation of China (No50671086) and Doctoral Innovation Funds of Southwest Jiaotong University. In this dissertation, the very high cycle fatigue tests of LZ50axle steel and35CrMo axle steel were conducted, the fatigue properties under constant and repeated two-step loading were investigated and the effect of surface treatment on fatigue strength was studied in detail. The main contents and conclusions of this dissertation as follows:(1) Research on fatigue properties of railway axle steel in very high cycle regime and reliability evaluation methodThe109cycles fatigue tests of LZ50axle steel and35CrMo axle steel under constant stress loading were carried out at room temperature in air with a four-axle cantilever-type rotary bending fatigue machine. The S-N curves were obtained and the fracture surfaces were observed and the properties of the steels in very high cycle fatigue regime were analyzed in detail. The results show that the LZ50axle steel and35CrMo axle steel have the conventional fatigue limit in the range of106and109cycles, while the35CrMo steel can fracture from surface after108cycles and this is considered to be caused by the very slow crack propagation. The S-N data of LZ50steel above the fatigue limit satisfy three-parameter equation while that of35CrMo steel satisfy two-parameter basquin equation. According to the observations of the fracture surfaces, it can be seen that all the cracks of the LZ50steel and35CrMo steel specimens originate from the material surface caused by the crystal slip. When high stress amplitude is applied, there are several crack initiation locations on the fracture surface, whereas, at the low stress amplitude there is only one crack source. Though the mean values of fatigue limit is larger for35CrMo steel, the standard deviation of fatigue limit is also large for35CrMo steel.In addition, the reliability strength evaluation is necessary when the railway axles are designed, whereas, it is impossible to carry out the test with large number of specimens, especially when it refers to the test with the full-scale axle. So the reliability evaluation method with small sample is needed for the high-speed railway axle design. Based on the S-N data obtained from the constant stress loading tests, the reliability evaluation method with small sample in the finite fatigue life region was proposed. The practicability of the proposed method is validated by staircase method fatigue tests and it is found that the result evaluated by the proposed method basically corresponds with that of the staircase tests.(2) Research on fatigue damage and very high cycle fatigue properties of railway axle steel under variable loadingDue to the fact that variable loading is applied to axles during the service, the fatigue cumulate damage of railway axle steel under variable loading is needed for the fatigue life evaluation. The repeated two-step loading tests on the medium carbon railway axle steel were conducted with the four-axle cantilever-type rotary bending fatigue machine. A method was proposed to evaluate the fatigue damage and the influence of the loading cycle ratio of high/low stress amplitude on fatigue damage was investigated. In this method by applying the cyclic stress-strain relationship of the material, the stress-life data were transformed to the plastic strain-life data, which satisfy the linearity. Afterward the fatigue damage under repeated two-step loading is evaluated with the plastic strain based on the strain linear cumulate damage rule. The results show that the fatigue limit disappears under the repeated two-step loading and the fatigue damage under repeated two-step loading can be well evaluated based on the modified plastic strain-life curves. Meanwhile, the fatigue damage is significantly influenced by the loading cycle ratio of high/low stress. When the ratio becomes larger, the strain-life data shift to short life region, otherwise they shift to long life region.(3) Research on effect of surface treatment on fatigue strength improvement of railway axle steel in very high cycle regimeThe surface treatment methods applied to the axles mainly include the surface cold treating, such as shot peening and rolling, and surface heat treating, such as induction hardening, carburizing, nitrocarburizing. Now, both of them are applied to the high speed railway axles. In this study the LZ50steel and35CrMo steel are treated by microshot peening and nitrocarburizing, respectively, and the109cycles tests are conducted to investigate the fracture behaviors and fatigue properties after surface treatment. For the microshot peened specimens, the reliability and relaxation mechanism of surface compress residual stress as well as the factor that controls the fatigue strength improvement were analyzed; meanwhile the match of the microshot peening process with material properties was discussed. For the nitrocarburized specimens, the effect of surface layers on fracture behaviors and fatigue properties in very high cycle fatigue regime was investigated in detail. The results show that the depth of the surface hardening layer induced by the microshot peening is small and the surface compress stress induced by the treatment can relax largely during the fatigue process. So the fatigue strength was not improved much by the microshot peening treatment. Whenas large depth of hardening layer can be obtained and there is no residual stress relaxation, so the fatigue strength was improved largely after the nitrocarburization treatment. All the specimens fractured from surface after microshot peening treatment, while they could fail from the internal defect or inclusion after the nitrocarburizing treatment. Therefore, it can be seen that the heat treatment such as nitrocarburization is more suitable for the application to the full-scale axles, but the alloy axle may fail from the internal inclusion in the long fatigue life regime after the surface treatment. Besides, the axial loading tests for gas nitrocarburized LZ50steel and35CrMo steel were carried out. The fracture behaviors and fatigue properties of the steels under axial loading were investigated. Meanwhile the fatigue strength improvement of nitrocarburized railway axle steel under axial loading was estimated.(4) Research on effect of inclusion size on fatigue strength of railway axleThe35CrMo alloy axle steels cannot fracture from internal inclusion under rotary bending loading, while they can fracture from the internal inclusions after nitrocarburization. Meanwhile, the stress gradient in full-scale axle is much smaller than that of the small specimen and the risk volume of full-scale axle is much lager than that of the small specimen. So it is possible for the full-scale axles to fracture from the internal inclusion. In order to ensure the safety of the railway axle, it is necessary to investigate the inclusion size in the35CrMo axle steel and analyze the effect of inclusion size on fatigue strength of full-scale axles in very high cycle fatigue regime. In this study, the ultrasonic fatigue tests were conducted by applying large risk volumes specimens with straight section. The inclusion sizes in the specimens were investigated and the maximum inclusion size in large volume was estimated according to the statistics of extreme values (SEV) method. The quantitative effect of inclusion size on fatigue strength of full-scale axles was analyzed based on the fracture mechanics.