表面凹坑缺陷对高速列车车轴钢疲劳性能影响研究

发布时间：2018-11-03 10:36

【摘要】：运行安全是高速动车组关注的首要问题,高速动车组车轴的疲劳寿命直接关系到高速动车组的行车安全。本论文针对现役S38C表面感应淬火高速动车组车轴在异物高速冲击下,其表面凹坑缺陷对疲劳寿命的影响开展细致研究,旨在为评价该类表面凹坑缺陷对车轴的疲劳寿命影响和提出该类表面凹坑缺陷的修复方法提供参考依据。高速动车组车轴设计寿命一般为30年或者设计服役循环周次约为2×109。通常,造成车轴疲劳断裂的主因是车轴表面损伤,这类表面损伤主要有:局部应力腐蚀、冶金缺陷与夹杂、过盈配合压装表面微动损伤等。随着高速动车组在中国客运专线上的大量载客运行和动车所维修维护工作的跟进,发现高速运行中的高速动车组车轴由于异物冲击,造成大量的车轴表面凹坑缺陷。目前,尚无针对此类缺陷对车轴疲劳寿命影响的系统研究。为系统评价车轴表面异物冲击造成的凹坑缺陷对车轴疲劳性能的影响,本论文开展了如下研究工作:首先,根据形貌特征对S38C车轴表面的高速冲击造成的凹坑缺陷进行分类,采用硬度计、光镜、透射电子显微镜和电火花直读光谱仪分析车轴表面和心部的微观组织和化学成分,在此基础上设计实验;其次,设计电火花加工缺口和人工压痕两种缺陷,结合疲劳试验机、硬度计、光镜、透射电子显微镜、扫描电子显微镜和EDS、XRD等手段研究两种缺陷对三种不同表面硬度梯度的车轴钢标准疲劳试样疲劳性能的影响;再者,截取S38C车轴上含有表面淬火层的四点弯曲疲劳试样,研究电火花加工缺口和人工压痕分别对其疲劳性能的影响,同时,设计非标的方法评价了 S38C车轴基体和表面感应淬火层疲劳裂纹扩展速率和门槛值。最后,对激光熔覆修复车轴表面电火花加工缺口和人工压痕工艺进行了探索性研究。本论文主要结论如下:1. S38C车轴表面异物冲击凹坑缺陷分为垂直撞击形成的沟槽和切向撞击产生的擦痕两类,车轴表面硬化层组织为高密度位错和晶格畸变的回火马氏体,心部组织为纯净珠光体和铁素体;2. S38C车轴钢人工压痕标准试样和电火花加工缺口标准试样疲劳极限均随着缺陷投影面积的增加而降低,含有电火花加工缺口试样的疲劳极限符合Murakami公式计算结果,人工压痕试样疲劳极限可用修正后的Murakami公式进行描述;3.经表面感应淬火的S38C车轴钢标准试样疲劳极限均随着人工压痕尺寸的增大而降低,先淬火回火再引入人工压痕标准疲劳试样的疲劳极限降幅较大;4. S38C车轴钢氮碳共渗标准试样疲劳极限相对于未处理试样提高了 12%,疲劳裂纹萌生于试样表面疏松氧化层;人工压痕造成化合物层破裂,疲劳极限随着人工压痕尺寸增大迅速降低,直至低于未处理人工压痕试样;抛光去除氮碳共渗化合物硬层可有效降低人工压痕对试样疲劳强度的影响。5.车轴表面淬火层的四点弯曲标准疲劳试样的疲劳极限随着电火花加工缺陷深度的增加而降低,人工钨钢球压痕对试样疲劳极限没有影响。钨钢球破碎产生的二次压痕底部具有微裂纹,试样疲劳极限降低。表面回火马氏体层的裂纹扩展门槛值为1.43MPa√m,远低于基体的 5.66 MPa√m;6.激光熔覆修复车轴表面凹坑缺陷带来的新缺陷导致试样疲劳性能差异。基于以上研究结果可知,对于塑形较好的S38C车轴钢标准疲劳试样,局部材料在人工压痕形成过程中产生大量的位错强化了缺陷处材料性能,可用引入强化常数Q修正后的Murakami公式预测含此种缺陷试样的疲劳极限。对于表面感应淬火回火形成的回火马氏体,在受到塑形变形时晶粒纳米化,塑形降低,受到冲击时多方向剪切变形形成微裂纹。车轴表面淬火层裂纹扩展门槛值低于基体材料,在循环应力作用下微裂纹极易扩展造成此类车轴失效。
[Abstract]:Operation safety is the primary concern of high-speed motor train unit, and the fatigue life of high-speed motor train unit is directly related to the driving safety of high-speed motor train unit. In this paper, the influence of surface pit defects on fatigue life of high-speed motor train EMUs at high speed under high-speed impact of foreign objects is studied in this paper. The purpose of this paper is to provide a reference basis for evaluating the effect of pit defects on the fatigue life of the axle and the method for repairing the defect of the surface pit. The design life of the axle of the high-speed motor train unit is generally 30 years or the design service cycle is about 2 mm109. In general, axle fatigue fracture is caused by the damage of axle surface, which mainly includes local stress corrosion, metallurgical defect and inclusion, over-interference fit pressing surface fretting damage and so on. With the follow-up of the operation and maintenance of the high-speed motor train unit in China's passenger dedicated line, it is found that the axle of the high-speed motor train unit in high-speed operation results in a large number of axle surface pit defects due to the impact of foreign matters. At present, there is no systematic study on the effect of such defects on the fatigue life of axles. In order to evaluate the effect of pit defects on axle fatigue performance caused by foreign body impact on axle surface, this paper carried out the following research work: Firstly, the pit defects caused by the high speed impact of S38C axle surface are classified according to the topographical features, and the hardness tester and light microscope are adopted. The microstructure and chemical composition of axle surface and core were analyzed by transmission electron microscope and electric spark direct reading spectrometer, and the experiment was designed on this basis; secondly, two defects of electric spark machining notch and artificial indentation were designed, combined with fatigue testing machine, hardness tester and light microscope. The effects of two kinds of defects on fatigue properties of axle steel standard fatigue specimens with different surface hardness gradient were studied by transmission electron microscope, scanning electron microscope and EDS, XRD, etc. The fatigue crack growth rate and threshold value of S38C axle matrix and surface induction hardening layer were evaluated by the design of non-target method. Finally, the notch and artificial indentation process of laser cladding repairing axle surface were studied. The main conclusions of this thesis are as follows: 1. S38C axle surface foreign matter impact crater defects are divided into two types of grooves formed by vertical impact and scratches generated by tangential impact, the hardened layer of axle surface is tempered martensite with high density dislocation and lattice distortion, and the core part is pure pearlite and ferrite; 2. The fatigue limit of the S38C axle steel artificial indentation standard test specimen and the electric spark machining notch standard specimen decreases with the increase of the defect projection area, and the fatigue limit of the sample containing the electric spark machining notch conforms to the calculation result of Murakami formula. The fatigue limit of the artificial indentation test specimen can be described with the modified Murakami formula; 3. The fatigue limit of S38C axle steel subjected to surface induction hardening is reduced with the increase of the size of the artificial indentation. the fatigue limit of the S38C axle steel nitrocarburizing standard sample is improved by 12% relative to the untreated sample, the fatigue crack is formed on the loose oxide layer on the surface of the sample, the artificial indentation causes the compound layer to crack, the fatigue limit decreases rapidly with the increase of the size of the artificial indentation, The effect of the artificial indentation on the fatigue strength of the specimen can be effectively reduced by polishing the hard layer of the nitrogen-carbon co-infiltration compound until it is lower than that of the untreated artificial indentation sample. The fatigue limit of the four-point bending standard fatigue test specimen of the axle surface quenching layer decreases with the increase of the depth of the electric spark machining defect, and the indentation of the artificial tungsten steel ball has no influence on the fatigue limit of the sample. The bottom of the secondary indentation produced by crushing the tungsten steel ball has micro-cracks, and the fatigue limit of the specimen is reduced. The crack propagation threshold value of the surface tempered martensite layer is 1. 43MPa/ m, which is much lower than that of the matrix. A new defect in the surface pit defects of the laser cladding repairing axle leads to a difference in fatigue performance of the specimen. Based on the above results, it can be seen from the above research results that, for the S38C axle steel standard fatigue test specimens with better shape, a large number of dislocations are generated during the process of artificial indentation formation, The fatigue limit of the sample containing such defects can be predicted using the Murakami formula after correction of the reinforcement constant Q. For the tempered martensite formed by quenching and tempering on the surface, the grain is nano-shaped when subjected to shaping deformation, the shaping is reduced, and the micro-cracks are formed by the multi-directional shear deformation at the time of impact. The crack propagation threshold value of the surface quenching layer of the axle is lower than that of the matrix material, and the micro crack is easy to expand under the action of cyclic stress to cause such axle failure.
【学位授予单位】：西南交通大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：U270.33;TG142.12

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