【作者】 吕博；

【导师】 张福成；

【作者基本信息】 燕山大学 ， 材料学， 2009， 博士

【摘要】 本文首先对我国实际铁路线路上使用失效的158组高锰钢辙叉进行统计,分析高锰钢辙叉失效规律;对10棵高锰钢辙叉的实际使用情况进行跟踪测试,研究辙叉工作表面断面相貌、表面硬度和外形尺寸在服役过程中的变化过程;解剖失效高锰钢辙叉,研究其亚表层的硬度分布、微观组织结构以及微观纳米力学性能的演变规律。在实验室高温管式炉中,利用吹氮和变质复合处理方法对高锰钢钢液进行精炼处理,在此结果的基础上,研究纯净致密高锰钢辙叉的制造工艺技术。同时,对模拟高锰钢辙叉心轨的小试样进行爆炸硬化试验,研究高锰钢爆炸硬化工艺参数及其材料行为,由此,对高锰钢辙叉进行爆炸硬化处理。将纯净致密高锰钢辙叉和爆炸硬化处理的高锰钢辙叉分别上道试验,并得到了广泛的应用。结果表明:高锰钢辙叉失效分为三个阶段,变形磨损、摩擦磨损和疲劳磨损。疲劳辙叉亚表层内存在与踏面平行的裂纹群,裂纹群形成位置在亚表层深度2～3.5mm范围内。提高高锰钢辙叉使用寿命的途径是提高其内在质量、增加其初始硬度以及改善其加工硬化能力。高锰钢辙叉在服役过程中,表面形成纳米晶层,其形成机理是,高锰钢辙叉在滚动接触应力反复作用过程中,当其中的变形储存能达到一定程度后,纳米晶粒直接从变形组织中通过应变诱发动态再结晶形成。高锰钢辙叉表面由粗晶到非晶转变的吉布斯自由能差为7.3 kJ/mol,而粗晶到纳米晶转变的吉布斯自由能差总是正值,这意味着这种转变不可能自发形成,必须有外界作用才可以发生,并且是由粗晶到细晶再到纳米晶的细化过程。高锰钢辙叉在服役过程中表面温升越小越容易形成纳米晶组织。辙叉亚表面变形组织没有形成白亮蚀层(WEL),其原因是,高锰钢在变形过程中不存在碳化物的回溶过程。高锰钢的微观弹性模量主要决定于其内部的空位密度,与位错等其它缺陷关系不大。高锰钢辙叉承受滚动接触应力作用下,疲劳裂纹的形成机制是大量的空位聚集、合并形成微孔,大量的微孔连接形成裂纹。利用原子直径较大的重金属对钢铁材料进行再合金化处理,其抗滚动接触疲劳性能应该较高。利用专用变质剂和底吹氮精炼技术处理高锰钢钢液,结合V法真空造型工艺技术制造纯净致密高锰钢辙叉,致使其中的P、S、[H]和[O]分别降低2.7、5.2、2.5和4倍,而[N]含量增高23倍。变质剂的最佳成分为(wt%):CaO25%,CaF2 25%,Re-Mg合金50%组成的粉末状混合物,粉末的平均粒度为50～100目,其加入量为1wt%。吹氮净化处理时,氮气的平均压力为0.6MPa,流量为1 m3/t?h,吹氮处理时间为15min。纯净致密化处理提高了高锰钢辙叉的屈服强度、抗拉强度和冲击韧度等力学性能,使高锰钢辙叉使用寿命提高30%以上。利用厚度为3 mm的RDX炸药对高锰钢辙叉工作表面爆炸预硬化2次,并且两次起爆点错位20 mm以上,可获得表面硬度为380HV、硬化层深度为40mm,而且硬度梯度平缓过渡的理想硬化效果,从而使高锰钢辙叉的实际使用寿命提高30%以上。高锰钢辙叉表面爆炸硬化的机理是表层变形量较大的区域为形变孪晶和位错强化、内部变形量较小的区域主要为位错强化,高锰钢爆炸硬化表面变形机理是原位塑性变形。更多还原

【Abstract】 158 groups of failed Hadfield steel crossings used in the railway system of China were analyzed statistically to study the failure mechanism of the crossing. 10 Hadfield steel crossings were undertaken tracking test to research the change processes of the morphologies of cross-section, the hardness and the size of the working crossing surface during their services processes. Thus, the failed Hadfield steel crossings were anatomized to study the hardness distribution, the microstructures and the properties of micro/nano-mechanical behavior of their subsurface. In the laboratory a high-temperature tube furnace was used to study the experimental parameters of pure Hadfield steel, the Hadfield steel liquid was refined by blowing nitrogen and compound modification approach. Based on the results, the manufacturing technology of the pure Hadfield steel crossing was studied. Besides, the pure Hadfield steel crossing was tested in the actual railway. At the same time, explosion hardening experiment was carried out with simulated point Hadfield steel crossing small samples to study the technical parameters of the explosion hardening. Based on the results of the research in laboratory, explosion hardening of the Hadfield steel crossing was undertaken. Furthermore, the Hadfield steel crossing subjected to explosion hardening was used in actual railway system.The results show three stages for the Hadfield steel crossing failure including deformation wear, friction wear, and fatigue wear. There are crack groups existing in the subsurface of the fatigue crossing in parallel with the tread base. The location of the crack groups’formation is in the range of 2~3.5 mm depth of the subsurface. The way to improve the lifetime of the Hadfield steel crossing is to improve its intrinsic quality, to increase its initial hardness and to improve its work-hardening ability. During the service process of the Hadfield steel crossing, the nanocrystalline layer is formed in the surface. Its formation mechanism is that the nanograins are formed directly through the dynamic recrystallization in the deformation structures when the stored deformation energy reaches certain degree in the repeated process of the rolling contact. The difference of the Gibbs free energy is 7.3 kJ/mol for the transformation from coarse-grain to amorphous in the surface of the Hadfield steel crossing. However, the difference of the Gibbs free energy is always positive for the transformation from coarse-grain to nanocrystalline. This means that such transformation can not be formed spontaneously and the external power must be included. The refined process is from coarse-grain to fine-grain and finally to nanocrystalline step by step. The smaller the surface temperature raises the easier formation of the nanostructure during the service process of the Hadfield steel crossing. The deformation structures do not form white etching layer (WEL) in the subsurface of the crossing. The reason is that there is no re-dissolution carbide process during the deformation process of the Hadfield steel.The micro elastic modulus of the Hadfield steel mainly depends on the vacancy density and has little to do with other defects, such as dislocation. Under the rolling contact stress for the Hadfield steel crossing, the formation mechanism for the fatigue crack is a lot of vacancy gathering and combining to form microporous. Then a large number of microporous connect to form cracks. The anti-rolling contact fatigue performance of the steel materials should be higher through alloying process treatment with larger diameter atom heavy metal.Using the special modifier and the refining technology of bottom blowing nitrogen to deal with the Hadfield steel liquid combining with V process of vacuum sealed molding technology, the high-purity-compact Hadfield steel crossing is made, which results in the content of P, S, [H] and [O] decreasing by 2.7, 5.2, 2.5 and 4 times respectively, whereas, the content of [N] increasing by 23 times. The best component(wt%)of the special modifier is CaO 25%,CaF2 25%, and Re-Mg alloy powder mixture 50%. The average particle size of the powder is 50~ 100 meshes with addition of 1wt%. During the process of the purification treatment with nitrogen blowing, the average nitrogen pressure is 0.6 MPa, the flow rate is 1 m3/t?h, and the nitrogen blowing processing time is 15 min. The pure-dense treatment improves the yield strength, tensile strength, impact toughness and other mechanical properties of the Hadfield steel crossing, which increases the lifetime of the Hadfield steel crossing more than 30%.The work surface of the Hadfield steel crossing is treated with two pre-hardening explosions by using a thickness of 3 mm of RDX explosive. Moreover, the point dislocation for the two initial-explosion is above 20 mm, which is available for the 380HB surface hardness, 40mm hardened layer depth, and the ideal explosion hardening effect of the hardness gradient smooth transition. Thus, the actual lifetime of the Hadfield steel crossing is increased by more than 30%. The surface explosion hardening mechanism of the Hadfield steel crossing is that there are the deformation twinning and dislocation strengthening in the region of larger surface deformation, whereas the dislocation strengthening mainly in the internal region of smaller deformation. The explosion hardening surface deformation mechanism of the Hadfield steel is the plastic deformation in-situ.更多还原