Mechanical properties and fracture toughness of rail steels and thermite welds at low temperature

Brittle fracture occurs frequently in rails and thermite welded joints, which intimidates the security and reliability of railway ser- vice. Railways in cold regions, such as Qinghai-Tibet Railway, make the problem of brittle fi＇acture in rails even worse. A series of tests such as uniaxial tensile tests, Charpy impact tests, and three-point bending tests were carried out at low temperature to investigate the mechanical properties and fracture toughness of U71Mn and U75V rail steels and their thermite welds. Fracture micromechanisms were analyzed by scanning electron microscopy （SEM） on the fracture surfaces of the tested specimens. The ductility indices （percentage elongation aider frac- ture and percentage reduction of area） and the toughness indices （Charpy impact energy Ak and plane-strain fracture toughness Kic） of the two kinds of rail steels and the corresponding thermite welds all decrease as the temperature decreases. The thermite welds are more critical to fracture than the rail steel base metals, as indicated by a higher yield-to-ultimate ratio and a much lower Charpy impact energy. U71Mn rail steel is relatively higher in toughness than U75V, as demonstrated by larger Ak and Klc values. Therefore, U71Mn rail steel and the corresponding thermite weld are recommended in railway construction and maintenance in cold regions.

Rail temperature variation under heavy haul operations

There currently does not exist in industry a reliable method for the detection of rail foot flaws.Like their head-based counterparts,foot flaws result in broken rail with potentially catastrophic consequences.A proposed area of research for the detection of these flaws is thermography,a non-contact method of measuring and analysing infrared emissions from an object under test.In industry,active excitation thermography is the most common,requiring an excitation source.This paper will present a temperature measurement system and a method of transient temperature extraction from the running rails for the effects of a passing train to evaluate heat transfer in the practical rail environment.The outcomes of these results will provide future direction in the development of a rail heat transfer model and determine if train passage provides enough active excitation for a thermography-based detection technique.

Effect of Preheating Temperature on the Mechanical and Fracture Properties of Welded Pearlitic Rail Steels

The effect of preheating temperature on the mechanical and fracture behavior, hardness, and the microstructure of slot welded pearlitic rail steel were studied. Railhead sections with slots were preheated to 200℃, 300℃, 350℃ and 400℃?before gas metal arc filling to simulate defects repair. Another sample, welded at room temperature (RT) with no preheat, was studied in comparison. The parent rail steel has ultimate strength, yield strength and strain to failure of 1146 MPa, 717 MPa and 9.3%, respectively. Optimum values of these properties for the welded rail steels were found to be 1023 MPa, 655 MPa and 4.7%, respectively, for the 200℃ preheat temperature. On this basis, the optimum weld efficiency was found to be 89.2%. The average apparent fracture toughness KI for the parent rail was 127 MPa.m0.5, while that for the optimum welded joint (200℃ preheat) was 116.5 MPa.m0.5. In addition, the average hardness values of the weld, fusion zone, and heat affected zone (HAZ) were 313.5, 332 and 313.6 HB, respectively, while that for parent rail steel was about 360 HB. Dominance of bainite and acicular ferrite phase in the weld microstructure was observed at 200℃ preheat.

Novel Analytical Model of Mean Temperature and Experimental Research on the Rail Universal Rolling

For building the analytical model of mean temperature in rail universal rolling, the cross-section of workpieces and the profile of horizontal roll and vertical roll are simplified rationally. The mean temperature of the web of rail, the top of rail and the base of rail are considered individually. The temperature rises for plastic deformation and friction incorresponding deformation zone, the temperature drop for contact is calculated on the base of variation principle and energy conservation law. Then the mean temperature is obtained. For verifying the theoretical model, the 18 kg/m light rail universal rolling experiments are accomplished in Yanshan University Rolling Laboratory, China and the surface temperature is measured. The surface temperature is not exact enough to express the true temperature and the mean temperature can show the status of the true temperature basically. So the mean temperature can be used to express the true temperature and this theoretical model and its results can be applied as an important reference to control the temperature of rail universal rolling and the heat treatment of the rolled rail.

Research on Hot Plasticity of Continuous Cast Blank for Heavy Rail between BNbRE and BNbRE(V) at High Temperature

The text determines the hot plasticity of CC blank for heavy rail between BNbRE and BNbRE(V) under 1300～850 ℃ by using Gleeble 1500D. It also makes comparative analysis of the microstructure and fracture of the tensile specimens by scanning electronic microscope. The result indicates that hot plasticity is almost zero when temperature is between 1200～1300 ℃. While hot plasticity of BNbRE(V) is better than BNbRE between 975～1200 ℃. Plasticity decreases with the reducing temperature from 975 ℃. Hot plasticity of BNbRE(V) reaches the minimum because of the precipitation of vanadium in the specimens reaches the maximum at 850 ℃.Vanadium largely affects the plasticity of steel in brittle temperature region Ⅲ and the embrittlement extends to lower temperature.

Sepiolite:A new component suitable for 380 km/h high-speed rail brake pads

To enhance the high-temperature adaptability of copper-based composite materials and C–C/SiC discs,this article innovatively introduces a method of replacing graphite with sepiolite,resulting in the successful fabrication of samples with exceptional mechanical and friction properties.The results reveal that moderate incorporation(less 6%)of sepiolite provides a particle reinforcement effect,resulting in an improvement of mechanical properties.Interestingly,the addition of sepiolite causes a change in the traditional saddle-shaped friction curve due to high temperature lubrication.Meanwhile,the primary advantage of sepiolite lies in its superior abrasion resistance,evident in the increased friction coefficient and altered wear mechanisms with higher sepiolite content.The wear resistance is optimal at 200 Km/h(400℃).Particularly,the unique composition of the friction layer(outermost layer:a composite film consisting of B2O3,sepiolite,graphite,and metal oxide films;intermediate layer:metal oxide films)plays a pivotal role in improving friction stability.Finally,there are significant optimizations in the GA algorithm,especially GA-GB model has the best prediction effect on the maximum friction temperature.

ANALYSIS OF THERMAL-ELASTIC STRESS OF WHEEL-RAIL IN ROLLING-SLIDING CONTACT

A coupling thermo-mechanical model of wheel/rail in rolling-sliding contact is put forward using finite element method. The normal contact pressure is idealized as the Hertzian distribution, and the tangential force presented by Carter is used. In order to obtain thermal-elastic stress, the ther-mal-elastic plane stress problem is transformed to an elastic plane stress problem with equivalent fictitious thermal body force and fictitious boundary distributed force. The temperature rise and ther-mal-elastic stress of wheel and rail in rolling-sliding are analyzed. The non-steady state heat transfer between the contact surfaces of wheel and rail, heat-convection and radiation between the wheel/rail and the ambient are taken into consideration. The influences of the wheel rolling speed and wear rate on friction temperature and thermal-elastic stress are investigated. The results show the following： ① For rolling-sliding case, the thermal stress in the thin layer near the contact patch due to the friction temperature rise is severe. The higher rolling speed leads to the lower friction temperature rise and thermal stress in the wheel; ② For sliding case, the friction temperature and thermal stress of the wheel rise quickly in the initial sliding stage, and then get into a steady state gradually. The expansion of the contact patch, due to material wear, can affect the friction temperature rise and the thermal stress during wear process. The higher wear rate generates lower stress. The results can help under-stand the influence of friction temperature and thermal-elastic stress on wheel and rail damage.

国六柴油机轨压和喷油正时对柴油机性能的影响

为优化柴油机性能,基于一台配备了完整国Ⅵ后处理系统的高压共轨直列4缸柴油机,开展了柴油机负荷特性研究,在此基础上研究轨压和喷油正时对柴油机性能的影响。研究结果表明:以适当较小的轨压配合较为提前的喷油正时可以达到较为理想的柴油机油耗及排放;轨压和喷油正时对HC排放的影响较小;轨压对柴油机油耗、排温、NOx排放、CO排放的影响较大,且在中等偏高负荷时更为敏感,随着轨压增加了57 MPa,柴油机油耗、排温分别下降了16.1 g/(kW·h)、63℃,NOx排放上升了663×10^(-6),CO排放下降了785×10^(-6);喷油正时对烟度及柴油机氧化催化器前端温度T4、柴油机颗粒捕集器前端温度T5、选择性催化还原系统前端温度T6的影响较大,且在低负荷时更为敏感。随着喷油正时提前6.0°,烟度下降了0.43%,T4、T5、T6峰值出现在轨压为76 MPa且喷油正时为上止点前7.5°时,温度分别为315.3、338.4、329.3℃。

考虑材料温变特性的三维轮轨接触热分析

为研究材料温变特性对轮轨接触行为和摩擦温升的影响,提出了一种考虑材料温变特性的三维轮轨热力耦合模型,能够考虑纵、横向蠕滑率和自旋的影响,更为真实地模拟轮轨系统的服役状态.首先,研究了热力耦合建模方式对轮轨界面摩擦温升及接触应力的影响;随后,将该模型应用于地铁小半径曲线处车辆-轨道相互作用模拟.结果表明:当轮轨界面温度达到450℃时,轮轨接触应力显著降低,降幅可达20%;考虑热力耦合建模后,轮轨界面的预测温升明显低于不考虑热力耦合建模时的结果,在蠕滑率为0.16时,两者的差异可达51%;地铁车辆在小半径曲线线路上运行时轮轨摩擦温升因过大的蠕滑率与自旋会急剧增大到750℃,应考虑轮轨热力耦合建模以避免过高估计轮轨摩擦温升以及轮轨接触应力.

高原地区无缝道岔健康监测研究

研究目的:无缝道岔是无缝线路的关键技术之一,高原地区的气候复杂使其受力更为复杂,为保证列车的安全运营,有必要对其进行长期实时健康监测。本文针对高原地区无缝道岔构建实时健康监测系统,并对一组道岔进行为期2个月的在线监测,对监测期内的轨温、尖轨尖端位移及尖轨断轨进行相应分析。研究结论:(1)钢轨温度变化趋势与气温变化趋势相同,最高轨温始终高于日最高气温,平均差值为11.75℃,其出现时间总体来讲早于气温出现3 h左右;(2)最低轨温与气温值基本相同,其出现时间也基本相近,大致于早上7:00—8:00出现;(3)钢轨温度和大气温度之间存在较好的一次线性关系,当气温变化1℃时轨温变化约为1.5℃;(4)尖轨尖端的纵向位移变化趋势与气温和轨温变化趋势相同,与气温间呈一次线性关系;(5)本文研究可为高原地区无缝道岔的养护维修提供数据支撑。

钢轨轨底裂纹扩展影响因素分析

【目的】为分析钢轨材质、残余应力、温度应力在钢轨使用中的内外因对轨底裂纹扩展的影响开展研究。【方法】基于断裂力学理论结合轨底伤损实测,建立钢轨轨底裂纹扩展模型,对轨底伤损扩展失效时的临界裂纹尺寸、裂纹平均扩展速率、剩余寿命等指标进行量化分析。【结果】结果表明:钢轨材料断裂韧性与轨底初始伤损扩展的剩余寿命存在正相关关系,U95Cr H材质钢轨的轨底裂纹扩展剩余寿命小于U78CrV H材质小于U75V H材质;以某线路实际使用的钢轨材质U78CrV H为例,轨底残余应力由0增加至300 MPa,轨底裂纹扩展的临界尺寸增长约2.5%,剩余寿命下降约6%;残余应力为200 MPa时,温度应力由0增加至200 MPa,轨底裂纹扩展的临界尺寸变化不大,在1%内。【结论】研究结果可为轨底裂纹扩展研究和养护维修提供参考。

基于销盘载流摩擦平台的电磁发射轨道表面防护

为避免电磁发射轨道表面转捩烧蚀损伤,对轨道表面防护工作进行了研究。首先,采用实验室自研的销盘试验机开展了极端工况载流摩擦实验;然后,利用COMSOL软件分析了实验过程瞬态温度场的特征;最后,考虑在盘试样表面采用耐高温的钼涂层,仿真对比了处理前后的温度场分布情况。研究表明:因失接触产生的高能电弧是影响滑动电接触瞬态温度场的主要因素之一,会对无处理的盘试样表面造成可达20μm深的烧蚀坑;进行钼涂层处理后的表面温度更高,处理前后的表面温差最高可达数百摄氏度,主要因为钼的热导率较低;在轨道表面处理选材时,要统筹考虑耐高温性能和导热性能。该研究成果对于延长轨道使用寿命具有重要意义。

基于轨道不平顺的无缝线路稳定性统计分析

为弥补现有无缝线路稳定性设计未区分轨道平顺性等级的不足,基于轨道不平顺数值模拟方法和无缝线路稳定性有限元分析方法,提出考虑轨道不平顺和道床横向阻力随机性的无缝线路稳定性概率统计分析方法。针对不同半径的铁路线路,研究轨道不平顺、道床横向阻力大小及位置分布随机性对钢轨允许温升分布规律的影响,并与现有无缝线路稳定性分析方法计算结果进行对比。研究结果表明:当道床横向阻力服从正态分布时,单独考虑道床横向阻力大小随机性时,钢轨允许温升近似服从极值I型分布;单独考虑轨道不平顺随机性、同时考虑道床横向阻力大小及位置分布随机性或综合考虑上述3个因素随机性时,钢轨允许温升均服从正态分布;以不少于95%保证率定义的钢轨允许温升标准值及均值随着线路平顺性的改善而增加;不同曲线半径条件下,现有“等波长-有限元”确定的钢轨允许温升与美国五级谱对应钢轨允许温升标准值相近,与其他轨道谱对应结果则存在较大差异;在综合考虑轨道不平顺、道床横向阻力大小及位置分布三因素随机性条件下,建立表征钢轨允许温升分布的均值、标准差及标准值与曲线半径间的显式映射关系,使无缝线路稳定性分析中考虑线路平顺性等级成为了可能。研究结果为进一步优化铁路无缝线路极限状态法设计提供了参考。

基于COMSOL的城轨弓网电弧多物理场仿真研究

弓网电弧产生的高温是烧蚀弓网系统材料的主要原因,研究弓网电弧发生机制,掌握弓网电弧温度分布规律,有助于减少电弧对弓网系统材料的烧蚀损伤,增加弓网系统服役寿命.文中基于磁流体动力学理论,根据弓网电弧数学模型设置相关初始条件,采用有限元软件COMSOL对弓网电弧数学模型进行求解,分析弓网电弧温度分布及电弧两极表面的温度分布,并探究弓网电弧持续时间与弓网电弧温度的关系.结果表明,弓网电弧中心区域温度最高,接触线表面温度远远超过接触线材料的熔点;弓网电弧持续时间与弓网电弧温度呈正相关.

济阳黄河公铁两用特大桥施工关键技术

济阳黄河公铁两用特大桥主桥为(84+144+228+240+300+120+60)m非对称、多跨、不等高、矮塔钢桁梁斜拉桥,采用塔梁固结、塔墩分离的结构体系。主梁采用N形双主桁、双层桥面布置,上层通行6车道公路,下层通行双线铁路。主墩桩基利用黄河枯水期采用大型旋挖钻机成孔,位于黄河主河槽的主墩承台采用锁口钢管桩围堰成套施工技术,其余主墩采用钢板桩围堰施工。主墩承台大体积混凝土结构采用自动测温及内部循环冷却水控制的智能化温度控制系统,承台混凝土未出现有害裂纹。钢桁梁利用门吊吊装杆件,设置墩旁托架和临时墩,采用双悬臂拼装+单侧悬臂拼装方式架设,结合钢塔两侧跨度不对称及跨黄河险工河段、施工条件受限等特点,通过定点精确配重+移动配重+索力调整,确保了钢桁梁线形满足设计要求、架设过程安全稳定。钢塔利用组合起重设备和定位工装分节组拼,钢横梁利用塔顶吊架整体提升,保证了钢塔安装精度。

城市轨道交通变电站主变压器室火灾顶棚下方烟气温度特性数值分析

[目的]随着地铁用电负荷的增大,湿式变压器等油浸式设备在过载情况下工作极易引起火灾,威胁顶棚结构安全。为了避免或降低变压器发生火灾的可能性,有必要研究火灾发生时,主变压器室内烟气的运动规律及顶棚温度的变化特性。[方法]以济南轨道交通3号线110 kV齐川变电站为例,通过理论分析和数值模拟,考虑通风条件、火灾热释放速率和火源高度等多变量的综合影响,对主变压器室发生火灾时,顶棚下的烟气温度特性进行研究。[结果及结论]机械通风开启时,室内气流组织会影响顶棚的烟气温度分布,补风导致火羽流倾斜,使最高温升点向排风口偏移,最高温升略低于机械通风关闭的情况。顶棚无量纲最高温升可分为两个区域:火焰未撞击顶棚时,无量纲最高温升同Q 2/5与(h ef-h 0)(Q为火灾热释放速率;h ef为火源表面到顶棚的距离;h 0为虚点源高度)的比值呈线性关系;火焰冲撞顶棚时,无量纲最高温升为常数,建立顶棚无量纲最高温升预测模型;基于模拟数据,提出弱火羽流驱动和强火羽流驱动时的顶棚射流温度衰减预测模型。

轮轨滑动磨损对应力和温度影响的研究

列车在启动、制动或通过曲线轨道、长大下坡道时,轮轨之间将出现滑动接触,这种现象在地铁中较为常见。滑动接触引起的磨损对接触区域应力和温度有着显著影响。根据数值计算方法和摩擦传热理论,建立轮轨接触磨损模型和摩擦传热模型,引入与磨损相关的接触应力,采用移动热源模拟轮轨之间的摩擦热,考虑温度对材料参数的影响,研究滑动磨损对应力和温度的影响。结果显示,随着滑动距离的增加,轮轨接触斑面积逐渐变大,接触应力从尖凸状变为扁平状,最大接触应力逐渐减小,接触应力分布趋于均匀。当磨损深度超过0.2 mm时,采用初始接触应力计算得到的轮轨节点温度值要显著高于采用与磨损相关的接触应力计算得到的温度值,并且随着滑动距离的增加,温度之间的差异逐渐变大。滑动接触时刻末,用初始接触应力计算的轮轨接触区域温度场中的最高温度比磨损相关的接触应力计算得到的最高温度分别高113.9℃和61.0℃。研究结果显示,磨损对应力和温度的影响较为显著,在计算应力和摩擦热时应充分考虑磨损对其的影响。

城市轨道交通车站空调冷冻水系统定温差变供水温度和大温差控制策略转换条件研究

[目的]为实现城市轨道交通车站空调冷冻水系统在定温差变供水温度和大温差两种工况下的整体能耗最低,需对这两种控制策略的转换条件进行研究。[方法]以郑州某城市轨道交通车站为例,基于TRNSYS(瞬时系统模拟程序)建立定温差变供水温度和大温差两种工况下的空调冷冻水系统模型,在满足空调冷冻水系统末端制冷量需求的前提下,分别研究冷冻水温差不变、供水温度为7~12℃,以及供水温度不变、供回水温差为5~10℃时对空调冷冻水系统能耗的影响。[结果及结论]两种控制策略的转换条件为:负荷率小于等于40%时,使用定温差变供水温度控制策略更节能;负荷率大于50%时,更适合采用大温差控制策略。该转换条件不仅能满足车站人员舒适度的要求,还实现了两种控制策略联合调控的目标,最终达到降低空调冷冻水系统整体能耗的目的。

Surface Temperature Change of U75V 60 kg/m Heavy Rail During Heat Treatment

The U75V 60 kg/m heavy rail samples were heated to 900 ℃ in a resistance furnace for a fixed duration of 50 min. Under this condition, the samples were austenitized totally. Then, the samples were dragged out of furnace and cooled for 25 s in the open air. After that, the samples entered into the air spraying channel, and were cooled from the top and both sides by compressed air. During this period, main technical parameters were changed such as the distance between spray nozzles and surface of rail head, air pressure, air spraying time and air temperature. So under laboratory condition, optimal heat-treating parameters of U75V 60 kg/m heavy rail were determined as the distance between spray nozzles and surface of rail head of 15 mm, air pressure of 0.26 MPa, cooling time of 80 s, and air temperature of 28 ℃. The surface temperature at different positions of heavy rail was measured before and after heat treatment, and the temperature changing law was determined. The self tempering occurred on the surface of rail head after heat treatment, and the tempering temperature became the largest （about 3 min） after heat treatment, separately 528, 524 and 536 ℃ at the center, top fillet and bottom fillet of rail head. The heavy rail was cooled in open air after heat treatment; during this period, the temperature gap on the surface of heavy rail became smaller and smaller, and was reduced to zero when being cooled for 20 min.

温度效应下小半径曲线桥梁轨作用及扣件阻力影响研究

为研究温度效应对小半径曲线桥梁轨相互作用的影响,以某小半径有轨电车曲线桥为背景,建立考虑无缝线路线形的线-桥-墩一体化空间有限元模型并进行验证。分析并对比直线桥与小半径曲线桥梁轨相互作用规律,研究了整体升温工况与局部升温工况下小半径曲线桥钢轨伸缩附加力、桥梁墩顶横向力和纵向力的大小及其分布规律,提出3种扣件布置方案并从减小钢轨伸缩附加力的角度进行对比。研究结果表明:相较于直线桥,整体升温情况下小半径曲线桥上钢轨横向位移及墩顶横向力均较大;对于小半径曲线桥,整体升温情况下,随着桥梁温度的升高,钢轨的伸缩附加力逐渐增大;采用整体升温工况进行设计检算时,钢轨所受伸缩力及墩顶纵向力与局部升温工况相比结果偏大;相较于铺设常阻力扣件,铺设小阻力扣件可以明显减小钢轨伸缩附加力,但纵向阻力过小容易导致钢轨折断时断缝过大。研究结果可以为有轨电车小半径曲线桥上无缝线路的设计提供理论参考。