U71Mn钢轨矫直断裂原因分析

针对U71Mn钢轨在矫直过程中出现断裂的现象,对钢轨断口部位的宏观形貌、显微组织、化学成分、力学性能和显微硬度等进行了观察和分析。结果表明:钢轨角部的外来夹杂物、网状渗碳体、针状渗碳体魏氏组织,以及钢轨角部由于擦伤出现的隐针马氏体和微裂纹,是造成钢轨矫直断裂的主要原因。最后探讨了解决措施。     

挤压道岔轨断裂原因分析

针对在挤压加工道岔轨时出现挤压爆裂和矫直断裂的情况，对断裂钢轨进行了化学成分分析、低倍及金相检验，并采用X-650扫描电镜及EDAX电子探针对断口进行了分析。分析结果表明，断裂钢轨化学成分满足标准要求，低倍组织和显微组织正常，断口呈脆性断裂解理特征，断口处氧化严重。由此认为，造成挤压时道岔轨爆裂和矫直断裂的原因是挤压前加热工艺不当所致。     

U75V焊接钢轨线路铺设时的断裂原因分析

某铁道线路进行大修换轨铺设新轨时,一支采用闪光焊接工艺焊接的长500m的钢轨在中部位置的焊缝附近发生横向断裂,采用宏观检验、化学成分分析,断口分析及微观组织分析等方法对钢轨断裂的原因进行了分析。结果表明：钢轨的冶金质量良好,钢轨发生断裂主要是因为其轨头踏面表层形成的马氏体组织在铺轨时的弯曲拉应力作用下,产生裂纹并快速扩展,导致钢轨发生脆性断裂;钢轨焊接时出现了高接头,为保证焊接钢轨的平直度对其进行了打磨,但打磨工艺控制不良,从而导致钢轨轨头踏面局部产生高温,发生马氏体组织转变。     

异型钢轨矫直断裂原因分析

某异型钢轨在经60kg／m热轧钢轨加工至50kg／m过渡轨后出现矫直断裂。经对钢轨断口的宏、微观检验和分析，认为，断裂裂纹起源于轨腰次表层；钢轨发生矫直断裂的原因是在加工异型钢轨的过程中，加热温度偏高，导致轨腰表层部分区域出现沿晶氧化，锻压时因轨腰表层受拉应力作用，沿晶氧化处形成微裂纹，随后矫直时轨腰三点弯曲受力，微裂纹缺陷处产生应力集中致使裂纹扩展，导致钢轨断裂。     

胶接绝缘钢轨用10.9级高强度螺栓断裂分析

某客运专线道岔胶接绝缘钢轨用高强度螺栓在安装使用1a(年)后出现少量断裂失效。采用宏观分析、化学成分分析、力学性能测试、金相检验以及扫描电镜断口分析等方法,对螺栓的断裂原因进行了分析。结果表明:该高强度螺栓断裂主要是由第一类回火脆性引起的,而淬火时形成的少量上贝氏体加速了螺栓的断裂。最后提出了改进措施。     

77MnA盘条断裂原因分析

通过宏观观察,夹杂物评级和显微组织观察,分析了77MnA盘条拉拔过程中断裂的原因.实验结果表明:马氏体的出现和基体为索氏体、屈氏体和少量的贝氏体的混合组织是断裂的主要原因;基体组织为混合组织和马氏体的出现是由于轧件出精轧机组进斯太尔摩控制冷却线时冷却温度过低造成的;同一直径钢丝心部的马氏体含量相对边部的马氏体含量多表明存在C、Mn、Cr元素的偏析,并提出改进措施.     

插销法研究自保护焊U75V钢轨的冷裂敏感性

采用插销试验法研究了JDHS-38#自保护药芯焊丝焊接U75V钢轨的冷裂敏感性,评估了预热温度、线能量、焊道数3个因素对U75V钢轨临界断裂应力的影响.结果表明,现场对钢轨施焊时,采取250℃的预热温度,14 kJ/cm左右的线能量,双层焊等措施可以使钢轨接头热影响区少产生甚至不产生焊接冷裂纹,使其临界断裂应力达646 MPa,与抗拉强度相当.若采用其他工艺参数,临界断裂应力较低,且当拘束拉伸应力高于抗拉强度时,于热影响区发生氢致断裂.焊接接头过热区组织主要为马氏体、贝氏体,断口显微形貌主要为河流花样、泥状花样及爪状花样.     

75kg/m热处理钢轨断裂原因分析

某线路发生一起PD3 75Kg／m热处理钢轨横向断裂。采用光学和电子显微镜及能谱仪对断裂钢轨进行了宏、微观分析。结果表明，钢轨轨头踏面表层为厚约2～3mm、硬度为498HV的马氏体组织；次表层为厚约3mm、硬度为312HV的回火索氏体组织；里层是硬度为380HV的细珠光体组织(即钢轨的正常组织)。钢轨轨头踏面表层的马氏体是用KD286焊条对钢轨进行堆焊所形成，次表层的回火索氏体是堆焊时热影响造成。在车轮力的作用下，表层与次表层的交界处产生裂纹，随后裂纹不断扩展而导致钢轨断裂。     

PG4热处理钢轨落锤断裂原因分析

采用体视显微镜、光学显微镜和扫描电镜等仪器分析了PG4热处理钢轨母材落锤试验不合格的原因。结果表明:落锤断裂钢轨的断裂均起源于轨底角圆弧过渡处的表面薄层马氏体组织区域;由于钢轨在输送过程中其轨底角与输送辊道轮缘根部磨损台阶接触形成滑动摩擦,摩擦热导致局部金属温度瞬时升高并达到奥氏体化温度,随后快速冷却生成了硬而脆的马氏体组织,致使钢轨母材在锤重1000kg和锤高6.4m条件下落锤1锤即发生断裂。     

等温淬火组织中残余奥氏体回火转变的研究 “兼论贝氏体回火脆性”

本文测定了30CrMnSiNi2A钢等温淬火后Ar回火动力学,进行回火组织的动、静态观察并研究了该钢的回火脆性和Ar的稳定性。结果表明:等温淬火后Ar的C曲线上支对应上(或反)贝氏体转变,下支对应下贝氏体或等温马氏体转变;热处理制度可以控制Ar的稳定性;Ar分解为上(或反)贝氏体、断续碳化物膜和回火冷却及形变时转变为马氏体是引起回火脆性的重要因素(特别是对含Ar较多的组织)。等温淬火后工件宜在250～300℃回火。     

不同热输入下F460钢焊接粗晶热影响区韧脆转变的内在机理

使用Gleeble 3800热模拟试验机模拟F460钢单道次焊接条件下焊接粗晶热影响区的热循环过程,通过光镜(OM)、扫描电镜(SEM)分析热影响区的显微组织、确定临界事件,通过ABAQUS软件计算临界解理断裂应力σf,进而系统分析不同焊接热输入E下韧脆转变温度变化的内在机理。结果表明:随着E的提高,焊接粗晶热影响区显微组织依次为少量板条马氏体和大量细密的板条贝氏体,板条贝氏体较多的板条/粒状贝氏体,粒状贝氏体较多的板条/粒状贝氏体,粗大的粒状贝氏体。原始奥氏体晶粒、贝氏体团的最大尺寸随着E的提高而变大。在完全解理断裂的冲击断口上,寻找停留在缺口尖端附近的残留裂纹,通过对比残留裂纹长度、原始奥氏体晶粒大小、贝氏体团尺寸,发现不同E下解理断裂的临界事件尺寸都是贝氏体团大小,而临界事件尺寸越小,韧脆转变温度越低。此外,通过有限元模拟缺口尖端的应力分布得到σf,σf越大冲击韧度越好,随着E的提高σf降低,故进一步说明随着E的提高韧脆转变温度Tk上升的内在机理。     

Development of bainitic rail steels with potential resistance to rolling contact fatigue

Rails are a major capital and maintenance cost for railways in North America. While manufacturers produce clean steels with high quality, most rails made today retain the basic carbon–manganese chemistry of traditional pearlitic rails. This paper describes the development of a bainitic rail steel with potential additional resistance to rolling contact fatigue damage. It is shown that rails can be produced in bainitic steel without the need for complex heat treatments after rolling, and that bainitic rails can have higher hardness and fracture toughness than pearlitic rails. Although small‐ and full‐scale tests indicate that the wear performance of bainitic steel depends considerably on test conditions, the indication is that bainitic steel rails can have significantly better rolling contact fatigue performance compared to pearlitic rails. Reasons for the superior fatigue performance are not fully understood, although a number of hypotheses exist. A conclusion is that continued research would be useful to understand quantitatively the physics and metallurgy of wheel/rail contact.     

热处理对特种设备用高强钢激光–电弧复合焊接头氢脆断裂行为的影响研究

目的 提高800 MPa级特种设备用低碳贝氏体高强钢激光–电弧复合焊接头的抗氢脆性能。方法 采用预充氢后慢应变速率拉伸试样的方法,定量评估焊态、焊后直接高温回火和焊后调质3种状态下800 MPa级低碳贝氏体高强钢激光–电弧复合焊接头的氢脆敏感性,结合扫描电镜下的初始微观组织和断裂特征,讨论抗氢脆性能的改善机理。结果 焊后调质处理有效消除了焊接热循环形成的马氏体组织,使接头各区域的微观组织趋于一致,接头的抗氢脆性能较焊态和直接焊后高温回火态的显著提高,断裂特征也从沿晶和穿晶的混合断裂转变为穿晶解理断裂。结论 焊后调质处理可以有效提高800 MPa级低碳贝氏体高强钢激光–电弧复合焊接头的抗氢脆性能。     

Banded microstructure in a low-alloyed eutectoid steel

The possible influence of banded microstructure on the fracture performance of a high-carbon steel is investigated using electron microscopy and X-ray diffraction techniques. The banded microstructure was found to be alternating layers of bainite and tempered martensite/retained austenite. Transformation of bainite was promoted by a combined effect of segregation of alloying elements, non-uniform thermal gradient across the steel, and the insufficient austenitisation. The transformation of bainite along the prior austenite grain boundaries (PAGBs) introduces a non-uniform strain distribution at bainite/martensite interface and a higher stress concentration, which may eventually lead to the failure of the heavy section in a brittle intergranular manner. Results provide insights to the importance of understanding the bainitic transformation in such eutectoid steel systems.     

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240,360,and 400 ℃.The results show that martensite,bainite,and retained austenite (RA) are the main microstructural phases.The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix,and the transformation of ultrafine bainite into coarse lath bainite and granular bainite,respectively.Thick filmy RA was distributed between bainite laths.The polygonal martensiteaustenite islands and blocky RA formed along the grain boundaries.The average carbon concentration in the matrix decreased with the temperature increase,while the impact toughness initially increased and then dropped with temperature.The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps.The microcracks disappeared in the RA on the prior austenite grain boundaries.On the other side,the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns.The polygonal martensite-austenite islands or blocky RA constrained the microcracks.After austempered at 400 ℃,the brittle fracture was dominant,showing river patterns,and the microcracks propagated through the granular bainite without any resistance.     

The role of Cu and Al addition on the microstructure and fracture characteristics in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels with superior toughness

The effects of Cu and Al addition on the microstructure and fracture in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels with superior toughness were studied and compared with the X70 pipeline base steel counterpart. The microstructure in base steel was dominated by a small fraction of acicular ferrite and predominantly bainite. However, acicular ferrite microstructure was obtained in Cu-bearing steel, which nucleated on complex oxide with outer layer of MnS and CuS because of Cu addition. The microstructure in Al-bearing steel consisted of bainite with ultrafine martensite–austenite constituent, which was refined by Al addition. CGHAZ in Cu-bearing and Al-bearing steels had superior impact toughness and ductile fracture, which were attributed to acicular ferrite and ultrafine martensite–austenite constituent, respectively.     

Fracture behaviour of 8%Ni 980 MPa high strength steel at various temperatures Part 2 – COD tests

Abstract

The fracture behaviour of 8%Ni 980 MPa grade high strength steel is investigated by combining experimental results of crack opening displacement (COD) tests at various temperatures with detailed microscopic observations of fracture surfaces and crack configurations in unloaded specimens. The results reveal that this high strength steel possesses high toughness with a transition temperature around ?150°C. Even though at a very low temperature (?196°C), cleavage cracking dominates the fracture process and the crack does not propagate immediately through the entire ligament: a ‘pop-in’ extension is observed in macroscopic tests, and the microscopic fracture mode is quasi-cleavage. It is found that resistance to crack propagation is provided by three barriers: original austenite grain boundaries, bainite colony boundaries and interlayers between bainite laths. These barriers manifest themselves by tear ridges with dimples on the fracture surfaces. At higher temperatures, the fracture mechanism is dominated by fibrous rupture, associated with a ‘dimpled’ fracture surface and some individual quasi-cleavage facets.     

Microstructure and mechanical properties of as rolled high strength bainitic rail steels

Abstract

Bainitic steels are emerging with the potential to replace eutectoid steels to achieve longer rail life. Bainitic steels of two compositions were made and processed into rails 13 m in length. The properties of these rails were evaluated in the as rolled slow cooled condition. The results from bainitic rail steels were compared with those for 880 MPa grade pearlitic rail steel. It was found that in the as rolled slow cooled condition rail properties such as fracture toughness, Charpy impact energy, and endurance limit were superior to those of 880 MPa grade pearlitic rail steel.     

厚规格多相组织X80管线钢组织控制与断裂行为研究

目的 研究厚规格X80管线钢的力学性能及断裂行为与微观组织之间的关系。方法 通过硬度实验、拉伸实验、夏比冲击实验及落锤撕裂实验（DWTT）,研究X80管线钢力学性能,利用OM和SEM观察微观组织和断口形貌。结果 多边形铁素体（PF）、粒状贝氏体（GB）、准多边形铁素体（QPF）、针状铁素体（AF）和贝氏体铁素体（BF）多相组织的结合保证了钢板优异的强韧性。在落锤撕裂实验裂纹扩展过程中,裂纹扩展至QPF时呈平直扩展,裂纹扩展至GB时,较为曲折迂回,说明GB能有效阻碍裂纹扩展。细小的马氏体-奥氏体（M-A）组元和AF同样能够有效阻碍裂纹的扩展。结论 研究结果可为厚规格多相组织X80级管线钢的服役使用提供理论依据。     

U75V重轨钢弯曲疲劳裂纹扩展行为EI北大核心

为明确珠光体钢轨的疲劳裂纹扩展行为,测定U75V重轨钢轧态和热处理态两种条件下的三点弯曲疲劳裂纹扩展速率,采用光学显微镜、扫描电镜、EBSD对钢轨的微观组织、片层、断口形貌及裂纹扩展轨迹进行观察。结果表明:轧态和热处理态钢轨的疲劳辉纹平均间距分别为253,215 nm,轧态钢轨的疲劳断口呈现解理台阶与河流花样形貌,且河流花样趋于合并,而热处理态钢轨的疲劳断口呈现大量的解理台阶及较多的微裂纹和撕裂棱,河流花样以支流为主;热处理态钢轨的疲劳裂纹扩展速率远低于轧态,到达裂纹失稳阶段也较滞后;轧态和热处理态钢轨的疲劳裂纹扩展都是以穿晶断裂为主的穿晶断裂和沿晶断裂混合扩展方式进行,轧态和热处理态钢轨的珠光体片层间距分别为272,148 nm,其中热处理态钢轨的珠光体片层细密且方向多样,存在显著的珠光体团簇,裂纹扩展轨迹中出现较多的分支裂纹和裂纹桥接现象,对扩展起到阻碍作用,是热处理态钢轨抗疲劳裂纹扩展能力优于轧态的重要原因。