Theoretical Model of Transformation Superlastic Diffusion Bonding for Eutectoid Steel

Based on current theories of diffusion and creep cavity closure at high temperature, a theoretical analysis of phase transformation diffusion bonding for T8/T8 eutectoid steel is carried out. The diffusion bonding is mainly described as two-stage process: Ⅰ The interfacial cavity with shape change from diamond to cylinder.Ⅱ The radius of the cylindrical cavity are reduced and eliminated gradually. A new theoretical model is established for the process of transformation superplastic diffusion bonding (TSDB) on the basis of a theoretical model for isothermal superplastic diffusion. The model can predict the bonding quality which is affected by technological parameters, such as limit cycling temperature, the compressive stress, the numbers of thermal cycles and temperature cycling through the phase transformation in the thermal cycling and so on. Results show that the maximum temperature, the compressive stress, the numbers of thermal cycles and the rate of temperature changing speed in the thermal cycling have an important influence on TSDB process. Meanwhile, reasonable technological parameters chosen from theoretical analysis is in good agreement with those obtained from experimental results.

Effect of hot-rolling deformation on the microstructure of pearlite of eutectoid steel

The effect of austenite deformation on the pearlite microstructure of eutectoid steel w as investigated by hot uniaxial compression tests using a Gleeble 1500 thermo-mechanical simulator. The results show ed that the deformation of the austenite phase accelerated the transformation of pearlite,leading to a smaller colony size and a smaller interlamellar space of pearlite; meanw hile,the orientation of pearlite lamellae became more heterogeneous,and the lamellar thickness decreased. Larger strain of austenite w as observed to accelerate the spheroidization process of carbides w ithin pearlite colonies.

Microstructural evolution of lamellar cementite in eutectoid steels by cold rolling

The pearlitic transformation and the deformation behavior of lamellar cementite after cold rolling in eutectoid steels Fe-0.76%C-0.137%Mn(mass fraction) were studied by means of Formastor-F (Full Automatic Transformation Testing Instrument) and field emission scanning electronic microscopy (FESEM) observation. Fine and coarse pearlite were obtained in the eutectoid steels austenitized at 900℃ for 15min, then hold at 620℃ for 90 s and 690℃ for 7 h, respectively. The deformation behavior of cold rolled lamellar cementite could be classified as: cleavage fracture, inhomogeneous slip, fragmentation, thinning or necking, and homogeneous bending. The cementite lamellae with the thickness of more than 100 nm could be deformed plastically.

Investigation into the Deep Drawability of 0.1%C Eutectoid Steel

The phenomenon of anisotropism in most rolled products necessitates that the rolling direction that enhances desirable mechanical property is established. In this paper, the comparative deep drawability of as-received and annealed mild steel containing about 0.1%C was investigated. The flat steel sample was divided into two and classified as as-received and heat treated respectively. The heat treated sample was obtained by annealing at 950°C after been soaked for 5 hours and deep drawn at ambient temperatures (35°C - 42°C). From both samples, circular specimens were machine- blanked parallel to the rolling directions inclined at 0°, 45° and 90° respectively and were prepared for deep drawability test while rectangular specimens were prepared for tensile test. Both specimens, as-received and annealed were then sub- jected to tensile, cupping and microstructural analyses. Results show that the contribution to increased formability at 90° rolling direction seems to have come from the spheroid-like pearlite grains induced during annealing while the sta- bility of spread observed was achieved through a modest increase in strength. Thus, the resistance of annealed eutectoid steel to cupping is quite minimal at 90° to the rolling direction. The desirable drawability characteristics developed by the annealed eutectoid steel specimen are: cup-height, 30 mm maximum and ear, 6.4% maximum.

Effect of deformation parameters on the austenite dynamic recrystallization behavior of a eutectoid pearlite rail steel

Understandings of the effect of hot deformation parameters close to the practical production line on grain refinement are crucial for enhancing both the strength and toughness of future rail steels.In this work,the austenite dynamic recrystallization(DRX)behaviors of a eutectoid pearlite rail steel were studied using a thermo-mechanical simulator with hot deformation parameters frequently employed in rail production lines.The single-pass hot deformation results reveal that the prior austenite grain sizes(PAGSs)for samples with different deformation reductions decrease initially with an increase in deformation temperature.However,once the deformation temperature is beyond a certain threshold,the PAGSs start to increase.It can be attributed to the rise in DRX volume fraction and the increase of DRX grain with deformation temperature,respectively.Three-pass hot deformation results show that the accumulated strain generated in the first and second deformation passes can increase the extent of DRX.In the case of complete DRX,PAGS is predominantly determined by the deformation temperature of the final pass.It suggests a strategic approach during industrial production where part of the deformation reduction in low temperature range can be shifted to the medium temperature range to release rolling mill loads.

Effects of Plastic Warm Deformation on Cementite Spheroidization of a Eutectoid Steel

The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 ℃ and in the strain rate of 0.01 s-1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.

Atom Probe and Mssbauer Spectroscopy Investigations of Cementite Dissolution in a Cold Drawn Eutectoid Steel

Mssbauer spectrum and three dimensional atom probes（3DAP） were combined to investigate the mechanism of cementite dissolution in a cold-drawn eutectoid steel at a true strain of 2.89.The experimental results suggest that the dislocations play an important role in the dissolution of the cementite by sweeping across the nano-scaled cementite,and transferring carbon from cementite to ferrite inducing cementite decomposition.The mechanism of cementite dissolution in the steel is discussed in association with the investigation of nonstoichiometric cementite structure after heavy deformation.

ELECTRON MICROSCOPE STUDY ON δ-EUTECTOID IN HIGH SPEED STEEL

The morphology and structure of δ-eutectoid and the transformation of δ-eutectoidic austenite in high speed steel have been studied under SEM and TEM.The δ-eutectoid in as-cast high speed steel is rod-shaped eutectoidic units with different orientations and is markedly surrounded by carbide shell.The eutectoidic carbide is rod-shaped M_6C of square eross section.The eutectoidic austenite may transform into pearlite structure during cooling.

Kinetics Analysis of Pre-Eutectoid Transformation of Austenite in Carbon-Manganese Clean Steel with Rare Earths

By means of rapid cool dilatometry, the influence of rare earths on kinetics of pre-eutectoid transformation of austenite of carbon-manganese clean steel was studied. While the content of rare earths were proper, the nucleation of pre-eutectoid ferrite on the grain boundary of austenite was accelerated and the incubation period of pre-eutectoid transformation was shortened. But while rare earths were excessive, opposite actions appeared. Rare earths decreased transformation velocity constant in the process of transformation from austenite to ferrite, which meant that the growth of per-eutectoid ferrite became slow. The reason above varieties was related to effect of rare earth on grain boundary constitution of austenite and diffusion of carbon element.

Simplified Thermodynamic Model for Pro-Eutectoid Ferrite Formation in Multicomponent Structural Steel

By introducing aparameter of difference in ferrite formation temperature between binary Fe-C and multicomponent system,and referring to the thermodynamic model for Fe-C binary system,a simplified thermodynamic model for pro-eutectoid ferrite formation in Fe-ΣXiC multicomponent structural steels(Xi=Mn,Si,Mo,Cr,Ni or Ti,etc)was suggested.The comparison of the calculated Ae3 temperatures with the measured data of steels 42 shows that the relative standard deviation and root-mean-square(RMS)error between them are only 0.71% and 8.92 K,respectively.However,the deviations between the same measured data and the values calculated from the superelement model are as high as 1.86% and 23.83 K,respectively.It can be concluded that the simplified thermodynamic model for pro-eutectoid ferrite formation in multicomponent structural steels is acceptable and the calculated Ae3 temperatures are in good agreement with the experimental data.

高强重载汽车大梁钢表面麻坑问题的研究与改进

高强重载汽车大梁钢表层氧化铁皮在开平矫直过程中破碎、脱落导致后续矫直过程中氧化铁皮压入形成大小不一的表面麻坑。本文研究精轧入口温度、轧制速度、冷却速率和卷取温度等工艺因素与氧化铁皮厚度之间对应关系。结果表明:精轧入口温度在1040℃以下时氧化铁皮厚度随着入口温度的上升而缓慢增加,在1040℃以上时氧化铁皮厚度随着入口温度的升高而急剧增加;随着轧制速度的加快,带钢表面生成的氧化铁皮变薄;较大速率冷却可以抑制氧化铁皮的长大、增厚;在560~630℃,卷取温度越高,氧化铁皮越厚。通过优化加热制度,提高轧制速度,优化层流冷却速率和卷取温度等措施,大梁钢表面麻坑缺陷得到有效控制。

New pearlitic steels for 21st century rail applications

To improve competitiveness,the nation's railroads have increased the axle loads and speed of the trains.This has led to a rapid decrease in the life expectancy of premium rails through accelerated wear,rolling contact fatigue and fracture.To counter this effect,the railroads need rails that exhibit better performance in these areas.A research program has been initiated to study the microstructural aspects of near-eutectoid steels that would improve these properties.The first phase of the work was to carefully characterize the existing commercial rail steels in terms of pearlite interlamellar spacing,steel cleanliness and the presence of pro-eutectoid cementite on prior-austenite boundaries.These characterizations were then correlated with both mechanical properties and overall rail performance.The second phase of the program was to develop a better microstructure through control of composition,thermomechanical processing and cooling path.This was achieved through the use of laboratory-melted heats of experimental near-eutectoid compositions and a computer controlled MTS compression machine modified for axisymmetric compression testing and subsequent controlled cooling.The optimum processing route for these new steels has been determined,and pilot-scale heats have been melted,hot rolled and cooled using the information gained from the MTS investigations.The mechanical properties of these new steels have been determined and the rail performance tests are being conducted using laboratory-scale evaluation.Ultimately,these new rail steels will be tested under commercial conditions on the TTCI test track in Pueblo,Colorado.This paper will report on the alloy and processing design and resulting properties of the steels developed in this research program.Guidelines for future rail compositions and processing to obtain improved properties and performance will be presented.

热镀锌双相钢钢板表面橘皮缺陷研究

目的研究一种热镀锌双相钢表面橘皮状缺陷的特征及形成机理。方法采用数字显微镜、扫描电镜和辉光光谱仪对正常和缺陷位置的镀锌态、去锌层态、冷轧态和热轧态的形貌和表层成分分布开展对比分析研究。结果橘皮状缺陷特征是由于锌晶粒异常长大至350μm,为正常晶粒的4倍。镀锌态缺陷区较正常区Mn含量高26%,纵截面观察基板表层有约2~4μm深的微裂纹,界面处检测出Al、O、Mn峰,存在Mn表面富集。去锌层态和冷轧态基板表层呈冷轧轧裂特征,存在大量垂直于轧制方向的10~30μm的横向微裂纹。卷取温度为650℃时,热轧态边、中位置氧化层厚度分别为6.5、5.8μm,较550℃时增加25%和23%。GDS检测表明,对应缺陷区热轧态发生共析转变,相构成以低氧含量的Fe_(3)O_(4)、FeO、Fe为主。实践证实,卷取温度下调至560℃以下时,可有效消除该缺陷。结论缺陷形成机理是,热轧钢卷在570℃以上温度卷取,经缓慢冷却至室温时,形成纯铁包覆Fe_(3)O_(4)的共析转变氧化铁皮,卷取温度高也使氧化层增厚,导致酸洗难度增大,氧化铁皮无法有效清除。酸洗后残存的氧化层和Mn富集导致表层协同变形能力弱,冷轧形成大量10μm级横向微裂纹。微裂纹处的凹凸表面诱发Fe-Zn相爆发生长,导致锌晶粒异常长大。厚带钢具有更多的内部热量,出锌锅冷速慢,也为镀后锌晶粒长大创造条件。

不同碳含量伪共析钢温轧组织与力学性能研究

通过SEM和拉伸实验研究了不同碳含量伪共析钢温轧后的组织与力学性能。结果表明:热水淬火后的伪共析钢中的铁素体随碳含量的增加而减少,珠光体含量增加。经500℃温轧后,35和45钢中的铁素体沿轧向呈长条状分布,而55钢中铁素体则为断续条带状分布。并且55钢中渗碳体破碎程度也最大,渗碳体分布也最为弥散,45钢中相较于35和55钢渗碳体破碎程度最小。随着碳含量的增加,钢的强度升高,塑性降低,屈服强度最高的55钢可达1308MPa,总延伸率最高的35钢可达6.4%。但是就综合性能而言,45钢的强塑比最佳,其性能甚至达到了低合金钢的水准。

共析钢的过冷奥氏体动态相变和组织超细化

通过在Gleeble-1500热模拟试验机上进行单轴热压缩实验,研究了共析钢中过冷奥氏体在A_1-A_(r1)之间变形时的组织演变,探讨了共析钢中复相组织球化超细化的机理．结果表明:随着应变量的增加,片层珠光体在原始奥氏体晶界形成的铁素体生长前沿形核长大,并向原始奥氏体晶内推进,直至相变完成．片层珠光体生成后继续变形,发生渗碳体的溶断和球化,及铁素体的动态回复再结晶等轴化等过程．渗碳体的球化有两种机制,其一是由于Gibbs-Thomson效应,珠光体中片层状渗碳体发生溶断和球化,生成的大颗粒渗碳体主要分布在铁素体晶界;其二是纳米级渗碳体粒子伴随铁素体动态再结晶在晶内重新析出．实验证实,在合适的变形工艺条件下可以得到亚微米级铁素体与亚微米、纳米级颗粒状渗碳体的双相组织．

过冷对共析钢动态相变和组织演变的影响

将共析钢在600-700℃的过冷奥氏体状态下进行单轴热压缩,获得了亚微米级别等轴铁素体+纳米级别弥散分布球化渗碳体的复相组织,其组织演变经历动态相变、片层渗碳体球化、铁素体动态再结晶和纳米级别渗碳体颗粒析出等过程.随着形变温度的降低,过冷度增加,相变动力学过程加快.片层渗碳体的球化程度由球化时间和球化速度控制,形变温度升高使渗碳体球化速度加快,但是相变开始时间及动力学延迟使得用于渗碳体球化和熟化的时间相应缩短,导致球化程度降低.铁素体再结晶和等轴化过程则主要受位错迁移、渗碳体颗粒钉扎的影响,形变温度升高导致较高的等轴化发展速度.

时效对冷拔珠光体钢力学性能的影响

用TEM观察了共析珠光体钢经真应变为2．89的拉拔变形以及不同温度时效后微观组织的变化,测量了时效温度对力学性能的影响．结果表明,大变形珠光体钢在473 K时效后获得相对于时效前(σ_b=2061 MPa)更高的抗拉强度(σ_b=2300 MPa),在673 K时效获得较均衡的强韧性能(σ_b=1856 MPa,δ=3．7％)．组织观察发现,大变形中溶解的渗碳体在时效过程中弥散析出,分析认为碳化物的析出阻碍位错运动,提高了该钢的力学性能．

共析钢的相变超塑性扩散连接

根据高温扩散和空洞蠕变闭合理论 ,在恒温超塑性扩散连接基础上建立相变超塑性扩散连接数学模型 ,可确定各工艺参数 ,诸如循环上限温度、压力、循环次数以及升、降温速率等对连接的影响 ,作为选择合适工艺参数的依据。经过对 T8共析钢相变超塑性扩散连接试验研究 。

共析珠光体钢激光冲击处理后的组织演变

采用SEM、TEM及显微硬度等方法研究了原始组织为层片状珠光体的共析钢激光冲击处理后的组织演变与硬度变化规律。结果表明:单道次激光冲击即可实现共析钢表层铁素体晶粒超细化,等轴铁素体晶粒尺寸约为0.8μm;激光冲击产生的冲击波诱发材料表层产生高应变速率的塑性变形,渗碳体层片具有较好的塑性变形能力,冲击波中心区域渗碳体层片球化完全,渗碳体颗粒粒径约为50nm,而冲击波边缘区域渗碳体层片主要发生弯曲、扭折和断裂;塑性变形导致大量位错的产生使得冲击波中心区域材料表层的硬度相比冲击前提高约20%左右。

不同组织共析钢室温拉伸变形的塑性失稳

通过不同热处理或热机械处理工艺获得了共析钢的4种不同组织,即2种片层间距的珠光体,球化珠光体以及超细晶(α+θ)复相组织。利用室温单轴拉伸实验、SEM及TEM分析了组织状态对共析钢力学性能和塑性失稳行为的影响,结果表明:细化片层间距可以改善片层珠光体组织的协调变形能力并提高强度,均匀伸长率有所降低的同时总伸长率提高。细片层珠光体的拉伸变形失稳符合塑性失稳判据,但不同片层间距的片层珠光体拉伸断裂均属于准解理断裂。与片层珠光体组织相比,球化珠光体和超细晶(α+θ)复相组织的协调变形能力强,虽然在均匀塑性变形阶段均存在少量渗碳体粒子开裂的情况,但拉伸变形失稳均符合塑性失稳判据,拉伸断裂为韧性断裂。与片层珠光体和球化珠光体相比,超细晶(α+θ)复相组织具有较好的强度-塑性配合。