汽车板QStE550TM应变疲劳性能研究

以3 mm厚汽车板用结构钢QStE550TM为研究对象,对其光滑试件进行了静态拉伸试验和不同应变水平控制下的应变疲劳性能测试,绘制了E-N曲线,并观察了疲劳断口形貌。试验结果表明,该汽车板屈强比和断层伸长率较高,具有良好的抗变形能力与加工成型性能。当应变幅小于0.8%时,循环寿命可达到104次以上,在低周应变循环中,该材料出现循环软化现象。疲劳断裂源于表面微裂纹的产生,疲劳源附近断口较为光滑,断口存在典型的疲劳辉纹。     

回火索氏体钢轨钢循环应变行为与位错结构关系的研究

本文用增量步进试验方法，研究了具有不同组织参数的回火索氏体共析钢轨钢的循环应力－应变行为。并通过透射电镜对位错结构的分析，探讨了回火索氏体循环软化机制。试验及分析结果表明，在本试验条件下，回火索氏体组织均发生循环软化。循环应变过程中组织中位错间的交互作用形成位错胞。回火索氏体强烈的循环软化主要是由于位错组态再分布，产生胞状亚结构，使内应力降低所致。     

预应变对Zr702/TA2/Q345R复合板低周疲劳行为的影响

研究了预应变对Zr702/TA2/Q345R复合板低周疲劳行为的影响。研究表明,预应变对Zr702/TA2/Q345R复合板微观组织的影响与各层材料的晶体结构有关。在Zr702纯锆和TA2纯钛层中,随着预应变水平提高,滑移带和孪晶明显增多。预应变促进了Zr702/TA2/Q345R复合板的循环软化变形,使应变范围、循环塑性变形程度提高而疲劳寿命降低。预应变后,材料的反向屈服极限降低,棘轮损伤程度因此降低。尽管预应变提高了复合板组成材料的抗裂纹扩展阻力,但复合板在预应变后应变幅提高,加剧了塑性损伤累积,因此疲劳寿命降低。     

应变幅对H13热作模具钢等温疲劳行为的影响

对H13热作模具钢试样进行600 ℃等温疲劳实验，通过显微维氏硬度计、金相显微镜（OM）、超景深显微镜和扫描电子显微镜（SEM）等设备研究了0.7%，0.9%和1.1%三种不同应变幅对疲劳行为的影响。结果表明:应力应变滞后回线呈现对称性，应变幅越大，滞回环面积越大。H13钢在实验中呈现循环软化的特征，应变幅越大，疲劳寿命越短，1.1%应变幅试样寿命约为0.7%应变幅试样的61.2%。应变幅的增加对裂纹萌生和扩展起促进作用，1.1%应变幅试样裂纹扩展最明显。高温非真空实验条件下，材料表面产生的氧化物也会促进裂纹扩展。疲劳后试样微观组织发生明显的长大和粗化，较大应变幅对碳化物析出有更大的助力，还会加速材料软化。有应变幅试样显微硬度远低于无应变幅试样。      

汽车齿轮轴用钢SCM440H扭转疲劳寿命及断裂特征分析

通过试验测定汽车齿轮轴用钢SCM440H经淬火和高温回火后的扭转疲劳寿命,并采用扫描电镜(SEM) 对疲劳断口进行观察、分析。结果表明,显微组织为回火索氏体的SCM440H齿轮轴用钢,在106的循环周次下,扭转疲劳强度约为253MPa;不同切应力幅下的扭转疲劳断口呈现韧性断裂特征,疲劳断口裂纹源处没有明显的析出物。     

Q235钢结构材料的超低周疲劳性能

对钢结构材料 Q235钢的超低周疲劳性能进行了研究。采用横向应变控制方法,保持频率1 Hz 恒定,在岛津电液伺服疲劳试验机上开展了试验钢的超低周疲劳试验。获得了循环应力响应特征曲线等实验数据,并在此基础上分别建立了试验钢基于塑性应变幅及应变速率的超低周疲劳寿命预测公式,且2种公式均能较好地对其寿命进行预测。通过电镜扫描(SEM),分析了试验材料超低周疲劳下的微观断裂机理。研究结果表明,试验材料在超低周疲劳与低周疲劳下的疲劳性能,如循环响应特征、寿命预测公式以及微观断裂机理等方面均存在一定的差异。     

基于应变控制的4Cr5MoSiV1热作模具钢热机械疲劳行为

采用MTS®热机械疲劳电液伺服试验机研究了4Cr5MoSiV1热作模具钢400~700℃范围内拉压对称机械应变控制的同相及反相热机械疲劳行为.结果表明:当应变幅为±0.50%时,4Cr5MoSiV1钢反相热机械疲劳寿命约为同相的60%;无论同相还是反相加载,应力-应变滞后回线均呈现不对称性,同相加载时表现为平均压缩应力,反相加载时表现为平均拉伸应力.两种加载方式下,最大应力与最大应变及峰值温度均不同步,在高温半周出现应力松弛现象.此外,高温半周呈现持续循环软化,而低温半周呈现初始循环硬化,随后持续循环软化的特征.同相加载时断口以主裂纹、撕裂脊和准解理特征为主,裂纹少而深;反相加载时断口以疲劳条纹和大量的凹坑特征为主,裂纹多而浅.      

平均载荷及载荷幅值对工业纯钛双轴保载疲劳行为影响

通过室温双轴保载疲劳试验，研究了不同平均载荷及载荷幅值对工业纯钛双轴保载疲劳行为的影响。结果表明，在相同保载时间下，平均应变及应变速率随着平均载荷和载荷幅值的升高而逐渐增大。当载荷幅值一定时，蠕变应变随着平均载荷的增大而增大，当平均载荷一定时，蠕变应变随着载荷幅值的增大反而减小。分析双轴棘轮与蠕变应变之间交互作用时，发现两者始终相互制约。断口分析表明，随着平均载荷和载荷幅值的增大，疲劳条带特征逐渐消失，韧窝及撕裂棱数量显著增加，断口呈现韧性失效特征。随着平均载荷或载荷幅值的增加，等效应变幅值增加，疲劳寿命逐渐降低。与平均载荷的影响相比，疲劳寿命对载荷幅值的变化更加敏感。分别利用最大主应变、最大剪应变、Mises等效应变、最大主应力及SWT模型进行双轴保载疲劳寿命预测，其中SWT模型预测精度最高。     

不同交货状态海上结构用钢板应变时效性能研究

结合组织和力学性能检测结果,对TMCP和正火两种交货状态的355 MPa屈服强度级别海上结构用钢板的应变时效行为进行研究.结果 表明:舞钢生产的TMCP和正火钢板均能满足标准中5％应变时效性能的要求.当应变率增大时,应变时效性能降低.最大应变率主要与塑性相关,具有稳定化组织的TMCP+回火钢板及正火+空冷钢板能承受较大...     

阻尼器用低屈服点钢BLY160的低周疲劳性能

对BLY160低屈服点钢低周疲劳性能进行了研究。采用轴向应变控制方法,在MTS809电液伺服疲劳试验机上开展低周疲劳试验,分别研究了在循环载荷作用下试验材料的循环应力响应特征、循环应力与应变的关系以及应变幅和应变速率对材料低周疲劳性能的影响。试验结果表明,BLY160钢具有良好的低周疲劳性能,完全满足耗能阻尼器用钢的需要。     

Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel

The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.     

Water–air online quenching process of 3Cr2Mo steel based on numerical simulation

A three-dimensional finite element model was established to investigate the water–air online quenching process of 3Cr2Mo steel with 130-mm thickness. The temperature, metallographic structure and stress–strain fields of the steel were calculated under single-pass continuous quenching, multi-pass continuous quenching and multi-pass interrupted quenching (MPIQ) processes. The results show that the three quenching processes can avoid the pearlite appearance, and MPIQ process could be more effective to decrease the brittleness of steel. Besides, MPIQ process is able to reduce stress–strain, minimise deformation and avoid cracking problem. The hardness and the metallographic structure were tested after MPIQ process and tempering. After tempering, the metallographic structures were all tempered sorbite, and the hardness difference of the whole steel was less than 3HRC with no cracks. It can draw the conclusion that the MPIQ process is a suitable quenching process for 3Cr2Mo steel.     

Load sharing of the phases in 1080 steel during low-cycle fatigue

By means of X-ray diffraction, the stress response of the individual phases in a 1080 steel were measured. Specimens with pearlitic, spheroidal, and tempered martensitic microstructures were subjected to low-cycle fatigue and the stress-strain hysteresis loops were separated into components for the carbide and matrix phases. Calculations of the microstresses formed by differential plastic deformation of the matrix and inclusions accurately model the spheroidite. Measured microstresses in the pearlite are smaller than the predicted values, probably due to yielding of the cementite and limitations on modeling the morphology. Work-hardening rates associated with the microstresses also qualitatively agree with the measurements. The tempered martensite cyclically softens with fatigue loading. The increased plastic strain range in the tempered martensite with cyclic softening is accompanied by an increase in the microstresses. These microstresses are significantly larger than predicted. Formerly Research Assistant, Department of Materials Science and Engineering, McCormick School of Engineering, Northwestern University     

The micro structure of high carbon low alloy steel for easy drawing

For better processing performance of high carbon low alloy steel wire rod,an investigation about the influence of cementite lamellar spacing on wire ’easy drawing’ performance is completed.It is pointed out that too thin cementite lamellar spacing(<80 um) reduces the strain hardening level of wire drawing, and reduce the torsion performance of drawn wire at same time.For the wire or wire rod from industrial production,compared with the micro-structure with troostite,the micro-structure with sorbite or sorbite mixed with pearlite is more suitable to the drawing process with high reduction ratio.     

Study of New On‐Line Pre‐Hardening Technology of 3Cr2NiMo Plastic Die Steel

The thermal property parameters of 3Cr2NiMo plastic die steel were tested and the two‐dimensional finite element model was established in the paper. The temperature fields of 3Cr2NiMo plastic die steel plate with the thickness of 130 mm were calculated and analyzed under three quenching processes. The results showed that for all of the three processes the cooling rates could avoid the pearlite transformation zone when the temperature of the steel plate was more than 500°C. When the temperature was less than 500°C, the cooling rate of the third process was slower, and the temperature difference was effectively alleviated, which avoided large transformation stress appearance. For 3Cr2NiMo plastic die steel with the thickness of 130 mm, the third process was the best process. In the paper, the hardness and the microstructures were tested after the third process and tempering. After tempering process, the structures were all tempered sorbite, and the tempering hardness difference in the whole steel plate was less than 3HRC, and there were no cracks. The study provided references for on‐line pre‐hardening process formulation and optimization of large plastic die steel.     

Microstructural Evolution of a Hypoeutectoid Pearlite Steel under Rolling-sliding Contact Loading

To study the microstructural evolution of pearlite steel subjected to pure rolling and rolling-sliding contact loading,a hypoeutectoid pearlite steel with composition and microstructure similar to BS11 was designed and twindisc tests of this pearlite steel were performed to simulate the wheel/rail system.After a series of twin-disc tests,optical microscope(OM)observation,scanning electron microscope(SEM)observation,X-ray diffraction(XRD),and micro-hardness tests were conducted to characterize the microstructure.Under the pure rolling contact condition,a large amount of reticular cracks emerged within 60μm below the contact surface of the samples after 120 000 revolutions.The largest deformation was approximately 200μm below the contact surface.Under the rolling-sliding contact condition,the nodularization of pearlite within 100μm below the contact surface was obvious.The microstructure and stress-strain distribution of the area within 2mm below the contact surface were investigated.The distribution of micro-hardness under the contact surface varied with contact conditions.Finite element method(FEM)was used to simulate the stress-strain distribution.The results of SEM,FEM,and micro-hardness tests indicated that under the pure rolling contact condition,the maximum plastic strain was approximately 200-400μm below the contact surface.Conversely,under the rolling-sliding contact condition,the maximum plastic strain emerged on the contact surface.Under the pure rolling contact condition,the distribution of micro-hardness was almost identical to that of the equivalent plastic strain.Under the rolling-sliding contact condition,the distribution of micro-hardness was affected by the equivalent plastic strain and tangential stress.     

Effects of Strain Rate on Low Cycle Fatigue Behaviors of High-Strength Structural Steel

 The low cycle fatigue (LCF) behavior of a high-strength structural steel was investigated in the strain rate range of 4×10-5-0. 12 s-1 (0. 001-3 Hz) under constant total strain (±1%) control. The cyclic stress response at all strain rates exhibited behavior of rapid softening in the early stage of fatigue life and subsequent saturation up to failure. It was found that the stress amplitude, the plastic strain amplitude, the plastic strain energy density and the fatigue life depend mainly on the strain rate. The strain rate of 0. 012 s-1 was found as a transition point where the LCF of the steel showed different behavior from low strain rate to high strain rate. The relationship between the time to failure and strain rate was expressed well by a power law relation. The fracture surfaces of the fatigue samples were characterized by using a scanning electron microscope (SEM) and the fracture mechanisms were discussed in terms of time-dependent deformation of the steel.     

The Research and Development of Hot Rolling Marine Sorbite Rail Steel

This article describes research and development results of the hot-rolled air-cooled fine lamellar pearlite (sorbite) rail,by the optimal regulation of chemical composition.Total cross section of rail is fine pearlite (sorbite),while resistant to marine atmosphere corrosion.     

重轨端部淬火温度场与相变的计算机模拟

运用有限元法对U71Mn钢(%:0.65～0.76C、0.15～0.35Si、1.10～1.40Mn、≤0.030V、≤0.025Ti)重轨端部电磁感应加热55 s,空冷8 s,喷风冷却42 s,空冷至常温的热处理过程进行了计算机模拟分析,计算机图形学理论动态显示轨端内部任意时刻的温度分布、组织分布。模拟结果表明,在该冷却条件下淬火后端部横断面组织为细片状珠光体(索氏体+托氏体),无马氏体、贝氏体等组织,与实验结果相吻合。     

Fatigue crack initiation and propagation in a quenched and tempered niobium bearing HSLA steel

The fatigue crack initiation and propagation behavior of a niobium bearing HSLA steel heat treated to give two tempered martensitic microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy, and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them. The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack.