Reverse-transformation austenite structure control with micro/nanometer size

To control the reverse-transformation austenite structure through manipulation of the micro/nanometer grain structure, the influences of cold deformation and annealing parameters on the microstructure evolution and mechanical properties of 316L austenitic stainless steel were investigated. The samples were first cold-rolled, and then samples deformed to different extents were annealed at different temperatures. The microstructure evolutions were analyzed by optical microscopy, scanning electron microscopy (SEM), magnetic measurements, and X-ray diffraction (XRD); the mechanical properties are also determined by tensile tests. The results showed that the fraction of stain-induced martensite was approximately 72% in the 90% cold-rolled steel. The micro/nanometric microstructure was obtained after reversion annealing at 820-870℃ for 60 s. Nearly 100% reversed austenite was obtained in samples annealed at 850℃, where grains with a diameter ≤ 500 nm accounted for 30% and those with a diameter >0.5 μm accounted for 70%. The micro/nanometer-grain steel exhibited not only a high strength level (approximately 959 MPa) but also a desirable elongation of approximately 45%.     

Effect of high temperature and high strain rate on the dynamic mechanical properties of Fe-30Mn-3Si-4Al TWIP steel

The dynamic mechanical properties of Fe-30Mn-3Si-4Al twinning induced plasticity (TWIP) steel were studied by the split-Hopkinson pressure bar (SHPB) at temperatures of 298–1073 K and strain rates of 700, 2500, and 5000 s?1. The TWIP steel indicates strain rate hardening effect between 700 and 2500 s?1, but it shows strain rate softening effect between 2500 and 5000 s?1. In addition, the strain rate softening effect enhances with an increase in deformation temperature. After deformation, the microstructures were studied by optical microscopy (OM). It is shown that the deformation bands become more convergence, a part of which become interwoven with an increase in strain rate, and the dynamic recovery and recrystallization are enhanced with an increase in both temperature and strain rate.     

IF钢与ELC钢织构及性能的对比

研究了经铁素体区热轧、冷轧和退火生产的ELC钢和IF钢的组织、织构及性能.结果表明,铁素体区热轧后,ELC钢中的大部分组织是变形带;而IF钢中已经形成了等轴晶粒.ELC钢的热轧织构中形成了较强的α织构和很弱的γ织构,织构主要组分集中在{001}〈110〉~{223}〈110〉;而IF钢中形成了强的γ织构和很弱的α织构.ELC钢的退火织构中主要是γ织构,但仍有较弱的α织构存在;IF钢的退火织构中只有很强并且均匀的γ织构,强度明显高于ELC钢的退火织构.与ELC钢相比,IF钢的平均塑性应变比(r值)高出0.6,延伸率高出5%,同时,抗拉强度升高,而屈服强度降低.     

Z3CN20—09M铸造奥氏体不锈钢的低周疲劳行为

采用径向应变控制研究了Z3CN20-09M奥氏体不锈钢在室温和350℃高温下的低周疲劳行为．Z3CN20-09M不锈钢表现为先硬化后软化的循环特性,但硬化的程度取决于温度和应变幅．随着应变幅的增加,Z3CN20-09M钢的低周疲劳循环寿命逐渐减短,而相同循环次数下应力幅也随之提高．温度对Z3CN20-09M钢的低周疲劳行为影响较大,与室温相比高温下的循环硬化程度更高,相同应变幅下高温的低周疲劳寿命也高于常温下的寿命．通过疲劳实验的原位观察发现,奥氏体内的滑移面、夹杂物及奥氏体和铁素体两相的界面是疲劳裂纹可能的形核位置,奥氏体和铁素体两相的不协调变形使相界处产生应力集中,导致疲劳裂纹容易沿两相界面扩展．     

Effects of annealing temperature on the microstructure and properties of the 25Mn-3Si-3Al TWIP steel

Microstructures and mechanical properties of the 25Mn twinning induced plasticity (TWIP) steel at different annealing temperatures were investigated. The results indicated that when the annealing temperature was 1000℃, the 25Mn steel showed excellent comprehensive mechanical properties, the tensile strength was about 640 MPa, the yield strength was higher than 255 MPa, and the elongation was above 82%. The microstructure was analyzed by optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Before deformation the microstructure was composed of austenitic matrix and annealing twins at room temperature; at the same time, a significant amount of annealing twins and stacking faults were observed by TEM. Mechanical twins played a dominant role in deformation and as a result the mechanical properties were found to be excellent.     

温度对X12CrMoWVNbN10-1-1钢组织及力学性能的影响

对X12Cr Mo WVNb N10-1-1耐热钢进行了300~600℃之间的高温力学试验,利用OM,SEM与TEM观察分析各温度下材料的微观组织及断口形貌,研究了温度对材料组织及高温力学性能的影响.结果表明:随温度升高,300~400℃,析出的脆性M3C相的数量和尺寸不断增加,且出现偏聚,析出强化和形变强化作用逐渐增强,塑性变形中,脆性相M3C处更容易出现应力集中,裂纹的产生、扩展更快,更易断裂,材料的塑性随之下降.400~600℃,碳化物发生转变,M3C相快速重熔分解,M7C3及M23C6开始析出,使得析出相的数量和尺寸下降,强化作用减弱;同时,动态回复作用越来越强,材料的强度快速下降,塑性快速上升.     

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.     

高铬铸钢轧辊热疲劳过程的组织和性能

针对现场出现的轧辊热疲劳裂纹较严重问题,研究了高铬铸钢轧辊热疲劳过程的组织和性能.通过约束热疲劳实验及高温拉伸实验,获得了热疲劳循环次数与主裂纹长度的关系,测试了高温状态下材料机械性能.用SEM,EDS分析了晶粒组织、成分偏析对热疲劳裂纹扩展的影响.结果表明:高铬铸钢轧辊的表面状态影响热疲劳裂纹萌生、扩展路径和方式;热循环上限温度及升温速度对裂纹扩展速率的影响较大;材料性能、组织变化等对高铬铸钢的热疲劳性能有明显影响.     

钢纤维混凝土桩承载特性模型试验

为研究钢纤维混凝土桩在竖向荷载作用下的承载力、桩身应变特点以及桩体的破坏现象,就不同直径的钢纤维混凝土桩和素混凝土桩进行了静载试验.沿桩体纵向布置应变片,监测其应变.试验完成后,开挖出桩体,对比分析钢纤维混凝土桩和素混凝土桩的破坏特点.结果表明:掺入钢纤维能有效增大桩体抵抗压缩变形的能力;钢纤维混凝土桩桩径由4.5cm增至7.0cm时,桩体抵抗压缩变形的能力增大,单桩极限承载力增大,但当桩径由7.0cm增至8.0cm时,桩体的压缩变形和极限承载力变化较小;2种桩体发生破坏时均为桩体两端出现裂缝甚至破损,钢纤维有效地提高了素混凝土桩的抗裂性能.根据试验结果,给出了设计钢纤维混凝土桩的几点建议.     

大足大佛湾石刻岩石微观与力学特性研究

为了给大佛湾石刻造像的病害治理提供依据,采用X射线衍射、光学显微镜、扫描电镜和岩石力学试验系统等对该区域内岩样的矿物成分、微观结构和力学特性进行分析。结果表明,石刻区域内岩石的矿物成分以石英、钠长石、方解石、黏土矿物为主,且由于黏土矿物和方解石的含量与分布不同,造成了砂岩外观颜色的差异;区域内岩层没有变形改造,砂岩内部存在较多微孔隙而泥质砂岩、泥岩的结构较为致密,且水对岩石的结构有溶蚀、淋滤等破坏;区域内岩石的力学性质受黏土矿物的影响较大,通过力学试验得出的基本力学参数显示,与天然状态相比,岩样在饱水状态下的强度明显降低。     

Microstructure and mechanical properties of the micrograined hypoeutectic Zn–Mg alloy

A biodegradable Zn alloy, Zn–1.6Mg, with the potential medical applications as a promising coating material for steel components was studied in this work. The alloy was prepared by three different procedures: gravity casting, hot extrusion, and a combination of rapid solidification and hot extrusion. The samples prepared were characterized by light microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis. Vickers hardness, tensile, and compressive tests were performed to determine the samples’ mechanical properties. Structural examination reveals that the average grain sizes of samples prepared by gravity casting, hot extrusion, and rapid solidification followed by hot extrusion are 35.0, 9.7, and 2.1 μm, respectively. The micrograined sample with the finest grain size exhibits the highest hardness (Hv = 122 MPa), compressive yield strength (382 MPa), tensile yield strength (332 MPa), ultimate tensile strength (370 MPa), and elongation (9%). This sample also demonstrates the lowest work hardening in tension and temporary softening in compression among the prepared samples. The mechanical behavior of the samples is discussed in relation to the structural characteristics, Hall–Petch relationship, and deformation mechanisms in fine-grained hexagonal-close-packed metals.     

楔横轧高铁车轴钢25CrMo4塑性损伤形成机理

为揭示楔横轧高铁车轴塑性损伤形成机理,采用近似分析方法对车轴钢25CrMo4试件在三个温度(1040,1100和1160℃)和两个应变速率(10和100s-1)下做不同形变量的压缩和拉伸,获取各应力曲线以及微观组织.结合微观组织对比应力曲线得出结论:压缩时的软化机制由回复和再结晶主导,而拉伸时的软化机制由塑性损伤主导;塑性损伤分为晶界损伤和夹杂损伤两种,且经历形核,长大和汇聚三个阶段;流线处是晶粒细化,晶界损伤及夹杂损伤最容易发生的变形带;夹杂损伤分为晶界和晶内两种;拉应力是造成楔横轧高铁车轴塑性损伤的主要原因.     

Effect of interface morphology on the mechanical properties of titanium clad steel plates

Interface morphology has important influence on the bond quality of titanium clad steel plates. The mechanical properties of titanium clad steel plates with wavy and straight interfaces were investigated by tensile-shear tests and bending tests. The interface morphology of the plates was examined by optical microscopy (OM) and scanning electron microscopy (SEM). The experimental results show that the shear strength of a wavy interface is higher than that of a straight interface. A wavy interface is the guarantee for obtaining high shear strength to provide a greater shear resistance. During the macrobending process, cracks appear in the swirl of the wave tip and ferrotitanium intermetallics. For in-situ observing the bending process by SEM, the wave tip of a wavy interface and the massive ferrotitanium intermetallics of a straight interface are places where cracks initiate and propagate. The results are the same as those observed in the macrobending process. Because of high hardness, the wave tip and the massive ferrotitanium intermetallics are hard in terms of compatible deformation.     

Hot deformation behavior of Super304H austenitic heat resistant steel

The hot compression tests of Super304H austenitic heat resistant steel were carried out at 800–1200℃ and 0.005–5 s-1 using a Gleeble 3500 thermal-mechanical simulator, and its deformation behavior was analyzed. The results show that the flow stress of Super304H steel decreases with the decrease of strain rate and the increase of deformation temperature; the hot deformation activation energy of the steel is 485 kJ/mol. The hot deformation equation and the relationship between the peak stress and the deformation temperature and strain rate is obtained. The softening caused by deformation heating cannot be neglected when both the deformation temperature and strain rate are higher.     

Macro-micro dynamic behaviors and fracture modes of roll bonded 7A52/ 7A01/7B52 aluminum laminates in high velocity deformation

Damage tolerance improvement in the lamination of aluminum alloy plates and composites has been reported in many studies. In the present study the macro-micro dynamic deformation behavior and related mechanisms of 7A52/7A01/7B52 laminated plates processed by hot roll bonding of 7A52, 7A01 and 7B52 plate at high strain rates and quasi-static compression has been investigated. The microstructure of the laminated plates was examined with backscatter diffraction (EBSD) techniques, scanning and transmission electron microscopies. The results showed that with increased strain rate, obvious strain rate hardening was observed in the single layer specimens. The peak flow stress of the multilayer samples was slightly higher than that of the 7A52 monolayer samples and much lower than that of the 7B52 monolayer samples at the same strain rate. Beyond the peak stress state, the strain hardening was replaced by thermal softening in the 7A52 layer, leading to low resistance of deformation and high tendency to facilitate deformation-induced adiabatic shear bands (ASBs) that consist of dynamic recrystallized grains. ASBs in laminated samples were deflected and bifurcated at the interface of 7A52 layer. In addition, bifurcated ASBs converged at the interface between the 7A52 and 7A01 layers. The high deformation resistance observed in the laminate under dynamic loading was a consequence of the high capacity for strain hardening in the 7A01 layer. This hardening effectively overcame the influences of thermal softening and dynamic recovery during dynamic loading. This study provides an understanding of the laminate's microstructure evolution during dynamic deformation and its relevance to the fracture modes of a multilayer structure under dynamic loading.     

热轧双相钢微观应力--应变模型

利用扫描电子显微镜和透射电子显微镜对热轧后双相钢的微观组织进行分析,用Image-Pro Plus软件测定双相钢微观组织中各独立相的体积分数. 根据多相材料中间混合法则和Swift方程,建立热轧双相钢微观应力-应变模型,并用DP590和DP780钢单向拉伸曲线进行验证. 结果表明,该应力-应变关系微观模型基本阐明热轧双相钢微观组织参数与宏观力学性能的内在联系,能够准确地描述材料的变形行为,同时很好地预测热轧双相钢宏观的拉伸曲线.     

Comparison between the surface defects caused by Al_2O_3 and TiN inclusions in interstitial-free steel auto sheets

Al_2O_3 and TiN inclusions in interstitial-free(IF) steel deteriorate the properties of the steel. To decrease the defects of cold-rolled sheet, it is important to clearly distinguish between the degrees of damage caused by these two inclusions on the surface quality of the steel. In this study, a nanoindenter was used to test the mechanical properties of the inclusions, and the distribution and size of the inclusions were obtained by scanning electron microscopy(SEM). It was found that when only mechanical properties are considered, TiN inclusions are more likely to cause defects than Al_2O_3 inclusions of the same size during the rolling process. However, Al_2O_3 inclusions are generally more inclined to cause defects in the rolling process than TiN inclusions because of their distribution characteristic in the thickness direction. The precipitation of Al_2O_3 and TiN was obtained through thermodynamical calculations. The growth laws of inclusions at different cooling rates were calculated by solidification and segregation models. The results show that the precipitation regularity is closely related to the distribution law of the inclusions in IF slabs along the thickness direction.     

Q&P1180高强钢激光焊接头的抗氢脆性能研究

采用激光焊技术对汽车用Q&P1180高强钢进行焊接试验,分析焊接工艺参数对激光焊接头组织演变和性能的影响。选择综合力学性能较好的试样进行低应变速率拉伸试验,研究激光焊接头不同区域的微观组织对Q&P1180高强钢氢脆敏感性的影响。结果表明,当激光功率为2.5～3.5 kW、焊接速度为4.0 m/min(或激光功率为3.0 kW,焊接速度为3.6～4.2 m/min)时,Q&P1180高强钢激光焊获得了全焊透及表面成形良好的接头。不同激光功率和不同焊接速度下,焊缝区和热影响区的硬度均高于母材区的,且在焊缝的两边都存在一个软化区。在激光功率为3.0 kW、焊接速度为4.0 m/min时,激光焊接头的综合性能最佳。在预充氢的慢应变速率拉伸试验中,随着充氢电流密度的增大或充氢时间的延长,其抗拉强度和塑性逐渐下降。其中,当充氢电流密度为2 mA/cm²、充氢时间为5 min时,氢脆敏感性因子为78.0%,具有较高的氢脆敏感性。     

铁路钢桥高强度紧固螺栓断裂分析及防护

为探求高强度紧固螺栓发生断裂的原因,对某铁路钢桥上断裂的紧固螺栓进行了失效分析。利用扫描电子显微镜、能量色散X射线谱、金相显微镜、洛氏硬度仪、X射线荧光光谱仪、电子万能试验机等技术手段,对铁路钢桥失效螺栓的断口形貌、断口微区成分、显微组织、硬度、螺栓用钢的成分以及力学性能等方面进行分析与研究。结果显示：螺栓用钢的化学成分、硬度及力学性能均符合国家标准要求;螺栓断口起裂区存在树枝状裂纹,裂纹内部有含硫的腐蚀产物,且螺栓工作时所受外力为交变载荷。因此,螺栓的断裂是在交变载荷下发生的腐蚀疲劳断裂。螺栓断裂的初始裂纹源位于螺杆上螺纹的根部或螺柱与螺母连接位置的根部,为由硫元素引起的应力腐蚀裂纹。此研究为防止螺栓发生应力腐蚀裂纹及衍生病害提供了理论依据。     

Damage prediction of 7025 aluminum alloy during equal-channel angular pressing

Equal-channel angular pressing (ECAP) is a prominent technique that imposes severe plastic deformation into materials to enhance their mechanical properties. In this research, experimental and numerical approaches were utilized to investigate the mechanical properties, strain behavior, and damage prediction of ECAPed 7025 aluminum alloy in various conditions, such as die channel angle, outer corner angle, and friction coefficient. Experimental results indicate that, after the first pass, the yield strength, ultimate tensile strength, and hardness magnitude are improved by approximately 95%, 28%, and 48.5%, respectively, compared with the annealed state, mainly due to grain refinement during the deformation. Finite element analysis shows that the influence of die channel angle is more important than that of outer corner angle or friction coefficient on both the strain behavior and the damage prediction. Also, surface cracks are the main cause of damage during the ECAP process for every die channel angle except for 90°; however, the cracks initiated from the neighborhood of the central regions are the possible cause of damage in the ECAPed sample with the die channel angle of 90°.