Friction and wear behavior of several hard materials

Sliding friction, abrasion and erosion tests were performed on several materials: cemented carbides, partially stabilized zirconia (Mg–PSZ), electroless Ni–P coatings and SAE 4140 steel as reference material. Sliding friction test was carried out in a pin-on-disk system. A micro-abrasion test was performed using the ball cratering methods. The erosion test consisted an air stream carrying abrasive particles of SiC, impinging flat samples. Sizes of wear scars were determined by optical microscopy and laser profilometry. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) were employed to observe morphological and chemical features of worn surfaces. The sliding pin-on-disk results of Ni–P coating showed smooth variation of the friction coefficient (mean values of 0.16–0.21), as well as the lowest wear rate compare with the other evaluate materials. Experimental results of the micro-abrasion test showed linear behavior between wear volume and sliding distance for all the evaluate materials. Cemented carbides showed the lowest wear rates followed by SAE 4140 steel, Mg–PSZ and Ni–P coating, respectively. Erosion test values ranked evaluated materials from lower to higher eroded volume as follows: SAE 4140 Steel, Ni–P coating, cemented carbides and Mg–PSZ ceramic.     

A comparison between magnetic properties and grain size for WC/Co hard materials containing additives of Cr and V

The magnetic properties of cemented carbides are often used as an indirect measure of the microstructure. For this purpose, models relating coercive force with WC-grain size have been developed. In this study, it is investigated how well magnetic models describe the microstructure of cemented carbides (WC/Co) for samples containing different kinds of commonly used grain growth inhibitors, varying carbon content, and cooling conditions. We found that small amount of V (0.2 wt%) and Cr (0.4 wt%) influence the magnetic properties differently. Even so, commonly applied models relating magnetic properties with WC-grain size may be applied, but the importance to account for variation in saturation magnetisation in the analysis is stressed.     

Detection and measurement of surface cracks in ferromagnetic materials using eddy current testing

Eddy current testing is used to detect and quantify defects in electric conductive materials. Defects investigated are specifically mechanical, chemical and microstructural heterogeneities. In this scope, artificial defects simulating superficial cracks have been achieved in order to establish an eventual relationship between the orientation, the shape and the size of the defect in one hand, and the signature that it generates in the impedance plane on the other hand. We determine in the first stage the operating conditions for the minimization of all signals that may perturb the crack signal such as the background one. We have been able to detect easily a surface crack on ferromagnetic steel in a large range of the frequency. Curves allowing the discrimination between signals corresponding to the presence of cracks and those corresponding to a lift-off have been established. The limit depth has as well been determined and we found that this limit has no relationship with the depth of penetration of the eddy current. The operative parameters such as the diameter and the working mode of the probe, the excitation frequency and the setting of the measurement equipment have been optimized for the testing of the quality and the reliability of critical ferromagnetic steel parts.     

J55套管钢铸坯角部横裂纹成因与防止措施

对马钢板坯铸机生产的 J55套管钢铸坯角部横裂纹成因与防止措施进行了探讨,并简述了相关试验情况。     

吉林和大庆地区钢制轻烃球罐多发性裂纹修复技术

作者曾对近年来吉林和大庆地区钢制轻烃球罐运行中频频发生的多发性裂纹进行了大量取样分析,证明它们基本属硫化氢应力腐蚀开裂,很难控制和修复.通过对球罐上述裂纹性质及机理分析、系列试验研究(包括修复技术理论基础、修复技术路线、修复材料和修复工艺试验、热处理作用和热处理工艺试验、表面复合复层作用与复合复层材料及工艺试验、浸渍试验和模拟硫化氢应力腐蚀试验、工程试验等),提出并发展了一种整体综合处理技术--排氢处理、软化组织的补焊、局部或整体热处理、喷涂特定的表面复合复层等.该技术已获工程应用,成功地修复了数十台容积120～1000m3的受损伤球罐,抑制了该类运行球罐H2S应力腐蚀开裂趋势.     

Mechanism of biaxial pre-stress method on welding residual stress and hot cracks controlling

Based on the conventional uniaxial pre-tensile stress method during welding, this study presents a new method of welding with biaxial pre-stress. With the help of numerical simulation, experiments were carried out on the self-designed device. Except for the control on residual stress and distortion as-welded, the experimental results also show its effect on the prevention of hot cracks, thus this method can make up for the disadvantage of the conventional pre-stress method. Hot cracks     

激光熔覆复合涂层裂纹产生原因及控制研究进展

综述了激光熔覆复合涂层裂纹产生的原因及影响因素,并从基材预热、熔覆后涂层整体回火和重熔、梯度结构、工艺参数优化、熔覆材料及添加剂成分控制和辅助技术等方面归纳了控制裂纹的方法和途径。     

废气处理燃烧炉壳裂纹原因及焊接修复

针对材质为0Cr18Ni9的奥氏体不锈钢燃烧炉,分析高温环境使用产生裂纹的原因及焊接性能,通过选择合适的焊接材料和制定合理的工艺措施,成功修复裂纹缺陷,保证0Cr18Ni9奥氏体不锈钢的高温使用性能和要求。     

热影响区连续孔隙状裂纹的表征及产生机理

以凝固温度范围极宽的合金为对象,研究焊接热影响区的液化现象,表征该区域中出现的连续孔隙状裂纹,并分析其产生机理. 结果表明,连续孔隙状裂纹的产生与母材中残余第二相颗粒的连续分布、热影响区中出现的严重液化现象及液相的流失密切相关. 其产生机理在于:焊接加热时,第二相颗粒加剧了晶界液化的程度,并形成连接热影响区和焊接熔池的液相通道;冷却过程中,由于熔池区熔体体积收缩促使热影响区中的液相流向熔池;凝固末期,通道因胞状晶生长而被堵塞,导致液相无法从熔池回填,从而在热影响区形成连续孔隙状裂纹.     

Near-threshold propagation of mode II and mode III fatigue cracks in ferrite and austenite

The near-threshold behavior of mode II and mode III long fatigue cracks in ferritic (ARMCO iron) and austenitic (X5CrNi18-10) steel were experimentally studied using various samples specially prepared to obtain the effective threshold values ΔK_IIeff,th and ΔK_IIIeff,th. In both investigated materials, the effective thresholds for mode III were ～1.7 times higher than those for mode II. Three-dimensional topological data obtained by the examination of fracture surfaces using stereophotogrammetry were utilized to identify crack growth micromechanisms. In austenite, mode I branching of both the mode II and mode III cracks started at the very onset of crack growth. On the other hand, all cracks in ferrite propagated in crystallographically assisted local mixed mode I + II + III with mode II dominance. These experimental results can be understood in terms of crack growth micromechanisms according to a deformation model in ferrite and a decohesion model in austenite. The dissimilarity of growth mechanisms in ferrite and austenite may be attributed to a different number of available slip systems in body-centered cubic and face-centered cubic metals.     

炼钢厂转炉托圈裂纹产生原因分析及改造措施

本文通过对不同钢厂同类设备使用及运行情况的科学分析比较,确定了包钢炼钢厂转炉托圈出现裂纹的原因,并采用技术可靠、施工简单、投资不大的改造措施,消除了由转炉托圈裂纹而带来的事故隐患,从而使整个转炉投入正常的运行。     

TiNi形状记忆合金与不锈钢激光焊接头裂纹分析及防止措施

采用激光焊对TiNi形状记忆合金与不锈钢异种材料进行焊接,用扫描电镜和激光共聚焦显微镜研究了TiNi合金/不锈钢接头裂纹及断口特征,分析了焊缝裂纹的形成机理,并提出了防止裂纹的措施.结果表明,裂纹多以微裂纹的形式出现于焊缝中心和TiNi合金侧熔合区.焊缝中存在大量的脆性化合物是产生裂纹的内在原因,接头受到拉伸应力是产生裂纹的必要条件,焊缝裂纹是二者共同作用的结果.通过焊接区加镍和钴中间层材料、改变激光光斑位置、焊接区施加轴向力及优化激光焊接参数的方法均能在一定程度上改善焊缝金属的裂纹敏感性,其中加金属中间层效果更为明显,加镍和钴中间层后,接头抗拉强度分别达到372和347 MPa,比未加中间层的接头的抗拉强度分别提高98.9%和85.6%.     

Equipment for remote recording of cracks in materials and welded joints

A plasma-arc welding technique is designed for joints with the technological beads necessary to fasten flanges in thin-walled large-sized cylindrical and spherical constructions. A high-temperature plasma-gas stream penetrating down to the bottom of the bath damages the oxide film when welding aluminium products.     

Ultrasonic detection and sizing of compressed cracks in glass- and carbon-fibre reinforced plastic composites

Nondestructive evaluation of compressed cracks is a major challenge. A quantitative study of the effect of crack-tip closure on the pulse-echo ultrasonic sizing of delaminations in fibre-reinforced polymer-matrix composites (FRP) is presented. In particular, this study focuses on the interaction of ultrasound with a closed crack or kissing disbond, and their effect on the ultrasonic inspectability of FRP laminates consisting of carbon and glass plies. The compression of laminar cracks in these two different laminate types is clearly detectable via both pulse-echo and through-transmission ultrasonic measurements, but the reflected ultrasonic pulses in the two material types exhibit markedly different behaviour. The glass-fibre laminates show a drop in the reflected signal for crack openings up to approximately half the crack growth load, whereas the corresponding carbon-fibre laminates show the expected increase in the reflected signal as the crack opens. The origins of the observed effect of crack closure on the reflection and transmission of ultrasound are analysed in detail to ascertain possible mechanisms responsible for these effects.     

On TGO creep and the initiation of a class of fatigue cracks in thermal barrier coatings

The initiation of a class of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to thermal gradient mechanical fatigue testing is investigated. The coating system is based on a NiCoCrAlY bond coat and a partially yttria stabilized zirconia top coat. To explain the development of the cracks of interest, the thermo-mechanical response of the bond coat and the thermally grown oxide (TGO) is examined and quantified through finite element analyses. The models include non-linear and time-dependent behavior such as creep, TGO growth stress, and thermo-mechanical cyclic loading. The simulations suggest that stress-redistribution due to creep can lead to tensile stresses in the TGO during TGMF testing that are large enough to initiate the cracks investigated.     

硬脆材料超声辅助磨削技术研究现状及展望

硬脆材料具有高强度、高硬度、低密度等特点,用传统方式加工时损伤大、效率低,而超声辅助磨削具有磨削力小、加工质量优等优点,在硬脆材料加工领域具有独特优势。从不同振动维数及振动方向与磨削表面位置关系的角度出发,总结了不同类型超声辅助磨削加工的机理及特性,在此基础上探讨了硬脆材料超声辅助磨削的延性域加工机理、超声振动参数与磨削用量匹配性的研究现状,并对今后超声辅助磨削领域应重点关注的研究方向进行了展望。      

Grain level analysis of crack initiation and propagation in brittle materials

A study on the accuracy of cohesive models for capturing dynamic fragmentation of ceramic microstructures is presented. The investigation consists of a combined experimental/numerical approach in which microcracking and damage kinetics are examined by means of plate impact recovery experiments. The numerical analysis is based on a 2-D micromechanical stochastic finite element analysis. The model incorporates a cohesive law to capture microcrack initiation, propagation and coalescence, as well as crack interaction and branching, as a natural outcome of the calculated material response. The stochasticity of the microfracture process is modeled by introducing a Weibull distribution of interfacial strength at grain boundaries. This model accounts for randomness in grain orientation, and the existence of chemical impurities and glassy phase at grain boundaries. Representative volume elements (RVE) of ceramic microstructure with different grain size and shape distributions are considered to account for features observed in real microstructures. Normal plate impact velocity histories are used not only to identify model parameters, but also to determine under what conditions the model captures failure mechanisms experimentally observed. The analyses show that in order to capture damage kinetics a particular distribution of grain boundary strength and detailed modeling of grain morphology are required. Simulated microcrack patterns and velocity histories have been found to be in a good agreement with the experimental observations only when the right grain morphology and model parameters are chosen. It has been found that the addition of rate effects to the cohesive model results in microcrack diffusion not observed experimentally.     

宝钢湛江连铸板坯角裂的控制研究及设计实践

板坯连铸机生产包晶钢、中碳钢、含硼或铌微合金钢容易产生角裂,将导致对应的轧材出现边部翘皮和边部裂纹,为了不影响轧材质量,连铸工序必须对角裂进行火焰清理,加大了生产成本。2013年宝钢决定将罗泾两台单流2 300 mm板坯连铸机搬迁到宝钢湛江钢铁基地。原罗泾连铸机所生产板坯的角裂问题严重,针对该问题,中冶赛迪和宝钢从弯曲段辊列、扇形段辊缝精度控制、二冷分区、二冷宽度控制、动态二冷模型、动态轻压下模型等方面做了全面的优化和改进,解决了原罗泾连铸机生产板坯易产生角裂的难题,包晶钢、中碳钢、含硼或铌微合金钢板坯角裂发生率从90%以上降低到1%以下。着重阐述在设计阶段针对角裂问题所做的工作。     

Study on axial-feed mirror finish grinding of hard and brittle materials in relation to micron-scale grain protrusion parameters

An axial-feed mirror finish grinding of hard and brittle materials is proposed by controlling grain protrusion parameters. In this grinding, the grinding wheel feed is along the wheel axial direction rather than in the traditional wheel cutting direction. The objective is to understand how micron-scale grain protrusion parameters influence ductile-mode grinding and ultimately to realize efficient mirror finish grinding using a coarse diamond grinding wheel. In this study, the grain tip truncation (GT-truncation) was performed after dressing to improve grain protrusion topography. First, a formation model of axial-feed ground surface was constructed to analyze the effect of grain protrusion parameters and grinding parameters on the critical cutting depth transferred from brittle-mode removal to ductile-mode removal; then GC dressing and GT-truncation of #180 diamond grinding wheel were experimentally performed to investigate surface roughness and ductile-mode grinding behavior with reference to grinding parameters and grain protrusion parameters; finally, a truncated coarser #60 diamond grinding wheel was employed for mirror finish grinding to observe active grain number and grain protrusion angle. Theoretical analysis shows that this ductile-mode grinding is dominated by active grain number, active grain protrusion angle, wheel rotating speed and axial-feed speed, but it does not depend on the depth of cut assumed to be less than the grain protrusion height. Experimental results indicate that the GT-truncation may increase active grain number and grain protrusion angle for ductile-mode grinding when the axial-feed speed decreases to some extent. Moreover, the micro tip radius of diamond grain also influences the ground surface. It is confirmed that by increasing active grain number and grain protrusion angle synchronously, a truncated #60 diamond grinding wheel can be applied for efficient mirror finish grinding of the SiC ceramic plate at the axial-feed speed of 50 mm/min and the tool path interval of 0.1 mm.     

硬脆材料的电镀金刚石线锯超声切割锯切力研究

基于冲量理论和振动加工理论,采用叠加原理建立了电镀金刚石线锯超声切割锯切力数学模型.从理论上分析了电镀金刚石线锯施加超声振动后锯切力与普通锯切力的区别,并进行了超声振动与普通锯切力对比试验研究.结果表明:锯切力的大小随线锯往复频率的提高而降低,随侧向压力的增加而增大.通过比较发现,超声振动锯切力比普通锯切力减小20%～30%.同时,对超声振动锯切过程中单颗金刚石磨粒切削运动轨迹进行了仿真分析,进一步解释了施加超声振动后锯切力减小的原因.