On the influence of binder content and binder composition on the mechanical properties of hardmetals

In order to achieve an understanding of damage mechanisms and the role of the binder phase in the fatigue behaviour of hardmetals, two series of hardmetals with 7, 15 and 27 wt% of Co and CoNiFe binder phases were investigated in a reversed bending stress apparatus. Microstructural investigations were carried out using SEM and TEM. Finite element simulations based on real microstructure cut-outs were used to analyse the stress and strain states. While both series of hardmetals exhibit inert strengths which depend strongly on the binder content, their fatigue behaviour is totally different. For the WC–Co grades, no significant influence of the binder content on the fatigue behaviour was found. In contrast to this, the fatigue behaviour of the WC–CoNiFe grades showed a clear dependence on the binder content. The microstructural investigations revealed that the mechanism of stacking fault formation and phase transformation in the Co binder phase, together with a stress concentration in the binder pools are responsible for the fatigue behaviour of the WC–Co grades. In WC–CoNiFe grades, no stacking faults or transformed zones were observed. In addition, the CoNiFe binder phase reveals a very ductile flow nature, which results in large plastically deformed zones in the binder phase.     

静载荷对30CrMnSiA在中性及酸性溶液中腐蚀速度的影响

    用电化学测量技术通过加载时30CrMnSiA在中性溶液中自腐蚀电位和阳极极化曲线负移,及在酸性溶液中电化学阻抗谱的不同变化,研究了静载荷对腐蚀速度的影响.并提出外力是否影响腐蚀速度,应视腐蚀的控制步骤而定的观点.结果表明,当腐蚀体系阴阳极都为活化控制时,外力增加腐蚀速度;当腐蚀体系控制步骤为扩散控制时,外力不影响腐蚀速度;对吸氧腐蚀而言,在外力较大时,还需考虑外力是否导致阴极控制过程的转变.     

焊接残余应力对高强度钢低匹配对接接头静载强度的影响

采用有限元方法研究了焊后拉伸条件下高强度钢等匹配和低匹配对接接头内部应力的变化情况。结果表明,焊后横向和纵向拉伸载荷增至临界失效载荷期间,等匹配和低匹配接头的焊缝区和母材区应力均一直持续增加,但焊缝及近缝母材区应力在焊接残余应力基础上的增加较远端母材区缓慢;最终近缝母材区的应力明显高于远端母材区,未表现出内应力完全调匀的特征;这意味着由于焊接残余应力的存在,高强度钢宽板等匹配焊接结构的静载强度可能略有损失,而高强度钢宽板低匹配焊接结构更将在焊缝低强的影响下损失更大的静载强度。     

Inconel 625熔敷金属抗Cl?腐蚀行为分析

为了分析Inconel 625熔敷金属在Cl?腐蚀液中的腐蚀行为,采用静态浸没腐蚀和慢应变速率拉伸的方法,研究了不同温度下熔敷金属浸没在熔盐中的腐蚀行为以及室温应力腐蚀开裂行为. 通过X-ray diffractiong(XRD),S-3400N扫描电子显微镜系统地研究了熔敷金属的腐蚀产物物相组成、腐蚀形貌和元素分布. 结果表明,在静态浸没腐蚀时,两种温度下熔敷金属腐蚀失重均呈现出递增的变化趋势,但是不同时间区间的增加幅度有所不同. 前10 h熔敷金属的腐蚀失重缓慢增加,10 ~ 60 h腐蚀失重激增,可以发现10 h是腐蚀行为的分界点. 700 ℃下熔敷金属的耐蚀性优于900 ℃,其原因在于700 ℃时熔敷金属表面出现致密MgO壳层,能够阻碍腐蚀反应进行;而900 ℃时,由于CrCl₃的存在,出现“破壳效应”,破坏MgO壳层,使其耐蚀性下降. 对于慢应变速率拉伸而言,腐蚀介质中试样的抗拉强度比在空气介质中降低22.97%,应力腐蚀开裂敏感性指数为30.39%,说明腐蚀介质中的Cl?会增大试样的应力腐蚀开裂敏感性.     

预腐蚀疲劳寿命影响系数模型研究

针对腐蚀环境下飞机结构疲劳寿命评定问题,研究了恒幅应力水平下的地面停放预腐蚀影响系数C模型，根据统计分析推导出C曲线的关系式;疲劳试验数据分析结果建立了预腐蚀影响系数C模型.结果表明，随着腐蚀时间的增加,疲劳寿命影响系数C不断下降;同一时间下,应力水平S高,影响系数C大;应力水平S低,影响系数C小；在一定的腐蚀疲劳条件(时间、应力水平)下,可求出任一给定可靠度p时的Cp值和疲劳寿命预测值.     

Near-nano WC–Co hardmetals: Will they substitute conventional coarse-grained mining grades?

Development of nanostructured hardmetals is a task of great importance. Nevertheless, in spite of some “euphoria” with respect to nanograined hardmetals, their potential application ranges are yet not clear. In some works, near-nano and nano hardmetals are believed can potentially substitute conventional medium- and coarse-grained WC–Co grades. In the present work near-nano hardmetals with WC mean grain size of nearly 200 nm and Co contents of 10–33 wt.% were produced and examined with respect to their hardness, fracture toughness, transverse rupture strength and wear-resistance. The near-nano hardmetal with 10% Co having a hardness of 20 GPa and fracture toughness of 9.5 MPa m^1/2 is characterised by exceptionally high wear-resistance obtained by use of the ASTM B611 test in comparison with an ultra-fine grade with 10%. The wear-resistance of the near-nano hardmetals in the ASTM B611 test significantly decreases with increasing the Co content and the wear rates of the difference between the wear rates of the grades with 10% and 33% Co is equal to nearly 44 times. The near-nano hardmetals with 25%, 28% and 33% Co having a moderate hardness and high fracture toughness corresponding to conventional coarse and ultra-coarse-grained mining grades have a very low wear-resistance in laboratory tests on concrete-cutting, granite-cutting and percussion drilling of quartzite. A number of grades with the very similar hardness of 13 ± 0.2 GPa, WC mean grain sizes varying from 0.2 to 4.8 μm and Co contents varying from 3% to 25% were produced and examined by use of the ASTM B611 test. The wear-resistance of the near-nano grade with 25% Co is found to be lower by more than three times compared to the coarsest grade with 3% Co at almost the same hardness. In this case, in spite of the very similar hardness of all the samples, the proportion of the soft binder phase on the surface subjected to abrasive particles when performing the test is significantly higher for the near-nano grade compared to the coarse- and ultra-coarse grained hardmetals. Thus, near-nano and presumably nano hardmetals are expected to never substitute conventional medium- and coarse-grained mining grades. The only application range, where near-nano and nano hardmetals can potentially substitute conventional grades, is an application range of hardnesses of above 18 GPa.     

Microstructure and mechanical properties of ultrafine-grained hardmetals

The interest in ultrafine-grained hardmetals as woodcutting tool materials derives from their excellent mechanical properties compared with those of conventional hardmetals. The aim of this work was to determine the mechanical properties of ultrafine-grained hardmetals and to correlate the measured effects with microstructural parameters. The ultrafine-grained hardmetals (WC grain size 0.3 μm) investigated consisted of different WC powders and different binder systems: Co and complex binder systems. The mechanical properties of ultrafine-grained hardmetals were tested under two different loading conditions: monotonically increasing and cyclic alternating bending loads. It could be shown that the binder systems of different compositions show different behaviours under cyclic loads. Ultrafine-grained hardmetals with Co binder exhibit high bending strength values, but high fatigue sensitivity. Ultrafine-grained hardmetals with complex binders show lower bending strength values but their sensitivity to fatigue is lower. This implies that different damaging mechanisms exist for ultrafine-grained hardmetals with Co and complex binders.     

A numerical model to investigate the role of residual stresses on the mechanical behavior of Al/Al2O3 particulate composites

Research is presented about the mechanical behavior of a 6061 aluminum alloy reinforced with alumina particles. In particular, the role of thermal-induced residual stresses on the mechanical behavior of this composite is analyzed. Experimental tests were carried out to evaluate the mechanical characteristics of this type of material under static and fatigue loading. Fractographies on broken specimens evidenced the failure mechanisms under different load conditions. Also carried out were measurements using the x-ray diffractometric (XRD) technique to determine the residual stresses due to the thermal treatment both in the matrix and in the particles. A microscale finite-element model (FEM) of this material was developed to investigate the actual stress state caused by the thermal treatment and an applied load. A comparison of the numerical results and the experimental observations helped to explain the fracture modes under static and cyclic loading and to determine the role of the residual stresses under both monotone and cyclic loads. These results suggest some treatment to improve fatigue strength of the material.     

A3钢在弹性形变范围内的应变电极行为

以A3钢为例,研究了外加力场对不同腐蚀体系电化学行为的影响.探讨了外加力场对腐蚀速度的作用,认为外加力场并不会影响扩散过程的腐蚀速度,但是会加速活化过程以及混合控制过程的腐蚀速度.在扫描电镜下观察了受外力和腐蚀交互作用之后的金属表面形貌.并提出对局部腐蚀严重的腐蚀体系需要控制外加力场,以免造成局部应力集中过大,萌生裂纹的现象.     

Model single point abrasion experiments on WC/Co hardmetals

WC based hardmetals are widely used in applications where abrasion resistance is important.This paper describes the utility of scratch tests as model single point abrasion tests to evaluate the response of WC/Co hardmetals to the type of conditions that will occur in abrasion. The different types of experiments were single and multiple pass tests, and multiple pass tests in corrosive media.The resultant scratches were observed by stepwise SEM imaging, stereographic image correlation and 3D reconstruction, and EBSD analysis.It was found that many of the features observed in the model tests paralleled those seen in other abrasive tests on WC/Co materials such as build up of plastic damage in WC grains, fracture and fragmentation of WC grains, deformation of the Co binder phase and re-embedment of WC grains in the surface layers of the material. For tests in the presence of corrosive media, the binder phase was removed early in the sequence of scratches, leading to breakdown of the structure of the material.Stereo reconstruction was shown to be a valuable way of visualising and measuring the physical dimensions of scratches, providing a future route for the quantification of damage in these model experiments.     

In-situ observation of hardmetal deformation processes by transmission electron microscopy. Part I: Deformation caused by bending loads

WC-based hardmetals have a unique combination of different properties including high hardness, fracture toughness, abrasion resistance, strength, fatigue resistance, etc. Such properties are achieved due to extraordinary features of tungsten carbide, which is characterized by a certain degree of plastic deformation before failure when loading. The major objective of this work is to examine hardmetal deformation processes leading to the formation and movement of crystallographic defects in a hardmetal lamella as a result of its in-situ bending directly in a transmission electron microscope. The deformation is found to result in the formation of different crystal lattice defects in the Co-based binder and WC grains. Mainly dislocations form in the tungsten carbide grains. The dislocations start moving when increasing the applied bending load. The deformation of the binder phase is found to result in the formation of mainly lamellae and stacking faults in the Co crystal lattice. As a result of the formation, interaction and movement of the crystal lattice defects in the WC phase and binder phase a significant rate of plastic deformation of the hardmetal lamella was achieved under bending loads without its breakage.     

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.     

The Determination of the Porosity of Electro Deposits

Newly developed steels with tensile strength of up to 150 Ions p.s.i. are shown to fail by embrittlement under constant load, well below their nominal ultimate strength. This effect is usually attributed to hydrogen embrittlement. The paper describes a programme of tests on one specific type of ultra-high tensile steel. Small specimens (tensometer No. 12), notched in their centre, are subjected to constant stress in specially designed rigs. Various types of electrolytic or vapour-deposited coatings are applied to the specimens. The tests are conducted in non-corrosive or corrosive conditions. It is concluded that with stresses of the order of 80% of fracture strength, specimens electroplated with cadmium generally fail in 10–100 hr in the absence of corrosion, whilst zinc vapour-coated specimens do not appear to be sensitive to delayed fracture. Nickel-chromium plated specimens give inconsistent results, whilst nickel-plating usually behaves in much the same way as zinc vapour- coated specimens.

If a corrosive agent, such as 3% sodium chloride spray, is applied, rapid failure of uncoated and coated specimens takes place at stresses of approximately 70% of the notched fracture strength. Of the metal coatings, cadmium-plated specimens are most sensitive, nickel-plated specimens the most resistant; hard silicone lacquer show the best results. There is evidence that failure is related to the mechanism of corrosion reaction rather than to the extent of corrosion. Other corrosive agents, such as tap water or solutions of ammonium carbonate, do not cause this accelerated failure even if severe corrosion takes place.     

高强钢低匹配等承载对接接头焊接残余应力调匀能力

借助有限元软件研究了高强钢对接接头中焊接残余应力的调匀情况.结果表明,因母材塑性储备不足,所有接头焊后横向、纵向拉伸峰值应力均在焊接残余应力的基础上不断增加,但接头各区域应力增速不同,最终残余应力均未完全调匀.低匹配静载ELCC接头高塑性、低屈强比的盖面焊道位于高值残余应力所在的焊缝及近缝区母材表面并分担载荷,使该区应力增速放缓,故其残余应力调匀能力优于等匹配接头.考虑焊接残余应力的ELCC接头静载失效更可能发生在母材区,这与ELCC接头的设计目标相符.明确了静载ELCC设计可以忽略焊接残余应力的影响.     

基于有限元的制动模架静强度与疲劳强度分析

以某动车组车下悬挂式制动模架为研究对象,对4种超常载荷组合工况以及4种运营载荷组合工况进行仿真分析。依据EN12663标准和相关设计要求,进行制动模架的静强度分析;依据EN12663和BS7608等相关疲劳设计标准,进行制动模架的疲劳强度分析。结果表明,在4种超常载荷组合工况下,工况2的Mises峰值应力最大,应力值为233.6 MPa,节点位于制动模架横梁与中间板连接焊缝处,该值小于模架基于安全系数S修正后的许用应力300 MPa,因此该模架的静强度满足要求;在4种运营载荷组合工况下,工况2的最大主应力峰值最大,应力值为29.65 MPa,节点位于制动模架横梁与中间板连接焊缝处,该值小于BS7608标准中F级焊接接头疲劳许用应力40 MPa,由此可以评定该模架的疲劳强度符合标准。     

CORROSION FATIGUE BEHAVIOUR OF 1Cr18Ni9Ti STEEL IN BOILING 45% MgCl_2 SOLUTION

<正> 我们适应工程上的需要,研究了1Cr18Ni9Ti奥氏体不锈钢在沸腾的高浓度MgCr_2水溶液中的腐蚀疲劳行为。 试样从热轧棒材上切取,经1100℃,10min,W.Q.后加工而成,标距部分为直径3长15mm,表面光洁度▽8。又从管材上切取15×3×1.5mm的片状试样,表面光洁度▽6。试验用钢的化学成分(wt-％)为;C 0.05,Mn 1.84,Si 0.53,S 0.021,P 0.026,Cr 18.40,Ni 10.30,Ti 0.45,Fe余量。 试验在专用的蠕变-疲劳试验机上进行,交     

Properties of WC–8Co hardmetals with plate-like WC grains prepared by plasma-assisted milling

WC-8C0 hardmetals with different proportions of prismatic WC grains and plate-like WC grains were directly produced through sintering the W-C-8C0 elemental powder mixture which was fabricated by dielectric barrier discharge plasma(DBDP)-assisted milling.The morphology of prepared WC-8C0 hardmetals,geometry and the preferential orientation of plate-like WC were investigated by X-ray diffraction(XRD) and scanning electron microscopy(SEM) analysis.The results demonstrate that the microstructure and mechanical properties of the sintered hardmetals are related to the morphology of W grain which is dependent on DBDP-milling time.The DBDP for 1 h(DBDP-1 h)-milled W-C-Co powder contains granular W particles that tend to form prismatic WC grains,while the DBDP for 3 h(DBDP-3 h)-milled powder contains lamellar W particles that generate plate-like WC grains.By adjusting the weight ratio of DBDP-1 h powder and DBDP-3 h powder in W-C-8C0 mixture,the proportion of plate-like WC in the hardmetals can be controlled,and relatively high transverse rupture strength(TRS) is obtained as the proportion of plate-like WC grain in the hardmetals is about 35%in present experimental condition.     

双剪统一强度理论在厚壁圆筒分析中的应用

运用双剪统一强度理论对厚壁圆筒进行了屈服应力函数和极限载荷的分析,得到了一系列公式,而这一理论可以运用在挤压凹模的塑性分析中。在这些计算公式中,当拉压异性系数α和中间主应力影响系数b取不同值时,可以简化成按不同的屈服准则分析的计算结果。运用双剪统一强度理论可以更好地发挥材料的强度潜力,取得更大的经济效益。     

稀土在硬质合金中的应用研究

从选择稀土的加入形态、方式和数量入手,研制成功了稀土添加剂并以WC—Co(YG型)及WC—TiC—Co(YT型)合金为应用对象,研究了稀土对合金性能及组织结构的影响.结果表明,稀土的加入明显地提高了两类合金的抗弯强度、耐磨性和冲击性能,降低了切削力和摩擦系数.含稀土的硬质合金组织状况得以改善,加入稀土后粘结相中α—Co的含量提高,并与S、O等杂质形成化合物,从而产生了净化相界的作用.