Influence of Fe-Ni-Co binder composition on nitridation of cemented carbides

Nitridation of cemented carbides leads to the formation of wear resistant Ti(C,N)-enriched outer-surfaces (fcc-rich outer-surfaces). In this work the influence of the binder phase composition on the kinetics of formation of fcc-rich outer-surfaces was investigated. Sintered cemented carbides were nitrided at 1200 °C in a 5 bar N₂ atm for 2 and 4 h. The thickness of the nitrided fcc-rich outer-surfaces increases with the addition of Fe to the Co and Ni binders. Correlations between nitrogen diffusion coefficients and nitrogen solubility levels in Fe-Ni-Co systems are investigated to understand the mechanisms governing the formation of graded layers on nitrided cemented carbides with Fe-Ni-Co binders.     

Kinetics of formation of graded layers on cemented carbides: Experimental investigations and DICTRA simulations

Kinetics of formation of fcc-free layers on Co-W-Ti-Ta-Nb-C-N cemented carbides was investigated by experimental methods and DICTRA simulations. The layer formation obeys a parabolic law, indicating a diffusion-controlled process. For DICTRA simulations, the influence of the mobilities for all diffusing elements in the liquid binder phase at the sintering temperature was investigated. The best agreement between experimental and simulations was obtained considering that the mobility of all metallic elements is two times slower compared with the mobility of the non-metallic elements.     

Influence of nitridation on surface microstructure and properties of graded cemented carbides with Co and Ni binders

The influence of a nitridation treatment on the microstructure and the properties of (W,Ti)C-based cemented carbides with Co and Ni binders is investigated. Nitridation results in the formation of a very fine-grained (Ti,Ta,Nb)(C,N) phase (γN-phase) with inclusions of binder and WC clusters in the near-surface region of the cemented carbides. Rietveld refinements of X-ray diffractograms reveal differences in the domain size and the microstrains of the γN-phase for cemented carbides with Co-binder and Ni-binder. Nitridation reduces the domain size and increases microstrains in the γ-phase. The chemical composition, the morphology and defects of the binder were investigated by TEM/EDX before and after the nitridation treatment. The results reveal that a high nitrogen activity leads to the preferred dissolution of the core-rim structure of the mixed cubic carbide grains during the formation of the γN-phase. The nitridation heat treatment improves the wear and corrosion resistance of cemented carbides with both Co-binder and Ni-binder.     

Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides

In this work, the effects of 1.0 wt.% additions of Mo and Co on the microstructure and properties of WC-TiC-Ni cemented carbides were investigated using scanning electron microscope, mechanical properties tests, corrosion resistance and abrasion resistance tests. The results show that 1.0 wt.% Mo addition can refine the WC grains and increase the hardness. Moreover, with the addition of minor Mo, the corrosion resistance and abrasion resistance of alloys improved significantly. The addition of 1.0 wt.% Co can inhibit the growth of WC grains, improve the density and hardness slightly, and enhance the abrasion resistance of cemented carbides. However, the minor Co has negative effect for the corrosion resistance.     

Preparation of Co powders for cemented carbides in China

Cobalt has been used as a supreme binder in cemented carbides. As the largest production nation of cemented carbides in the world, large amounts of cobalt powders, about 1200 tons in 2005, are consumed in China market. It is known that many factors, both in terms of composition and processing techniques, affect cemented carbides properties that are correlative with the ingredient, particle size, particle shape and microstructure of the cobalt powders. The studies on preparation of Co powders for cemented carbides in China are reviewed in this paper. The fabrication method, particle size and shape of cobalt powders are described and discussed according to the processing techniques. The typically chemical and physical properties of cobalt powders in Chinese production are introduced briefly. In order to further improve the cemented carbides quality, more research works on the microstructure and composition of doped alloying elements cobalt powders should be made in China in future.     

Influence of microstructure on ultraprecision grinding of cemented carbides

The influence of microstructure on the ultraprecision grinding response of a series of cemented carbides for spherical mirrors was characterized by means of optical and laser interferometry, atomic force microscopy, scanning electron microscopy and X-ray diffraction. Surface roughness, form accuracy, grinding-induced residual stress and material removal behaviors were studied as a function of tungsten carbide (WC) grain size. In connection with the removal mechanisms in ultraprecision grinding, microindentations performed on each material showed similar deformation patterns, all in the plastic regime. The microstructure of WC-Co materials was found to have little influence on the nanometre surface roughness and submicron form accuracy. However, the X-ray stress measurements indicated that the microstructure of carbide materials had a significant influence on the grinding-induced residual stresses; i.e. an increase in grinding-induced residual compressive stress with an decrease in WC grain size. No grinding-induced cracks were observed in the ground cemented carbide surfaces. The material removal in ultraprecision grinding was considered to occur within the ductile regime. The formation of microgrooves and plastic flow regions via slip bands of WC grains along the cobalt binder without visible resultant microfracturing of WC grains were the dominant removal mechanisms.     

Antifriction characteristics of quasi-nano alumina reinforced with Zr-O-B compounds against cemented carbides

In this paper, Zr-O-B compounds that strengthened quasi-nano alumina composites were produced by hot pressing techniques. The mechanical properties of the composite as well as its wear behaviors, coupled with a cemented carbide ball in unlubricated conditions, were investigated systemically. SEM and EDX techniques were employed to observe the worn surfaces of the test specimens and antifriction mechanisms were simultaneously discussed. Analysis of the experimental data and observation on worn surfaces revealed that the improvement in the wear resistance of composites may be attributed mainly to the addition of Zr-O-B compounds in the alumina matrix. The mechanism responsible was explained as the formation of a lubricating film between the sliding couple, and the composition of this lubricating film was found to be ZrO₂ and B₂O₃. The generated lubricating film can decrease the friction coefficient of the composites, and prevents the wear rate from reaching the theoretical value.     

The influence of carbon content on formation of carbo-nitride free surface layers in cemented carbides

To increase crack propagation resistance in cemented carbide cutting tools, it is sometimes of interest to create tough surface zones in the substrates. A way to do this is to use so-called gradient sintering in the manufacturing of the cutting tool. In this sintering process a nitrogen and titanium containing cemented carbide is sintered in a nitrogen free atmosphere. The difference in nitrogen activity between atmosphere and cutting tool during sintering will create an outward nitrogen diffusion. Due to thermodynamical coupling between nitrogen and titanium, this gives rise to an inward titanium diffusion, which creates a surface zone depleted of hard cubic carbo-nitrides, and enriched in ductile binder phase. By varying the carbon content of the material, the nitrogen activity is affected, and this in turn affects the surface zone formation.

In this report, Ti(C,N)–(Ti,W)C–WC–Co, Ti(C,N)–NbC–WC–Co, and Ti(C,N)–TaC–WC–Co cemented carbides were studied. All three materials were produced in series with varied carbon content, in order to study the effect of carbon on gradient surface zone formation.     

WC-Co硬质合金结构参数对烧结表面残余应力的影响

通过建模分析和实际测量,对WC-Co硬质合金结构参数与烧结表面残余应力之间的关系进行了研究。结果表明,硬质合金的结构参数对烧结表面残余应力有很大的影响。相同WC粒径条件下,随Co相体积分数的增大,硬质合金烧结表面残余应力的数值增大。当Co相体积分数固定时,增大WC颗粒的粒径,可明显降低硬质合金烧结表面的残余应力。实际测量结果与分析结论相吻合。     

Effect of WC particle size on the microstructure, mechanical properties and fracture behavior of WC–(W, Ti, Ta) C–6 wt% Co cemented carbides

This study deals with the microstructure and mechanical properties of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides fabricated by conventional sintering. The conventional WC particles of 4 μm size and ultrafine particles of 0.2 μm were introduced in the system with varying ratio. The ratios of conventional WC particles to ultrafine WC particles were 2:1, 1:1, and 1:2. The microstructures of sintered WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides were sensitively dependent on the ratio of conventional WC particles to ultrafine WC particles. The rim phase increased with the increase in the amount of ultrafine particles. Hardness of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbide increased with increase in the amount of rim phase and decrease in the average grain size of WC particles. The bending strength showed the similar trend of the hardness. The fracture morphologies are reported. The fracture behavior changed from mixed mode to transgranular fracture mode, when the ratio of conventional WC particles to ultrafine WC particles was changed from 2:1 to 1:2.     

WC-Co梯度硬质合金的制备及渗碳对其组织的影响

采用光学金相检测、扫描电镜分析、能谱分析等方法对WC-6Co硬质合金渗碳处理后的成分和梯度组织结构进行分析.结果表明:对硬质合金渗碳处理后可形成显微组织和钨、钴含量的梯度分布,其特征是合金表层和次表层的η相已经完全消失,属正常的WC γ两相组织,合金的芯部依然是含η相的三相组织,中间形成了一个富钴层;碳原子的扩散和液相钴的流动是形成梯度的原因;在各渗碳温度下,合金的梯度结构厚度均随渗碳时间的增加而增加;在渗碳时间和渗碳温度相同的情况下,合金的梯度层厚度均随合金初始总碳含量的增加而增厚.渗碳处理后外表面的WC晶粒可能会产生一定的粗化现象.     

Influence of sulfur on the nature and morphology of carbides formed on the surfaces of Fe-Ni-Cr alloys at high temperatures

The influence of sulfur upon surface carbide formation on a range of Fe-Ni-Cr alloys has been investigated at a temperature of 1273 K. The addition of small amounts of H₂S, up to 100 ppm (range of = 2.2×10^–12 to 5.5×10 su–11 bar), to a H₂-CH₄ carburizing atmosphere (a_c=0.8) was found to significantly modify the behavior from that normally observed in the sulfur-free environment. The carburization of these alloys in the H₂-CH₄ atmosphere led to the formation of globular particles of M₇C₃ on the surface of the alloy, but the addition of H₂S affected the type, morphology, and distribution of the surface carbides. Initially, the lower carbon containing M₂₃C₆ was formed, which transformed to M₇C₃ at a rate determined by the concentration of sulfur in the environment. The morphology of the M₇C₃ was modified by the presence of sulfur, and the carbide exhibited a preferred crystallographic orientation in the [001] direction. Particles of manganese sulfide were formed on the commercial alloys at > 2.3 × 10^–11 bar at 1273 K, and these served as nucleation sites for carbides sothat, in contrast to the behavior in sulfur-free conditions, complete surfacecarbide layers were formed.     

Improvement of a-C:H film adhesion by intermediate layers and sputter cleaning procedures on stainless steel, alumina and cemented carbide

The adhesion of amorphous hydrogenated carbon (a-C:H) films deposited in a radio frequency (r.f.) plasma discharge on stainless steel, alumina and cemented carbide with different intermediate layers (Ni, Ti and TiC) and sputter cleaning procedures was studied. The composition of the carbon films and the intermediate layers as well as the interface between the coating and the substrate was determined by secondary ion mass spectroscopy (SIMS). The adhesion experiments were carried out using a scratch tester. Tested specimens were also studied by scanning electron microscopy (SEM) to reveal the morphology of the coatings and the scratches.

Without any intermediate layer, the a-C:H coatings generally had insufficient adhesion to the substrate materials studied. For stainless steel and cemented carbide substrates, the TiC intermediate layer and, for alumina substrates, the titanium intermediate layer gave the best adhesion values evaluated by the scratch test. Also, the sputter cleaning of the substrates prior to deposition was necessary for sufficient adhesion of the coating. The intermediate layers also change the failure mode of the coating in the scratch test in some cases.     

Cobalt layers crystallized on the WC(100) surface: Spin-polarized ab initio study

Ab initio methods of the density functional theory and pseudopotentials were used for studying the structure, electronic states, binding energy and separation energy of Co layers on the WC(100) surface. It has been found that the first monolayer repeats the geometry of the WC(100) surface with the same value of the lattice constant. Comparison of three cases - A) Co atoms are above C atoms; B) Co atoms are above W atoms; C) Co atoms are above the symmetry centers between WC pairs - shows that the A case is an energetically preferable one. The second Co monolayer also repeats the fcc-cubic WC geometry, however the distance between the first and second Co layers is less than the distance between planes in the WC cubic crystal. All Co layers are ferromagnetic ones with magnetic moments of 0.87-2.41 μ_B dependently on the case. The density of states (DOS) for the WC/Co system looks like the superposition of those for bulk fcc-WC and free Co layers.     

Effect of varying blends of finished RO, surface and ground waters on solid lead surfaces

PbO, PbCO₃, and Pb₃(OH)₂(CO₃)₂ were identified as primary corrosion products on lead coupons following three months of exposure to finished RO, surface and ground water by XRD and XPS. Based on these results, a Pb₃(OH)₂(CO₃)₂ solid phase equilibrium model was used as a function of water quality, was modified using diffusion theory and accurately predicted lead release for a majority of field observations. SEM photographs revealed sulfate and chloride affected the surface structure of corrosion scales and lead release.     

Microstructure and properties of hard layers formed by duplex surface treatment containing nickel and phosphorus on a titanium-base alloy

Microstructure and chemical composition of the hard Ni–P–Ti layer formed on the Ti–6Al–4V alloy after duplex surface treatment were investigated by light microscopy, X-ray diffraction, scanning electron microscopy and analytical/high resolution transmission electron microscopy. The investigation revealed a clear correlation between the micro/nanostructure of the multiphase layer with its micro-mechanical and tribological properties.     

Modelling the process of formation of a composite coating with SiC particles on the surface of aluminium alloys under the effect of powerful laser radiation

The process of penetration of SiC particles into the surface of an aluminium alloy, melted by the effect of powerful laser radiation, is investigated. The formation of these surfaces greatly increases the wear resistance of components made of aluminium alloys. The physical and mathematical models, describing the technological conditions for the penetration of particles to a depth of up to 1.5–2.0 mm, are investigated. The results of the calculations are confirmed by experimental data.     

完整晶面金刚石的粒度极限研究

本文以SPD6×1200型六面顶压机为金刚石的合成设备,分别采用常规及特殊工艺进行金刚石的合成。为了表征不同粒度金刚石的形貌特征,本文利用光学显微镜及扫描电子显微镜对单晶形貌进行了观察。结果表明：常规粒度金刚石晶面平滑度基本不随粒度变化,超细颗粒金刚石在8～10μm以上具有与常规粒度金刚石相同的特点,而当粒度降至5μm以下时合成的金刚石不再具有完整的晶形,且随粒度降低晶形变差的趋势加剧,从而得出高温高压法合成的完整晶面金刚石的粒度极限在5μm左右的重要结论。我们认为这一现象的主要原因是由于碳原子或其团簇在金刚石表面上各点的沉积,时间先后不同,从而导致了某一时刻晶体表面各点的厚度不同,对5μm以下的超细颗粒金刚石来讲晶面上不同点的相对差别较大,宏观体现为晶面残缺。     

鞍面力-位移分控多点成形起皱的数值模拟研究

在多点成形中,起皱是影响板材成形质量的重要因素。针对鞍面在力-位移分控多点成形方式下的起皱,以ABAQUS为模拟分析平台,模拟了鞍面的成形过程,并分析了其起皱缺陷。研究结果表明:在鞍面力-位移分控多点成形过程中,当板厚较小(1和1.5 mm)时,板材中心区域发生了较为严重的起皱,且优先变形的方向发生的起皱现象更明显;随着板厚和曲率半径的增大,起皱趋势减小。针对起皱现象探明了起皱的原因,提出了鞍面起皱的抑制方法,并进行了模拟计算和成形试验,结果表明,增大成形力和错位都可以抑制起皱,错位并结合增大成形力可以显著改善板材成形质量。