Corrosion damage in WC–Co cemented carbides: residual strength assessment and 3D FIB-FESEM tomography characterisation

Abstract

The effect of corrosion damage on cemented carbides was investigated. The study included residual strength assessment and detailed fractographic inspection of corroded specimens as well as detailed 3D FIB-FESEM tomography characterisation. Experimental results point out a strong strength decrease associated with localised corrosion damage, i.e. corrosion pits acting as stress raisers, concentrated in the binder phase. These pits exhibit a variable and partial interconnectivity, as a function of depth from the surface, and are the result of heterogeneous dissolution of the metallic phase, specifically at the corrosion front. However, as corrosion advances the ratio between pit depth and thickness of damaged layer decreases. Thus, stress concentration effect ascribed to corrosion pits gets geometrically lessened, damage becomes effectively homogenised and relatively changes in residual strength as exposure time gets longer are found to be less pronounced.     

Fine Grained WC–VC–Co Hardmetal

Abstract

Preliminary work on the preparation process and some properties of VC–WC–Co alloys containing approximately 10 wt–%VC and 10 wt–%Co are reported. Sinterability of the alloys proved to be better than expected and hardness higher than the hardness of WC–Co alloys of equal cobalt content. The toughness was found to be superior to that of WC–Co alloys of equal hardness. PM/0742     

Recycling of WC–Co from scrap materials

Abstract

WC-Co scrap generated by the cutting tool industries was electrochemically broken down to cobalt, which was deposited at the cathode and a mixture of tungsten oxide and tungstic acid was collected at the anode with an overall recovery efficiency of about 90%. The tungsten oxide/tungstic acid was reduced to produce nanostructural tungsten powders, which were subsequently carburised and chemically coated with cobalt to produce WC-Co powders. The powders synthesised were characterised for purity and size.The WC-Co powders, thus obtained were consolidated to near theoretical densities using a novel plasma pressure compaction (P²C) technique. The microhardness of the consolidated sample was measured to be 2200 HV, which is 20% higher than the reported literature values.     

Effective interface model for design and tailoring of WC–Co microstructures

Interface structures are a key feature in developing modern composite material solutions with ever improved performance. We present a nano-microstructural modelling approach for the tungsten carbide (WC)–Co system which can include the interface structures of WC–Co and various other phases present in the microstructure, utilising a methodology which combines imaging-based and synthetically generated nano-microstructures into an effective interface model for predicting the behaviour and properties of the resulting composite material. The effective model comprises of a local model of the WC/Co interface interacting with a larger-scale model of the WC–Co microstructure. The results provide a linkage between the interface character of cemented carbide microstructures and their properties, for example with respect to compressive strength, fracture toughness and wear resistance. The methodology presents a multiscale formalism for carrying out performance and application-driven evaluation and tailoring of composite interfaces and mesostructures, carried out on the basis of the emerging engineering material properties.     

Implementation of advanced characterisation techniques for assessment of grinding effects on the surface integrity of WC–Co cemented carbides

Grinding is a key step on the manufacturing process of WC–Co cemented carbides (hardmetals). In this work, an investigation of grinding effects on the surface integrity of hardmetals is conducted. It is done by combining diverse advanced characterisation techniques: X-ray diffraction, field emission-scanning electron microscopy, electron back scatter diffraction, focused ion beam – 3D tomography and transmission electron microscopy. The study is carried out in a fine-grained WC–Co grade. Besides ground state, polished surface finish condition is assessed for comparison purposes. It is evidenced that grinding induces significant alterations: 3D tomography illustrates microcracking exists down to 2.5?μm depth with a highly anisotropic distribution at the subsurface, large compressive residual stresses extending until subsurface levels of about 12?μm, and phase transformation of binder from the original fcc phase into the hcp one, as well as severe plastic deformation observed within the binder at the surface level.

Invited keynote papers from EuroPM2017, Milan.     

Fabrication of ultrafine and nanocrystalline WC–Co mixtures by planetary milling and subsequent consolidations

Abstract

In this work ultrafine and nanocrystalline WC–Co mixtures were obtained by low energy milling in planetary ball mill. The effect of the processing conditions on the reduction and distribution of the grain sizes and the internal strains level were studied. The characterisation of the powder mixtures was performed by means of scanning and transmission electron microscopy and X-ray diffraction analysis. Observations through SEM and TEM images showed a particle size below 100 nm, after milling. The X-ray diffraction profile analysis revealed a WC phase refined to a crystallite size of 19 nm.

The mixtures obtained have been consolidated and mechanical and microstructurally characterised. The results show improvements in resistant behaviour of the material consolidated from nanocrystalline powders, in spite of the grain growth experienced during the sintering. The best results were found for the material obtained by wet milling during 100 h, which presents values of hardness higher than 1800 HV.     

Fabrication of Functionally Graded WC–Co Cemented Carbides with Plate-Like WC Grains

Powder Metallurgy and Metal Ceramics - Functionally graded WC–Co cemented carbides (FG WC–Co) with plate-like WC grains were fabricated by carburizing of cemented carbides with no...     

Structure and Properties of Coarse-Grained WC–Co Alloys with an Especially Homogeneous Microstructure

Russian Journal of Non-Ferrous Metals - The structure and properties of coarse-grained WC–6% Co carbides with carbon deficiency from 0.11 to 1.31% relative to the stoichiometric ration...     

射频等离子体和热处理制备球形WC–Co粉末

以喷雾造粒WC–30Co粉末为原料,采用射频等离子体和后续热处理制备3D打印用球形WC–Co粉末,研究射频等离子体球化和热处理对粉末特性的影响。结果表明,射频等离子体球化效果显著,喷雾造粒粉末的球化率可达100%。球化后的粉末表面光滑、结构致密,存在一定数量表面粗糙的“费列罗”颗粒。射频等离子体处理使粉末的松装密度和流动性显著提高,同时导致WC严重分解和Co蒸发损失,球化粉末中含有大量C、W₂C和Co₃W₃C等有害相,Co质量分数降低至25.80%。后续热处理可很好地对球化粉末进行物相和成分调控。经900 ℃热处理后,粉末的物相组成重新转变为WC和Co,游离碳含量控制在合理的水平,并且粉末依然保持良好的球形度,具有较好的松装密度和流动性,可以满足3D打印对原材料的要求。     

WC–Co硬质合金摩擦磨损行为的分子动力学模拟

利用分子动力学模拟研究了WC–Co硬质合金在不同条件下的摩擦过程,分析了晶粒尺寸、摩擦载荷和滑动速率等因素对硬质合金摩擦磨损行为的影响,从原子尺度揭示了硬质合金发生摩擦磨损的微观机制。结果表明,随晶粒尺寸增大,相比于晶粒转动,Co相和WC中的位错滑移逐渐在摩擦引起的塑性变形机制中起主导作用。摩擦载荷增大会导致易变形的Co粘结相被挤出表面而首先去除,通过减小晶粒尺寸可以抑制Co相的挤出–磨损机制,进而提高硬质合金的抗滑动磨损性能。滑动速率升高会降低磨损速率,主要原因是在高速滑动过程中,亚表层各相中位错的形核扩展缺乏持续的驱动应力,位错密度较低,WC不易发生断裂,Co相被挤出表面造成的磨损程度明显减轻。     

Structure and Magnetic Properties of WC–50% Co Model Alloys Containing TaC Additives

The structure and magnetic properties of model high-cobalt WC–50% Co alloys with different carbon contents and TaC additions in the amount of 1.6–5.6 wt % are studied. Model alloys are fabricated by the liquid-phase sintering of powder mixtures at 1420°C, and their composition is described by the formula 50% Co + 50% WC + x% TaC + y% C, where x = 0, 1.6, 2.6, 3.6, 4.6, and 5.6 wt %; y = 0, 0.2, and 0.5 wt %. It is shown that precipitations of the (Ta,W)C phase are present in all studied alloys and (Ta,W)C precipitations are needle-shaped at a TaC concentration up to 3.6 wt %, while the (Ta,W)C grains become spherical at =3.6 wt %. The (Ta,W)C precipitations are arranged both in a binding phase and along the WC grain boundaries. The lattice parameter of the (Ta,W)C phase in alloys with a low carbon content lies in a range from 0.4438 nm for the alloy with 1.6% TaC to 0.4451 nm for the alloy with 4.6% TaC. It is established by the EDX analysis that the concentration of dissolved tungsten in a cobalt phase is independent of the TaC content and strongly depends on the total carbon content; it is 7, 12, and 17 wt % for alloys with high, elevated, and low carbon contents, respectively. The TaC addition in alloys with a low and elevated carbon content leads to an increase in the coercive force up to 875 A/m and a decrease in the magnetic saturation by 5–10 G m³/g. The experimental results made it possible to put forward a hypothesis on the possibility of formation of dispersed tantalum-containing precipitates in a binder phase.     

Some Trends in Improving WC–Co Hardmetals. I. Hybrid and Coarse-Grained Hardmetals

Powder Metallurgy and Metal Ceramics - The paper reviews and analyzes the areas for improving WC–Co hardmetals. One of the areas was initiated by Grygorii Samsonov in the mid-1950s and has...     

Relation between the residual induction and the temperature stability of sintered Nd–Dy–Fe–Co–B magnets

Relation between the temperature coefficient of induction of materials of the Nd–Dy–Fe–Co–B system at temperatures of 20–100°C and their residual induction has been found. It is shown that the residual induction is 0.6 T at the zero temperature coefficient of induction.     

Co—WC Pseudobinary Eutectic Reaction

Abstract

Using sphere-plate models made from copper it is shown that during sintering the dislocation density increases considerably in the contact region. Its distribution and time dependent variation can be analysed by means of the Rossel technique and described quantitatively. The same effect is observed during sintering compacts of electrolytic copper powder. The results of positron annihilation spectroscopy show the high dislocation densities generated in the heating phase to be reduced by non-conservative dislocation movement during the intensive shrinkage stage of sintering. Resulting densification mechanisms are discussed. PM/0344     

SOME RELATIONS BETWEEN THE STRUCTURE AND MECHANICAL PROPERTIES OF WC–TiC–Co ALLOYS

Abstract

The relationship between the mechanical properties and the structure of the sintered carbide WC–TiC–Co has been studied. Specimens containing 7 and 15% cobalt were sintered at temperatures between 1350 and 1550°C (1625 and 1825 K), for times varying from to 32 h. The structure was examined by electron. microscopy. Density, coercive force, hardnesss, transverse rupture strength, and energy of crack initiation were determined.

The rate of grain growth is governed by the rate at which the carbide dissolves in the cobalt phase. The activation energy for growth was found to be 120 ± 15 kcal/mol (500 ± 63 kJ/mol).

The coercive force is a linear function of the specific grain surface rather than of the specific cobalt surface. The hardness of alloys with different cobalt contents is a function of a single structure parameter [(1– f_β)/f_β]S_γγ, where f_β is the volume fraction of β phase and S_γγ is the specific grain-boundary surface.

It has been suggested that transverse rupture strength should vary as the square root of the specific grain surface. The present results tend to confirm this suggestion. The energy of crack initiation is highly dependent on the contiguity of the carbide phase.     

Effect of Bonding Temperature on Microstructure and Mechanical Properties of WC–Co/Steel Diffusion Brazed Joint

In this work, the diffusion brazing of AISI 4145 steel to WC–Co cemented carbide using RBCuZn-D interlayer with bonding temperature values of 930, 960, 990 and 1020 °C was studied. The microstructure of the joint zone was evaluated by scanning electron microscope (SEM) and X-ray diffraction (XRD). Vickers microhardness and shear strength tests were performed to investigate mechanical behaviors of the brazed joints. The XRD and SEM results indicated that with increase of bonding temperature, the elements readily diffused along the interface and formed various compounds such as γ, α and β and Co₃W₃C. The results also showed that with the increase of bonding temperature from 930 to 960 °C, a sound metallurgical bond was produced, however in higher bonding temperatures (990 and 1020 °C) a decrease in mechanical properties of the joints was observed which could be due to the excessive zinc evaporation, interface heterogeneity and voids formation. The maximum shear strength of 425 MPa was obtained for the bond made at 960 °C.