Tungsten Carbide Grains Research Articles

PIXE characterization of by-products resulting from the zinc recycling of industrial cemented carbides

By-product materials of the widely used zinc recycling process of cemented carbides have been studied. Scanning electron microscopy and micro-PIXE techniques have identified elemental concentrations, distributions and purity of by-product materials from an industrial zinc recycling plant. Cobalt surface enrichment, lamellar microstructures of varying composition, including alternating tungsten carbide (WC) grains and globular cobalt, and regions of excess zinc contamination were found in materials with incomplete zinc penetration. Liquid Co–Zn formation occurred above 72wt.% Zn at the furnace temperature of 930°C, and was extracted towards the surface of poorly zinc infiltrated material, primarily by the vacuum used for zinc distillation. Surface enrichment was not observed in material that was zinc infiltrated to the sample center, which was more friable and exhibited more homogeneous porosity and elemental concentrations. The result of incomplete zinc infiltration was an enriched surface zone of up to 60wt.%Co, compared to an original sample composition of ∼10–15wt.%Co. The impact on resulting powders could be higher or inhomogeneous cobalt content, as well as unacceptably high zinc concentrations. PIXE has proven it can be a powerful technique for solving industrial problems in the cemented carbide cutting tool industry, by identifying trace elements and their locations (such as Zn to 0.1wt.% accuracy), as well as the distribution of major elements within WC–Co materials.

Focus on Carbide-Tipped Circular Saws when Cutting Stainless Steel and Special Alloys

Circular saw blades are used exclusively for cut-off work, ranging from small manual feed operations, up to very large power fed saws commonly used for sectioning stock as it comes from a rolling mill or other manufacturing processes for long products. The teeth profile, as well as the tooth configuration are of fundamental importance for the blade performances; through a combination of blade rigidity and grinding wheel condition a good quality surface finish is attained for tools of commercial standard. The materials used for the production of circular saw blades are ranging from high speed steel to cemented carbides. In particular, cemented carbides, being characterized by high hardness and strength, are used in applications where materials with high wear resistance and toughness are required. The main constituents of cemented carbides are tungsten carbide and cobalt. Tungsten carbide imparts the alloys the necessary strength and wear resistance, whereas cobalt contributes to the toughness and ductility of the alloys. The WC-Co alloys are tailored for specific applications by the proper choice of tungsten carbide grain size and the cobalt content. The grain size of the tungsten carbide in WC-Co varies from about 40 µm to around 0.3 µm, the cobalt content from 3 to 30 wt%. The coarse grained hardmetals are mainly used in mining applications, the smallest grain size being about 3 µm and the minimum cobalt content 6 wt%. The grain size of tungsten carbide in the metal cutting industry, as well as for universal applications lies in the range of 1-2 µm. However, with the advent of near net shape manufacturing and thin walled components, the use of submicron carbide is growing, since their high compressive strength and abrasive wear resistance can be used to produce tools with a sharp cutting edge and a large positive rake angle.In this invited paper, a general overview on the actual trends in the choice of the best material when cutting special alloys will be presented and discussed. Based on the recent and past literature some examples of their up-to-date application, such as circular saws used to cut stainless steels and some high strength alloys, are talk over.

Drilling hard alloys based on high-temperature tungsten carbides

The influence of strength properties of a cobalt binder and tungsten carbide grains on the operational characteristics of hard-alloy picks of drilling bits is investigated. Results are used to develop new brands of hard alloys based on high-temperature tungsten carbides such as VK6S, VK10S, VK15S, and VK15K. It is established that the application of these alloys in the pick production gives the opportunity to increase the penetration speed due to the use of picks with a more aggressive design. It is shown that new alloy brands also make it possible to increase the resource of the instrumental accessories made of them.

On rock-drill hard steels based on high-temperature tungsten carbides

The effect of strength properties of cobalt binder and tungsten carbide grains on the operational characteristics of carbide drilling bits has been in- vestigated. The results were used for development of new grades of hard alloys based on high-temperature tungsten carbides: BK6C, BK10C, BK15C and BK15K. Application of the given alloys in the manufacture of bits is established to afford the opportunity to increase of mechanical drilling rate at the cost of using the bits of more aggressive design. New grades of alloys are shown to allow increasing the life of tool inventory.

Characterization of nano-cemented carbides Co-doped with vanadium and chromium carbides

Cemented carbides are ultra-hard composites used in a variety of applications that call for tough and wear-resistant materials. Conventional cemented carbide typically consists of grains of tungsten carbide (WC) embedded in a cobalt (Co)-based binder matrix. Often compounds are added that inhibit the growth of the WC grains and thus ensure a fine and homogeneous grain structure, to optimize hardness and toughness. Here, we report a detailed study of the structure and properties of cemented carbides that contain combined grain-growth inhibitors, i.e. vanadium carbide (VC) and chromium carbide (Cr3C2). The concentrations were varied and the mixtures were sintered at 1200°C and 1300°C to achieve optimum combination of properties. Alloy systems with a combination of both VC and Cr3C2 grain-growth inhibitors displayed superior mechanical properties compared to those in which only one inhibitor compound was present. The inclusion of VC produced lower densification with finer grains, whereas the incorporation of Cr3C2 produced a harder and tougher carbide. The compound WC–12Co–0.2VC–0.8Cr3C2 reached nearly full densification, with an average grain size of 50nm, a microhardness of 1690Hv30 and a fracture toughness of 12.5MPa∙m1/2. It is proven in this work that achieving a balanced concentration of both inhibitors is extremely vital to reach an optimum combination of properties.

Corrosion and subsequent erosion of hardmetals: Dependence of WC grain size and distribution, and binder composition

Choke valves used in oil and gas production can be exposed to serious corrosive conditions during oil well stimulation when concentrated acids are used. These valves are also exposed to extreme erosive conditions. Hardmetals with varying binder content (5–7 wt%), binder composition (cobalt and a mixture of cobalt, nickel, and chromium), tungsten carbide (WC) particle size (0.6–3.5 µm) and particle size distribution were tested. Six hardmetals were first tested for resistance towards pure erosion. The hardmetals were later tested by first corroding the test specimens for 4, 8 and 16 h in 3.7 wt% hydrochloric acid, that is, a pH of approximately 0, and then subsequently eroding them. A liquid jet erosion rig was used for erosion using silica particles at 74 m/s jet velocity and 90° impact angle. The effect of corrosion time, t, on the relative mass loss, MR, after erosion was found to follow the equation , where k is a constant. The relative synergetic effects registered when measuring mass loss due to corrosion with subsequent erosion are higher for the more erosion resistant materials. Reduced binder percentage reduces the relative synergy. A more varied tungsten carbide grain size gives a less synergetic effect. Although the synergetic effect is stronger the more resistant the hardmetals are towards pure erosion, the same ranking, as to the resistance to corrosion with subsequent erosion, is obtained for the three types of materials.

Orientation-dependent hardness and nanoindentation-induced deformation mechanisms of WC crystals

The orientation dependence of hardness and nanoindentation-induced deformation mechanisms of differently orientated tungsten carbide (WC) grains in WC–Co hardmetal were studied. Electron backscatter diffraction, atomic force microscopy and scanning electron microscopy investigations were performed to determine the grain orientation, and to study the surface morphology and the resulting deformation fields around the indents. The hardness of the differently orientated WC grains showed significant angle dependence from the basal towards the prismatic directions, but there was only a slight change in hardness between the two types of prismatic orientations ((101¯0) and (21¯1¯0)). Sink-in and pile-up effects, together with highly deformed regions and dislocation steps, were revealed around the imprints in the case of basal and prismatic orientations, respectively. A theoretical model is proposed in which the critical force for slip activation is determined as a function of orientation, based on the possible slip systems of WC. The predictions of the present model concerning the measured hardness values and the deformation field around the indents together with the sink-in effect are in good agreement with the experimental results.

Surface and subsurface deformation characteristics of cemented tungsten carbide under nanoscratching

Surface and subsurface deformation characteristics of cemented tungsten carbide during nanoscratching were systematically investigated. Nanoscratch tests were performed at various normal loads and different scratch velocities and the surface and subsurface after scratching were examined using scanning electron microscopy. The results indicated that the deformation of cemented tungsten carbide was significantly influenced by the microstructure of the material. The deformation of subsurface structure was caused by combination of plastic deformation of cobalt binders, relocation of tungsten carbide grains, fractures and chipping.

Use of FIB/SEM to assess the tribo-corrosion of WC/Co hardmetals in model single point abrasion experiments

This paper describes the results of experiments where single point model abrasion experiments were carried out under a range of conditions of applied load and different media. FIB/SEM revealed that in air, damage occurred by binder extrusion in conjunction with fragmentation and fracture of the tungsten carbide grains, with re-embedment of fragments of carbide in the binder phase to form surface layers of mechanically mixed material. In HCl the binder was preferentially removed leaving the WC grains unsupported leading to further fracture and breakdown of the surface of the hardmetal. FIB revealed subsurface cobalt dissolution. A cobalt–nickel binder material proved the most versatile in resisting surface degradation in both ambient air and in HCl.

Machinability of CBN Tool in Turning of Tungsten Carbide

Tungsten Carbide have extremely high hardness and wear-resistivity compared with conventional steel materials, and it is expected that the Tungsten carbide can be applied widely to dies and molds in the near future. In order to develop an efficient machining method of Tungsten Carbide for the dies and molds, series of cutting experiments were carried out to turn the sintered Tungsten Carbide materials with CBN tool. The selected sintered Tungsten Carbide workpieces are those containing Tungsten Carbide grains with mean grain size of 5μm, and 15wt%, 20wt% and 22wt% of Cobalt binder. The sintered CBN tool selected contains super-fine grains of CBN with mean grain size of 1μm. The cutting speed was varied from 10m/min to 60m/min, and the tool wear and the surface roughness were measured. It is concluded that the tool wear is less when cutting the sintered Tungsten Carbide containing larger amount of Cobalt binder. The surface roughness of about 2μm in Rz is obtained.

Fabrication of tungsten carbide–vanadium carbide core–shell structure powders and their application as an inhibitor for the sintering of cemented carbides

An innovative way to synthesize tungsten carbide-vanadium carbide core-shell structure powders, which are predicted to act as inhibitors for cemented carbides, has been explored. The core-shell structures were synthesized by chemically induced precipitation and carbonization reactions. The cemented carbides with a homogeneous microstructure were prepared at 1400 °C. The existence of tungsten carbide-vanadium carbide core-shell structure can significantly refine tungsten carbide grains, inhibit the abnormal growth of finer grains in the liquid sintering and improve the mechanical properties of the hard metals.

Adhesion and wear resistance of CVD diamond coatings on laser treated WC–Co substrates

Abstract A novel method using a high power diode laser to selectively remove cobalt binder from the outermost layer of Co-cemented tungsten carbide (WC–Co 5.8 wt.%) is hereby discussed. A continuous 940-nm wavelength diode laser source at power of 600–1500 W and scanning speed of 1 mm/s was used to perform the thermal treatments. Hot Filament-Chemical Vapour Deposition (HF-CVD) was then used to deposit PolyCrystalline Diamond (PCD) coatings. Increasing the laser fluence, Co-binder was progressively and selectively evaporated from the WC–Co substrate. The selective removal of the Co-binder from the outermost layer of WC–Co induces a significant corrugation of the substrate, very promising to deposit on it high quality diamond with improved adhesion. Indeed, too much high laser fluence must be avoided, as it would tend to induce the resintering or the local melting of the tungsten carbide grains.

Structure and Properties of Nano- and Microcomposite Coating Based on Ti-Si-N/WC-Co-Cr

Using the two technologies: plasma-detonation and vacuum-arc deposition, we fabricated two types of coatings: Ti‐Si‐N/WC‐Co‐Cr/steel and Ti‐Si‐N/steel. We found that the top coating of Ti‐Si‐N was nanostructured one with 12 to 15 nm grain sizes and H = 40 to 38 GPa hardness. A thick coating which was deposited using the pulsed plasma jet, demonstrated 11 to 15.3 GPa hardness, an elastic modulus (E) changing within 176 to 240 GPa, and tungsten carbide grain dimensions varying from 150 to 350 nm to several microns. An X-ray diraction analysis shows that the coating has the following phase composition: TiN, (Ti,Si)N solid solution, WC, W2C tungsten carbides. An element analysis was performed using energy dispersive spectroscopy (microanalysis) and scanning electron microscopy, as well as the Rutherford backscattering of 4 He + ion and the Auger electron spectroscopy. Surface morphology and structure were analyzed using scanning electron microscopy and scanning tunnel microscopy. Tests friction and resistance (cylinder-plane) demonstrated essential resistance to abrasive wear and corrosion in the solution. The decrease of grain dimensions • 10 nm occurring in the top Ti‐Si‐N coating layer increased the sample hardness to 42§2:7 GPa under Ti72‐Si8‐N20 at.% concentration.

The diamond-tungsten carbide polycrystalline composite material

The sintering of a diamond composite from diamond and tungsten powders of submicron and nanosizes at high pressure and temperature has been discussed. It has been shown that as a result of the diamond and tungsten interaction tungsten carbide particles, which are chemically bonded to diamond particles, form in spaces between them. Over the whole volume of the composite the structure has been observed, where tungsten carbide and diamond grains are regularly placed and are uniform in size. An addition of a coarse diamond component to the composite has been found to contribute to an increase of the composite wear resistance. The optimal sintering parameters and formulation of this composite have been defined.

Control of exaggerated tungsten carbide grain growth at the diamond–substrate interface of polycrystalline diamond cutters (PDC)

Exaggerated tungsten carbide grain growth is common at the interface between the diamond table and the cobalt-cemented tungsten carbide (WC–Co) substrate in polycrystalline diamond cutters (PDC). The exaggerated WC grains at the interface can grow as large as 50 μm with an aspect ratio of 50:1. These large grains can also grow as clusters. The presence of large WC grains/clusters creates weakness at the diamond–substrate interface and impairs the strength of the PDC tool. In the present investigation, we tried to understand the root cause of exaggerated WC grain growth at the interface. Our findings show that WC grain growth at the interface decreases with a decrease in the carbon/tungsten (C/W) ratio. By adding 5 wt.% pure tungsten powder to the diamond, the C/W ratio decreased and we found no WC grain growth. By adding fully stoichiometric WC, which has 6.13 wt.% carbon, grain growth was reduced but still observed. Sintering on a substrate having η-phase (carbon deficient phase) also decreased the C/W ratio, and we did not observe WC grain growth.

Novel hardmetal with nano-strengthened binder

The novel nano-structured hardmetal with nano-strengthened binder was developed. The microstructure of modern alloy consists of the rounded grains of tungsten carbide and the cobalt-based binder, which is reinforced by nanoscaled particles of the θ phase (Co2W4C). The microhardness of binder for the novel hardmetal exceeds significantly the values of this parameter for usual hardmetals due to reinforcement by nanoscaled particles. The simultaneous combination of microstructural features and nanograin reinforced binder leads to the fact that the novel alloy is characterized by large value of service durability, which is substantially higher than that in usual alloys with similar grain size and identical cobalt content.

Residual Stresses in TiC-based Cermets Measured by Indentation

The carbide composites are best known for their high hardness; evaluate bending strength and excessive, ceramic-like, brittleness. The titanium carbide based carbide composites (TiC-based cermets) are prospective candidates for tooling in sheet-metal forming (stamping) processes. The wear resistance (adhesive and functional) of some TiC-Fe/Ni cermets is better than that of hardmetals. Previous studies have shown that TiC carbide grains better absorb plastic deformation during cyclic loading (fatigue) and have lower fatigue sensitivity if compared to tungsten carbide grains in conventional hardmetal (WC-Co). The core-rim (double-carbide) structure of the carbide grains is characteristic for TiC-based cermets. The effect of core-rim structure on the behavior of the cermets is poorly investigated. Such a structure is highly dependent on the production technology (powder metallurgy routes) as the difference between the coefficients of thermal expansion of components is obvious cause of high residual stresses appearing after sintering of pressed powder parts. Although, the thermal expansion coefficients of TiC-based cermets are lower than those of WC-based hardmetals.The instrumented indentation technique is used to determine the scope of residual stresses in TiC-based cermets. The first steps are made for evaluation of the residual stresses proportions of the carbide and binder phases, core-rim structures in carbide composites.

Wettability of hardmetal surfaces prepared for brazing with various methods

Hardmetals belong to the materials hardly wettable by liquid brazing alloys, so they should not be brazed without removing the surface layer after sintering. In the paper, mechanical and chemical methods of preparing a hardmetal surface for brazing are discussed. Special attention is paid to the electrolytic etching method that gives very good energetic properties to surfaces of hardly wettable materials. Electrolytic etching of hardmetals consists in anodic dissolution of tungsten carbide grains in water solution of alkaline metal hydroxides. The α phase (WC) is removed from surfaces of carbide preforms, leaving the much better wettable β phase (Co-W-C). Cobalt does not show amphoteric properties, so it does not dissolve in bases. With regard to brazing, besides significant development of the surface, an especially profitable feature of this method is isolating the metallic cobalt phase, which allows easier wetting by brazing alloys. On the grounds of surface roughness measurements, parameters of electrolytic etching were determined to remove WC grains from surfaces of carbide performs and to obtain much higher Co fraction on the brazed surface. The presented results are based on measurements of wetting angles on B40 hardmetal surfaces, isolated α and β phases and C45 steel, as well as on EDX cobalt analysis on hardmetal surfaces.

Improving the Wear Behavior of WC-CoCr-based HVOF Coating by Surface Grinding

WC-CoCr-based high velocity oxy fuel (HVOF) coatings are being used for several components which are prone to severe erosion or abrasion. In this study, the HVOF coating was applied by liquid fuel-based equipment. These coated samples were subjected to surface grinding of various depths (100, 200, and 300 μm). Hardness test after surface grinding showed that the coating hardness increased by 33% after grinding to a depth of 200 μm (1472 Hv). The residual stress after different depths of grinding was measured using x-ray diffraction. It showed that the compressive residual stress of coating increased with grinding. Increase in hardness of the coating (after grinding) is believed to be due to the increase in compressive residual stress. The abrasive wear resistance increased after grinding to a depth of 100 μm thickness and remained constant during successive grinding. In contrast, the erosive wear resistance increased the most when the grinding thickness was 200 μm. It is concluded that the surface grinding of coatings helps in increasing abrasive and erosive wear resistance. The increase in microhardness of the coating is believed to be the reason for high wear resistance. SEM studies of worn out surface show carbide grain pull out due to removal of softer phase, i.e. cobalt and chromium, and is followed by tungsten carbide grain pull out.

Study on Micro Drilling — Rotating Bending Fatigue of Micro Carbide Drills

In micro drilling, the run-out of drill due to rotational motion error of spindle or eccentric chucking of drill causes machining error and larger radial forces which may break the drill. Furthermore, because the spindle speed is very high, rotating bending fatigue of drill may dominate the tool life. In the previous work, relationship between bending deflection and radial cutting force, and fatigue fracture of drill were investigated theoretically and experimentally. The objective of this paper is to investigate more in detail the relationship between the radial cutting force/deflection of drill and its fatigue life. A series of pseudo rotating bending fatigue tests was performed by using three types of ultrafine grain carbide drill blank with 0.1mm diameter. The main results obtained are as follows, (1) Rotating bending fatigue curve did not depend on cobalt content and tungsten carbide grain size. This differs from the results of conventional cemented carbides. (2) The fatigue fracture surface was dominantly occupied by fast fracture regions.