Tungsten Carbide Grains Research Articles

Effect of bimodal WC particle size and binder composition on the morphology of WC grains in WC–Co–Ni3Al cemented carbides

In addition to dispersed nanoparticles in the binder phase, the effect of Ni3Al on the growth mechanism of tungsten carbide (WC) grains is also essential for the fabrication of high-quality of cemented carbides. In this work, WC–Co–Ni3Al cemented carbides with different combinations of Co and Ni3Al binders was studied by using the original artificial bimodal WC grain size distribution. The WC particle sizes of medium 4 μm, fine 1 μm and ultrafine 0.2 μm were adopted. A series of WC–Co–Ni3Al alloys were firstly designed based on thermodynamic calculations and then liquid-sintered at 1450 °C for 1 h. Subsequently, the morphology and grain size distribution of WC grains were studied. By increasing the proportion of Ni3Al in the Co–Ni3Al composite binder phase, the solid–liquid interfacial energy increases significantly, which may provide a higher driving force for abnormal grain growth (AGG) through 2-dimension (2D) nucleation. However, the increase of Ni3Al proportion conversely greatly raises the energy barrier for the migration of W and C atoms into liquid binder phase, which finally retards the AGG of WC. Consequently, the AGG of WC decreases and the number of crystallites with blunt faces increases on WC particles with the increase of Ni3Al proportion, which can be well accounted for by the presently detected WC morphology evolution.

Measuring the fracture toughness of single WC grains of cemented carbides by means of microcantilever bending and micropillar splitting

The main goal of this work is to obtain a reliable value of the fracture toughness of single grains of tungsten carbide (WC). It is attempted by testing microcantilever and micropillar samples shaped out, by means of focused ion beam milling, of individual WC particles embedded within a WC-Co cemented carbide grade. Experimental testing included the use of a nanoindenter for inducing microcantilever bending and micropillar splitting, as well as sampling WC grains with basal and prismatic orientations. Experimental results are compared with those previously assessed through implementation of the indentation microfracture technique. Bending of notched microcantilevers yielded more consistent results than those measured out of micropillar splitting tests. In this regard, the average value of fracture toughness for single WC grains, within the two-phase interpenetrated network existing in cemented carbides, is found to be 5.6 ± 0.8 MPa·m1/2. Such a relatively high value – coherent with local plastic features evidenced in nanoindentation imprints - is in satisfactory agreement with results indirectly estimated from other macromechanical tests.

Effect of cryogenic treatment of cemented tungsten carbide tools for die application

This study investigated effects of cryogenic treatment and post tempering on microstructure parameters and mechanical properties of cemented tungsten carbide for die applications. Carbide grain size was increased after the cryogenic treatment resulted in an increased carbide contiguity. After tempering process, tungsten carbide grain size was reduced and more uniform in size distribution with less in WC-Co debonding. The fracture toughness of cryogenically treated carbide which evaluated from microstructure parameter was slightly increased from 20 to 21.1 MPa · m1/2. After tempering, the fracture toughness was almost unchanged compared to that of untreated sample and the number of tempering cycles did not affect the fracture toughness. However, hardness and scratch resistance were improved 10% and 60%, respectively, by cryogenic treatment. Increasing the number of post tempering cycles after applying the cryogenic treatment result in reducing hardness but did not affected the scratch resistance of the cemented tungsten carbide. In addition, results of ball on disc tribological testing indicated that wear resistance of the cemented tungsten carbide could be improved by cryogenic treatment due to the higher hardness and scratch resistance as expected. The wear volume was increased with the number of tempering cycles. Therefore, sliding wear resistance and scratch resistance applications in which material surface failure behaviour is mainly controlled by surface hardness, post tempering process might not be required.

Development of a Technique for Analysis of Tungsten-Containing Sludge Using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)

A technique for analysis of secondary tungsten-containing raw materials (tungsten-containing sludge) using inductively coupled plasma atomic emission spectrometry in combination with microwave autoclave sample preparation has been developed. The composition of the acid mixture and algorithm of microwave heating of the autoclave are chosen to provide a complete quantitative transfer of the sample into a convenient analytical form for subsequent ICP-AES determination of the main components (heating of the sample to 220°C in the acid mixture of NH4F, HNO3, and HCl provides a quantitative transfer of its components to be determined into the solution). Analytical lines free from spectral interference were selected to determine Ti, Cr, Fe, Co, Ni, and W. We used real industrial samples of secondary raw materials obtained in product manufacture from VK-type hard alloys [solid solution of tungsten carbide grains (WC phase) in cobalt (Co phase)] in the form of tungsten-containing powders, grinding sludge of solid sintered alloys, dust waste, defective mixtures, waste of ventilation systems, and carbide powders. Correctness of the determination of elements was proved in analysis of ferrotungsten standard samples upon determination of the matrix component (tungsten) and by the methods of additives and sample weight variation upon determination of the alloying elements and impurities. The method of internal standard (with Sc as the element of comparison) was used to improve the precision and accuracy of the tungsten determination. The relative standard deviation was thus reduced from 0.03 to 0.004. The developed precise rapid technique was tested in analysis of industrial samples of secondary tungsten-containing sludge of hard alloys. The technique is recommended to control the composition of the secondary tungsten-containing sludge of hard alloys in a wide range of concentrations.

Surface quality of cemented tungsten carbide finished by direct cutting using diamond-coated carbide end mill

• Superiority of direct cutting of cemented carbide with diamond-coated tool was shown. • Surface quality in direct cutting, electrical machining, and polishing was compared. • Specularity of finished surface similar to polishing was obtained via direct cutting. • Large compressive residual stress can also be obtained via direct cutting. • Higher transverse rupture strength can be achieved compared with polishing. This study addresses the finished surface quality of cemented tungsten carbide obtained via direct cutting with a diamond-coated carbide end mill under ultrasmall feed rates. The surface appearance, composition distribution, profile, roughness, residual stress, and transverse rupture strength were experimentally investigated. The finished surfaces obtained via electric discharge machining with two types of machining conditions and mechanical polishing processes were also evaluated to compare the surface quality. Two types of cemented tungsten carbide materials, TAS VC-70 and VM-40, were applied as the workpiece materials. A mirror surface can be obtained via direct cutting and mechanical polishing with both materials. The microstructure morphologies of the finished surface obtained via direct cutting were different from those obtained via mechanical polishing. In the case of VC-70 which has larger tungsten carbide grain size and higher cobalt content, the flat cross-section of the tungsten carbide grain under ductile mode cutting was conspicuous in direct cutting under small feed rate and depth of cut. In contrast, an extremely thin layer composed of fine grains of tungsten carbide and cobalt was observed on the finished surface in direct cutting under comparatively large feed rate and depth of cut. The surface roughness and profile of the finished surface resulting from direct cutting were comparatively rougher than those resulting from mechanical polishing, although the finished surface was significantly smoother than that resulting from electric discharge machining. The residual stress of the subsurface obtained via direct cutting showed larger compressive stress than that obtained via electric discharge machining and mechanical polishing under all cutting conditions and materials. Moreover, satisfactory transverse rupture strength can be obtained via direct cutting in both materials compared with other processes, and two times more transverse rupture strength can be achieved via direct cutting compared with the case of electric discharge machining in VM-40 cutting.

Evaluation of mechanical and metallurgical properties of cryo-treated tungsten carbide with 25% cobalt

In this study, the impact of cryo-treatment on mechanical and metallurgical properties of straight tungsten carbide (unalloyed) specimens manufactured with a binder content of 25% cobalt was evaluated. Wear rate of the untreated and cryo-treated specimens were compared in pin-on-disc dry sliding friction test and the mechanical properties like Vickers hardness and transverse rupture strength were studied. Metallurgical properties like saturation magnetization (Ms), coercivity (Hc), and microscopic evaluation to study the grain morphology, distribution, and presence of elements like η-carbides were evaluated to validate the outcomes of wear testing. The results show that cryo-treated specimens showed a significant reduction in wear rate. The coercivity and Vickers hardness showed an increase of around 9% and 2%, respectively in the cryo-treated specimens. From the metallurgical evaluation, the reduction in wear rate of cryo-treated specimens was credited to the martensitic phase transformation of the binder element. The microscopic evaluation confirmed that the morphology of tungsten carbide grains was unaffected.

On wear of WC–Co cutting inserts in turning of Ti6Al4V – a study of wear surfaces

ABSTRACT Titanium alloys are known to cause crater wear on cutting inserts. It is one of the life-limiting factors for the tool life and highly influenced by the cutting parameters. To increase productivity, there is a strong incentive to increase the production speed. At the same time, the tool life should be unchanged or even prolonged. The influence of cutting speed, feed and cutting length on wear was investigated in turning tests using cemented carbide inserts and titanium alloy Ti6Al4V as work material. The results showed a very strong influence from all parameters on crater wear, confirming previous findings reported in the literature. The resulting wear surfaces were studied in detail, revealing a crater filled with transferred titanium alloy. The tungsten carbide grains underneath were depleted of carbon, resulting in a nano-rough top surface rich in tungsten. In addition, the turning tests showed a threshold cutting speed, below which no crater wear occurred.

Properties of Borided Cemented Carbides with Various Binders

This paper explores the thermochemical treatment of cemented carbides (CC), specifically the boriding process. Six different types of CCs with different size of tungsten carbide (TC) grains were chosen as experimental materials. They contained binders of different chemical compositions. In five CCs, the binders were pure metals: cobalt (four of them) and nickel (one of them). In the six one, the binder was a complex Ni-Co-Cr-based alloy. Samples of the different types of CCs were prepared by grinding and polishing and then half of them underwent boriding process. The experiment aimed to find how boriding affects the final properties of CCs and their structure. Microstructural changes in the materials were examined using X-ray diffraction and optical and electron microscopy. Changes in mechanical properties and wear resistance were evaluated using hardness testing and the Ball on Disk test. The experimental results, for example, shown that CC with nickel binder had lowest wear resistance from all tested sorts of CC.

Oscillatory pressure sintering of binderless tungsten carbide

Tungsten carbide ceramics were fabricated by a novel oscillatory pressure sintering process and the influence of sintering temperature on the microstructure and mechanical properties of tungsten carbide ceramics was investigated. It was found that tungsten carbide would decarbonize to form W2C under high temperature environment. As the sintering temperature increased, the density and hardness of tungsten carbide ceramics increased apparently, and the growth of tungsten carbide grains became more obvious. The samples fabricated by oscillatory pressure sintering exhibited superior mechanical properties in comparison with some published data obtained by other pressure-assisted sintering methods. The optimum sintering temperature for tungsten carbide ceramics under oscillatory pressure was considered to be 1900 °C. Flexural strength, hardness and fracture toughness of the corresponding sample prepared at this temperature were found to be 1014 MPa, 26.98 GPa and 5.91 MPa·m1/2, respectively.

Microstructure and Properties of Ultrafine Cemented Carbides Prepared by Microwave Sintering of Nanocomposites

Ultrafine cemented carbides were prepared by microwave sintering, using WC-V8C7-Cr3C2-Co nanocomposites as a raw material. The effects of sintering temperature and holding time on the microstructure and mechanical properties of cemented carbides were studied. The results show that the ultrafine cemented carbides prepared at 1300 °C for 60 min have good mechanical properties and a good microstructure. The relative density, Vickers hardness, and fracture toughness of the specimen reach the maximum values of 99.79%, 1842 kg/mm2 and 12.6 MPa·m1/2, respectively. Tungsten carbide (WC) grains are fine and uniformly distributed, with an average grain size of 300–500 nm. The combination of nanocomposites, secondary pressing, and microwave sintering can significantly reduce the sintering temperature and inhibit the growth of WC grains, thus producing superfine cemented carbides with good microstructure and mechanical properties.

COMPARISON OF THE EFFECT OF THE APPLIED ENERGY ON THE PROPERTIES OF PROTOTYPES MADE FROM DIFFERENT TYPES OF POWDER MIXTURES

The paper deals with the influence of different types of WC-Co powder mixtures processed by SLM technology on the final properties of the prototype build.The powder mixtures used differed in the morphology of the powder particles, i.e. the shape, size and chemical composition of the individual powder particles. In total, two different types of WC-Co powder mixtures were used for the experiment. The study was focused on the evaluation of the effect of these powders and other parameters of SLM technology (applied energy) on the resulting properties of the prototype sample.The samples were subjected to metallographic analysis after their preparation. This analysis was performed using metallographic and scanning electron microscopes. The metallographic analysis was supplemented by analyzing the phase composition of the prototype samples and the changes in their chemical composition. The experiments enabled us to compare the differences in the properties of the prototype samples, especially in terms of their porosity, structural changes, changes in chemical and phase composition. The information obtained in relation to the parameters of the technology and the powders provided us with interesting data. The diffusion of the elements, especially carbon, meant the eta phase was formed in all of the samples, which is considered to be an undesirable phase in cemented carbides. However, its volume varied depending on the type of powder and the processing conditions. The smallest amount was recorded for samples made from a powder mixture with a free bond between the binder particles and tungsten carbide grains.

Исследование особенностей высокоскоростного спекания плазмохимических нанопорошков карбида вольфрама с повышенным содержанием кислорода

Spark plasma sintering (SPS) of plasma-chemical nanopowders tungsten carbide with a high oxygen content are studied. The results show that the nanopowder WC shrinkages during SPS is limited by grain boundary diffusion with abnormally low activation energies. It is established that the decrease in the activation energy can be caused by the influence of oxygen on the diffusion permeability of the tungsten carbide grain boundaries at the stage of intense compaction, as well as by abnormal grain growth at the stage of high-temperature sintering. The SPS kinetics of WC-W2C-WO3-W nanopowder compositions at the stage of intense compaction is controlled by the rate of oxide particle sintering with their simultaneous transformation into W2C particles, and then, by the plastic flow of W2C particles in the presence of W particles at the stage of high-temperature sintering. Using the SPS method, we obtained ceramics with a high density (98-99%), ultrafine-grained structure (the average grain size is less than 0.3 mm), having an increased hardness HV = 30.5 GPa with a Palmquist crack resistance of ~6.5 MPa·m1/2.

Influence of heat treatment on properties of SD251-PH1 composite produced by additive SLM technology

This study evaluates how heat treatment affects the structure and properties of the engineered composite material SD251-PH1. This material was formed by mixing two powder blends in a weight ratio of 90 wt. % WC-Co powder SD251 with 10 wt. % of a PH1 precipitation hardening powder of the stainless steel. Samples were prepared from this mixture on an SLM additive device. The printed samples were then divided into groups. Some of them were left in the initial state and the rest were used for heat treatment based on the standard AMS 5659. Light and electron microscopy metallographic analysis together with X-ray diffraction analysis were then used to evaluate structural and phase changes in the prototype samples. The main attention was paid to changes in the phase composition of the samples. In addition, changes in the size and shape of the pores and tungsten carbide (WC) grains were studied. The metallographic analysis was supplemented by an evaluation of the changes in their mechanical properties and wear resistance. Vickers hardness measurements and the ball on disc test were used for this purpose. The experiments showed that the selected heat treatment processes precipitated new structural phases which differed from the original sample structure in terms of shape, size, chemical composition, and mechanical properties.

Subgrain structure and dislocations in WC-Co hard metals revealed by electron channelling contrast imaging

In this study, electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) have been used to examine the substructure and dislocations in tungsten carbide (WC) grains in tungsten carbide‑cobalt (WC-Co) hardmetals. These complimentary scanning electron microscopy (SEM) diffraction techniques provide quantifiable information of the substructure without the difficulty of transmission electron microscopy (TEM) sample preparation and examination. Subgrain structures in WC grains have rarely been reported previously because of the sample preparation difficulty, but this study has found they can occur frequently and may provide information on grain growth during sintering. ECCI has also shown for the first time complex dislocation networks across large grains, indicating accumulation of stress in as-sintered materials. To identify the defects revealed by ECCI more precisely, WC grains with surface normals 0001,11¯00and112¯0,were identified using inverse pole figure orientation maps generated from EBSD data. ECC images from these grains reveal defects intersecting the surface and subgrains bound by dislocations. The combination of ECCI and EBSD allows for new insights into dislocation networks in a WC-Co hardmetal sample over a large, in this case 75 μm × 75 μm, field of view.

Development and characterisation of novel Cr-based hardmetals strengthened by nanosized tungsten carbide

This work highlights the promising mechanical properties and oxidation resistance achieved by novel Cr-based WC hardmetals having nanosized tungsten carbide grains, designed by combining thermodynamic calculations and experimental evaluation. The hardmetals are processed by a powder metallurgy route including mechanical milling of powders and subsequent consolidation by spark plasma sintering. The effect of the addition of different extra Fe and C contents on microstructure, mechanical properties, and oxidation resistance is fully investigated. Thus, the best mechanical performance is achieved when the formation of brittle ŋ-phases, M7C3 carbides and soft graphite is reduced or suppressed, while maintaining the nanosized WC grain size. More specifically, the Cr-based WC hardmetal with an extra 3 wt% Fe content and extra 0.5 wt% C content reaches the best combination of hardness and toughness fracture values (2219 HV30 and 8.2 MPam1/2). In addition, the oxidation resistance reached is higher than that of conventional Co-based or Ni-based WC hardmetals tested under the same conditions turning them into promising candidates for applications under aggressive oxidation environments.

T Dependence of the Mechanical Properties on the Microstructural Parameters of WC-Co

Abstract The effect of binder content and WC grain size on the mechanical properties is widely investigated in literature. An increase in binder amount and WC grain size leads to a decrease in hardness and an increase in fracture toughness. Actually, these correlations are related to the influence of binder content and WC grain size through the contiguity and mean binder free path, which are the microstructural parameters that affect the mechanical properties. The aim of this study is to verify the dependence of the two microstructural parameters that govern the WCCo mechanical behaviour, namely the contiguity and mean binder free path, on the mechanical properties of an extended range of WC-Co samples, which differ in terms of Co content and tungsten carbide grain size.

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

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 prepared from narrow-fraction tungsten carbide powder with a grain size of 5–15 μm are studied. It is established by the results of metallographic analysis that sintering temperatures in a range of 1390–1420°C provide the pore-free state of the alloy with the normal carbon content, but the samples have considerable porosity at its lowered concentrations. It is revealed that sintering temperatures of 1450–1475°C, irrespective of the carbon content, make it possible to prepare carbides with residual porosity lower than 0.02%. It is shown that alloys with a carbon deficit of 0.11–0.91% had a two-phase structure, while the alloy with a carbon deficit of 1.31% contained inclusions of the η phase in addition to WC and γ phase. It is established that the retardation of the growth of tungsten carbide grains is observed with a decrease in the carbon content during liquid-phase sintering. The concentration of dissolved tungsten is established by electron probe microanalysis. It was 10, 12, 15, and 19 wt % for carbides with a normal, low, mid, and high carbon deficit, respectively. The use of narrow-fraction tungsten carbide powders makes it possible to fabricate carbides with rounded grains having a shape factor of about 0.77. The alloy with a carbon deficit of 0.91% relative to the stoichiometric ratio had the best combination of hardness and crack resistance:11.1 GPa and 16.0 mPa m1/2.

Structure and properties of coarse-grained WC-Со hard metals with extra homogeneous microstructure

The structure and properties of coarse-grained WC-6%Co hard metals with carbon deficiency from 0,11 to 1,31 % obtained from narrow fraction tungsten carbide powder with a grain size of 5 to 15 pm were studied with respect to the stoichiometric ratio. According to the results of metallographic analysis, 1390 to 1420 °C sintering temperatures provide a non-porous alloy state with normal carbon content, while alloys with lower carbon content feature considerable porosity. It is found that hard metals with less than 0,02 % residual porosity can be obtained at sintering temperatures of 1450-1475 ° С regardless of the carbon content. It is shown that alloys with 0,11—0,91 % carbon deficiency have a two-phase structure, while the alloy with 1,31 % carbon deficiency contains n phase inclusions in addition to WC and γ phase. It is determined that lower carbon content slows down the tungsten carbide grain growth process during liquid-phase sintering. EDX analysis was used to determine the concentration of tungsten dissolved in the binder phase — 10, 12, 15 and 19 wt.% for hard metals with normal, low, medium and high carbon deficiency, respectively. Narrow fraction WC powders allow obtaining hard metals with rounded grains having a form factor of about 0,77. The alloy with 0,91 % carbon deficiency with respect to the stoichiometric ratio had the best combination of hardness and toughness (11,1 GPa and 16,0 MPa-m 1/2 ).

Fabricating Ultrathin Plate-Like WC Grains in WC–8Co Hardmetals by Increasing Discharge Intensity During Plasma-Assisted Ball Milling

The effects of plasma discharge intensity on the microstructure evolution of ball-milled tungsten (W)–carbon (C)–cobalt (Co) mixtures and the formation mechanism of ultrathin plate-like tungsten carbide (WC) grains prepared by ball milling with and without plasma discharge were investigated. It was found that increasing the plasma discharge intensity during ball milling obviously promoted the formation of a thin flake-like W phase because of the electroplasticity effect and simultaneously lowered the carburization temperature between W and C. A combination of high hardness and transverse rupture strength of 92.9 HRA and 3659 MPa, respectively, was obtained for the WC–8Co alloy fabricated by plasma milling at a gas pressure of 5 × 103 Pa with a dielectric barrier discharge layer thickness of 3 mm. These properties were mainly attributed to the markedly lowered activation energy of the WC phase and generation of highly oriented ultrathin plate-like WC grains by plasma milling. The combination of the flake-like structure of the plasma-milled W aggregate and high specific interfacial area and short diffusion distance of W/C were readily inherited by the ultrathin plate-like WC grains in the sintered WC–Co hardmetals.

Fracture energy based approach for cemented carbides grain debonding

Cemented carbide materials exhibit high hardness and fracture toughness. As a consequence they possess an excellent wear resistance that make them a material of choice for the manufacturing of cutting, drilling and forming tools. Cemented carbide materials are made of tungsten carbide (WC) grains embedded in a ductile metal phase, generally made of cobalt. The wear mechanism of the tungsten carbide and cobalt composites (WCCo) operates in several successive steps: removal of the cobalt binder in the surface vicinity, plastic deformation and/or fracture of WC grains, debonding of WC grains and finally galling of workpiece material in the cavities freed of the WC grains. The present study focuses on the debonding step of the WCCo wear mechanism.A micromechanical finite element is developed to study de sensitivity of a WC grain to be torn off the surface of a WCCo composite structure. The model considers a single WC grain embedded in a homogeneous elementary volume of WCCo material. The geometry of the WC grain is a rectangle which perimeter, width and length are based on statistical analyses of actual WCCo die surfaces used in cold forming. Cohesive elements are used to model the junction between the grain and the elementary volume. The grain is torn off the surface when the fracture energy calculated within the cohesive element is totally consumed. The loadings applied on the grain are based on actual cold heading forming sequences. Various grain size, shape, orientation and friction conditions are tested. Results show that grain orientation, friction and WCCo material toughness play a great role in WC grain removal. Equiaxed grains with small perimeter are less sensitive to debonding.