Hard Metal Alloys Research Articles

Análise de fases por difração de raios X de WC-10%Co dopado com terras-raras obtido sob alta pressão

O carbeto cementado WC-Co também denominado de metal duro é um material de grande importância tecnológica produzido por metalurgia do pó através de sinterização por fase líquida, partindo-se da mistura dos pós de carbeto de tungstênio (WC) e de cobalto (Co) em moinho convencional. Entretanto, para obtenção de metal duro com maior capacidade (produtividade, vida útil, etc.), uma nova rota alternativa de metalurgia do pó denominada de altas pressões e altas temperaturas vem sendo utilizada para produção destes materiais com resultados promissores. O presente trabalho tem como objetivo principal a análise de fases de WC-10Co dopado com terras-raras (La2O3, CeO2e Y2O3) preparadas sob condições de alta pressão. Esta liga de metal duro é um exemplo de material compósito largamente utilizado como ferramenta de corte e operações de usinagem em geral. Foram preparadas diversas misturas de WC-10Co contendo até 2 % em peso de terra-rara (La2O3, CeO2e Y2O3) da fase cobalto. Os pós previamente misturados foram compactados numa matriz cilíndrica (Φ = 7 mm) a uma pressão de 800 MPa e sinterizados a 1400 ºC durante 40 s sob alta pressão de 5,5 GPa. Os compósitos produzidos foram caracterizados por difração de raios X para identificação de fases cristalinas presentes. Além disso, foi feita análise comparativa entre o carbeto isento de terra-rara (amostras de referência) e carbetos dopados com elementos de terra-rara (La2O3, CeO2, e Y2O3). Para as condições estudadas os resultados mostraram que durante o processo de sinterização ocorreu à formação, mesmo que em pequenas quantidades, das fases Co3W3C2 e γ (carbeto tipo ε) nas peças de metal duro estudadas.

Thermoelastic Residual Stresses and Deformations at Laser Treatment

Abstract A thermoelastic model implying relaxation of stresses at melting is applied for materials with arbitrary thermoelastic properties and the melting point. The range of Poisson's ratio 0.17 - 0.34 is numerically studied. The residual stresses are independent of the space scale. In narrow remelted zones and beads the maximum longitudinal tensile stress is approximately twice as high as the transverse one. The calculations predict cracking of alumina, even with 1600 oC preheating, plastic deformation or cracking of hard metal alloys H13 and TA6 V, and no destruction of polystyrene and thestrongest grades of quartz glass. The calculation results can be used for predicting the thermomechanical stability of materials at laser treatment.

Optimization of multi machining characteristics in WEDM of WC-5.3%Co composite using integrated approach of Taguchi, GRA and entropy method

Wire electrical discharge machining (WEDM) is a well known process for generating intricate and complex geometries in hard metal alloys and metal matrix composites with high precision. In present work, intricate machining of WC-5.3%Co composite on WEDM has been reported. Taguchi’s design of experiment has been utilised to investigate the process parameters for four machining characteristics namely material removal rate, surface roughness, angular error and radial overcut. In order to optimize the four machining characteristics simultaneously, grey relational analysis (GRA) coupled with entropy measurement method has been employed. Through GRA, grey relational grade has been computed as a performance index for predicting the optimal parameters setting for multi machining characteristics. Using Analysis of Variance (ANOVA) on grey relational grade, significant parameters affecting the multi-machining characteristics has been determined. Confirmatory results prove the potential of present approach.

Analysis of Formation and Interfacial WC Dissolution Behavior of WC-Co/Invar Laser-TIG Welded Joints

During the valve fabrication, hard metal is welded to stainless steel or invar alloy for sealing purposes because of its good heat resistance operating at 500 °C. However, WC (tungsten carbide) dissolution in weld pool softens the hard metal and decreases mechanical properties near the hard metal/weld interface. In order to analyze the WC dissolution in welded joint, joining of hard metal and invar alloy was carried out using laser-tungsten inert gas hybrid welding method. Microstructures of the weld region, chemical composition were investigated using optical microscope, scanning electron microscopy, and EDAX, respectively. Mechanical properties such as microhardness and four-point bend strength test were performed. Larger and smaller WC dissolution and WC dissolution through transition layer based on thermo-dynamics were discussed. The results thus indicate that WC dissolution led to cellular microstructure, columnar crystal, and transition layer under the effect of laser beam and tungsten arc. WC dissolution was affected by metal ions Fe+, Ni+, Co+ exchange in W-M-C system, and WC grain growth was driven by forces caused by laser beam and tungsten arc in larger WC, smaller WC, and liquid Fe, Ni systems.

Use of Ti in hard metal alloys – Part II: mechanical properties

AbstractWC‐Co hard metal is a material of high hardness, high compressive strength and wear resistance while maintaining good toughness and thermal stability. Samples of nanosized WC powders with 10 wt% Co, WC‐10 wt% Ti, WC‐9 wt% Ti‐1 wt% Co were cold pressed at 200 MPa and sintered at 1500°C during 1 hour under vacuum of 10–2 mbar. The characterization of the sintered materials was performed by the measurements of densification, HV30 hardness, fracture toughness and compression strength. The results showed that it is possible to process a hard metal through a Powder Metallurgical conventional route from nanoscaled WC grains, using Ti (or a Ti‐Co mixture) as a binder phase, with good mechanical properties.

Dissolution of cemented carbide powders in artificial sweat: implications for cobalt sensitization and contact dermatitis

Skin exposure to cobalt-containing materials can cause systemic immune sensitization and upon repeat contact, elicitation of allergic contact dermatitis (ACD). Data on cobalt dissolution rates are needed to calculate uptake through skin and for development of models to understand risk of sensitization or dermatitis. The purpose of this research was to measure the dissolution kinetics of feedstock and process-sampled powders encountered in the production of hard metal alloys using artificial sweat. The physicochemical properties of each material were characterized prior to evaluation of dissolution behavior. Variations in artificial sweat solvent pH and chemistry were used to understand critical factors in dissolution. Dissolution of cobalt, tungsten, and tungsten carbide was often biphasic with the initial rapid phase being up to three orders of magnitude faster than the latter long-term phase. Artificial sweat pH did not influence dissolution of cobalt or tungsten carbide. Solvent composition had little influence on observed dissolution rates; however, vitamin E suppressed the dissolution of cobalt and tungsten carbide from sintered particles obtained from a chamfer grinder. There was no effect of particle size on dissolution of feedstock cobalt, tungsten, tungsten carbide, and admixture powders. Particle physicochemical properties influenced observed dissolution rates with more cobalt and tungsten carbide dissolving from chamfer grinder particles compared to the feedstock powders or admixture powder. Calculations using the observed dissolution rates revealed that skin exposure concentrations were similar to concentrations known to induce cobalt sensitization and elicit ACD. Observed dissolution rates for cobalt in artificial sweat indicate that dermal uptake may be sufficient to induce cobalt sensitization and allergic dermatitis.

Biological monitoring of tungsten (and cobalt) in workers of a hard metal alloy industry

To evaluate a combined biomonitoring approach based on both cobalt and tungsten determination in workers of the hard metal alloy sector. We enrolled 55 workers from a factory producing cutting tools for carpentry. Combined workroom air and biological monitoring of both cobalt and tungsten relied on inductively coupled plasma mass spectrometry determinations. Metals were determined on plasma, blood and urine samples. Urine samples from 34 unexposed subjects were also analyzed. Tungsten was determined in every collected sample. Workers showed significantly higher urinary tungsten levels than controls (pre-shift values of 4.12 vs. 0.06 microg/l on average; P < 0.0005). Both airborne and biological levels of tungsten prevailed among workers involved in wet-grinding activities. The element was excreted at higher urinary levels than cobalt and showed lower circulating (blood, plasma) concentrations. Exposure-dose relationships were apparent for tungsten biomarkers. Obtained results may contribute to the development of biomarkers of exposure to tungsten. The association of such biomarkers to traditional determinations of cobalt in blood and/or urine may substantially improve the exposure assessment of workers employed in cemented carbide industries.

Cobalt‐containing alloys and their ability to release cobalt and cause dermatitis

Cobalt, nickel, and chromium are important skin sensitizers. However, knowledge about cobalt exposure and causes of cobalt sensitization is limited. To study release of cobalt, nickel, and chromium from some cobalt-containing hard metal alloys and to test reactivity to the materials in cobalt-sensitized patients. Discs suitable for patch testing were made of some hard metal alloys. Cobalt, nickel, and chromium release from the materials was determined by immersion in artificial sweat (2 min, 1 hr, 1 day, and 1 week). Patch test reactivity to the discs and to serial dilutions of cobalt and nickel was assessed in previously patch-tested dermatitis patients (19 cobalt positive and 18 cobalt-negative controls). All discs released cobalt, nickel, and chromium. Some discs released large amounts of cobalt (highest concentration: 290 microg/cm(2)/week). Seven discs elicited three or more positive test reactions. The concentration of released cobalt was high enough to elicit allergic contact dermatitis in cobalt-sensitized patients. As the materials in the discs are used in wear parts of hard metal tools, individuals with contact allergy to cobalt may develop hand eczema when handling such materials.

Comparison between exhaled breath condensate analysis as a marker for cobalt and tungsten exposure and biomonitoring in workers of a hard metal alloy processing plant

Cobalt (Co), Tungsten (W) and Tungsten Carbides (WC) are major constituents of hard metal alloys. Whereas little is known about potential health hazards due to tungsten carbide exposure, occupational exposure to cobalt has been shown to induce a variety of respiratory diseases. Since the concentration of a potentially hazardous substance in the target organ is the most meaningful risk indicator in occupational medicine, the detection of hard metals in exhaled breath condensate (EBC) has been proposed to be a valuable instrument. The present study examines the correlation of Co and W concentrations in EBC and urine with one another and various spirometrical and clinical parameters to scrutinize this potential. A total of 62 subjects (90.3% males, age 40.6 +/- 9.2 years) were recruited from a hard metal processing plant in Germany. Examinations included the airborne workplace exposure, a complete spirometry, measurements of Co and W concentrations in EBC and urine with high resolution inductive coupled plasma mass spectrometry (HR ICP-MS) and graphite furnace atomic absorption spectrometry (GFAAS). Air concentrations ranged between 0.0019 mg/m(3) and 0.074 mg/m(3) for Co and 0.012 mg/m(3) and 0.021 mg/m(3) for W. Median urine concentrations and interquartile ranges of the exposed subjects ranged from 0.81 (0.0-1.46) microg/l for Co and 30.5 (14.5-57.7) microg/l for W. Median breath condensate metal concentrations and interquartile ranges ranged from 8.4 (5.0-13.9) microg/l for Co and 8.8 (4.4-18.5) microg/l for W. Urine concentrations of Co and W were closely related to the airborne workplace exposure that had been assessed by air monitoring. EBC concentrations of Co and W showed no correlations to urinary W and Co concentrations and the ambient monitoring results of the individual workplace, respectively. Cobalt EBC concentration was elevated in subjects who reported to have suffered from respiratory disease; both Co and W concentrations in EBC, however, decreased with increasing spirometrical signs of obstruction. According to our study, urinary concentrations of Co and W seem to be more reliable indicators of current workplace exposure than EBC concentrations. As far as new methods and exposure matrices for valid concentration measurements in respiratory organs and possible hazardous effects--especially of cobalt--in the lung are concerned, the present results are less clear-cut, and further research is required.

Pressure Aided Sintering of Ultrafine Powders of WC with Addition of Co

Hardmetal alloys are processed by liquid phase sintering, which consist of one refractory carbide, embebided in to a tough matrix of Co. These alloys extensively are used in applications that demand abrasion resistance and high hardness. The present work has as objective to analyze the sintering of ultrafine powders of tungsten carbide (WC) with the addition of 10%wtCo. The alloys had been prepared by using high pressure and high temperature technique. The sintering of the WC/10%pCo alloy was processed at the 3, 4 and 5 GPa, in temperatures of 1300,1400 and 1500°C, during 2 and 4 minutes. The analysis was made by microstructure, densification, structure, Vickers hardness and fracture toughness. The best results was density = 98.9%, hardness HV30 =10,77GPa and fracture toughness KIC = 15,57 MPa.m1/2.

WC/Co-composite powders via hydrothermal reduction of Co3O4-suspensions

Abstract Due to the strong influence of the cobalt distribution in the green powder compact on the quality of the sintered hardmetal alloy, intensive powder milling and a balanced ratio of WC to Co powder particle size are usually required to achieve uniform carbide-microstructures. In the past many attempts have been made to reduce or avoid powder milling and to improve the quality of the powder mixture by chemical means. The research described below focused on a new route to carbide coating comprising oxide-pressure-precipitation of submicron to nano-sized Co 3 O 4 -suspensions from cobalt solutions, mixing with WC-suspensions and subsequent hydrothermal reduction of those suspensions in hydrogen atmosphere. The effects of WC addition and catalyst addition on the progress of cobalt oxide reduction are discussed. The resulting cobalt and composite powders were characterised via powder diffraction, SEM and TEM imaging. Sintering tests confirmed the uniform Co distribution.

Structure and hardness of a hard metal alloy prepared with a WC powder synthesized at low temperature

The structure and hardness of a WC-10 wt% Co alloy prepared with an experimental WC powder are compared with those of another alloy of the same composition produced under the same conditions and prepared with a commercial WC powder. The experimental WC powder was synthesized by a gas–solid reaction between APT and methane at low temperature and the commercial WC powder was conventionally produced by a solid–solid reaction between tungsten and carbon black. WC-10 wt% Co alloys with the two powders were prepared under the same conditions of milling and sintering. The structure of the sample prepared with the experimental WC powder is homogeneous and coarse grained. The structure of the sample prepared with the commercial powder is heterogeneous. Furthermore the size and shape of the WC grains are significantly different.

Mechanical properties and microstructures of hot-pressed MgAlON–BN composites

The relationship between the mechanical properties and microstructure of hot-pressed MgAlON–BN composite materials was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and X-ray diffraction (XRD) techniques. The phase compositions of hot-pressed samples prepared from starting mixtures of Al 2O 3, AlN, MgO and h–BN consisted of MgAlON phases as a matrix and BN phases as the second phase. The density, bending strength at room temperature, fracture toughness and Vickers hardness were measured. The results indicated that the density, strength and Vickers hardness decrease with increasing h–BN content due to the non-reactive nature and layered structure of h–BN. The fracture toughness, however increased with increasing h–BN addition, reaching a maximum of 3.64 MPa m 0.5; it decreased with further increase of BN content. The increase of fracture toughness was attributed to the presence of microcracks and the decrease was considered to be the discontinuous microstructure of the MgAlON phases. Temperature dependence of bending strength remained constant at low temperature, followed by an increase at 800 °C and then, dropped quickly. The increase in the bending strength of the composite was attributed to the decrease of residual stress and to the interwoven microstructure of the composites which prevented grain boundary slip and reduced the attenuation rate of high temperature strength. The machinability of the composites was examined. The results indicate that the composite materials with BN content more than 15 vol.% exhibit excellent machinability and could be drilled using conventional hard metal alloy drills.

A Computer Simulation of the Hard-Metal Structure: the Interpretation of Structural Features

The intercept length measurement of WC grains in hardmetal alloys is simulated in computer. The distribution of intercept length is related to the distribution of WC grain size, but the latter can not be measured from a sintered alloy. It must be interpreted from the intercept length measurement. By using simulation, the complex grain size distributions of real alloys can be decomposed into more simple distributions, as the monomodal ones. The analysis of the monomodal structures is used to interpret the more complex structures. It is found that the lognormal intercept length distribution measured in hardmetal alloys in practice does not necessarily correspond to a lognormal grain size distribution, as commonly reported. The lognormal-like format of the intercept length distribution curve of real alloys is a natural trend of structures with multimodal grain size distributions.

Machinable Al 2O 3/BN composite ceramics with strong mechanical properties

Al 2O 3/BN composite ceramics with nano-sized BN dispersions ranging from 0 to 30 vol.% were successfully fabricated by hot-pressing α-Al 2O 3 powders with turbostratic BN (t-BN) coating, which was prepared through chemical processes using boric acid and urea. SEM observations revealed that the nano-sized hexagonal BN (h-BN) particulates were homogeneously dispersed within Al 2O 3 grains as well as at grain boundaries. Vickers hardness of materials decreased with an increase in BN content. The fracture toughness was improved but the fracture strength had a small decrease, in comparison to Al 2O 3 monolithic ceramics. The nanocomposite ceramics with BN content more than 20 vol.% exhibited excellent machinability, which could be drilled using conventional hard metal alloy drills. Drilling rates and normal forces demonstrate the ease of machining of these materials. The preliminary information on the relationship between microstructures and properties are provided. The mechanism of material removal is also discussed.

Synthesis of niobium carbide at low temperature and its use in hardmetal

A new method of synthesizing niobium monocarbide at a low temperature (950 °C) and in a short time (2 h) is described. Conventionally, niobium monocarbide (NbC) can only be produced at 1600–1800 °C for longer periods. The method consists of the carburization of a niobium complex by a gaseous (H 2+CH 4) mixture. The method of preparation of the precursor and its carburization are presented. Precursor and carbide are characterized by XRD, SEM, TG/DTG and granulometry. Shape, mean size and size distribution of the NbC particles are similar to those of the niobium precursor. Reactions during carburization do not alter these characteristics. NbC particles are very large and porous. NbC powder is used in a hardmetal alloy as a WC grain growth inhibitor. Its performance is compared to that of a commercial NbC powder. NbC synthesized by the new technique exhibited a slightly higher efficiency in inhibiting the WC coarsening, evidenced by the SEM of the hardmetal structures and hardness measurements.

Production and indentation analysis of WC/Fe–Mn as an alternative to cobalt-bonded hardmetals

An iron–manganese (Fe–Mn) alloy-bonded tungsten carbide hardmetal, WC/Fe–13.5 wt.% Mn, was produced in various combinations of binder phase by using powder metallurgical routes. This was done in order to provide lower-cost hardmetals and to offer a nontoxic alternative to cobalt (Co)-based materials. The outcome of this effort has been the preparation, at acceptable sintering temperatures, of a Fe–Mn bonded WC with a fully austenitic binder phase. Vickers indentations on the hardmetal substrates produced Palmqvist-type cracks. The variation of Vickers hardness with sintering temperature was investigated for WC bonded with 15 and 25 wt.% Fe–Mn alloys. Mechanical property evaluations showed that the alloys possess higher hardness and slightly lower fracture toughness values than the equivalent of Co-bonded grades. This fact could lead to the use of such hardmetal alloys in mining and wear-resistant applications with better reliability.

Hard Metal-Induced Disease: Effects of Metal Cations in Vitro on Guinea Pig Isolated Airways

Inhalation of dust from hard metal (HM), a mixture of tungsten carbide, cobalt, and other metals, can cause interstitial alveolitis, fibrosis, and asthma in the workplace. Some effects of HM could occur after the metals dissolve in the lung. We examined whether chloride salts of metals in HM alloys can elicit responses or modify reactivity to methacholine (MCh) or responses to electric field stimulation (EFS) in guinea pig tracheal strips. In unstimulated strips, Co2+, Cd2+, and Ni2+ evoked contractions (>3 × 10−6 M), while Ta5+, Zn2+, Cr2+, and Cr3+ caused weak relaxations (>10−5 M). In strips contracted with MCh (3 × 10−7 M), Co2+ and Ni2+ also caused relaxation in lower concentrations while the other metals caused weak relaxation only in high concentrations (>10−4 M). The metals were generally without effect on reactivity to MCh, except that Cd2+ inhibited and Ni2+ potentiated some responses. The effects of selected metals (10−6 M; Cr3+, Ni2+, Cd2+, and Co2+) on EFS-induced contractile and relaxant responses were examined (±MCh; ±10−6 M indomethacin (Indo), 30 min). No metal had any effect on the excitatory nonadrenergic, noncholinergic-mediated contraction phase. Cd2+ and Ni2+ inhibited cholinergically mediated contractions of unstimulated strips (+Indo), whereas Cr3+ both inhibited (−MCh, −Indo) and potentiated (−Indo,+MCh; +Indo, +MCh) contractile responses. Cr3+ was the only metal to inhibit the inhibitory nonadrenergic, noncholinergic-mediated relaxation phase (±MCh; −Indo). Co2+ had no effect at all. The results suggest that smooth muscle tone and nerves in the airways could be targets of cationic metals after they dissolve in the lung.

Diamond deposition on hardmetal substrates after pre-treatment with aluminium or aluminium compounds

Diamond coated WC-Co hardmetal alloys are widely used as cutting tools for non-ferrous materials. A disadvantage is the poor adhesion of diamond layers on hardmetal, which is mainly caused by cobalt used as binder material. Various attempts have been made to reduce cobalt on the substrate surface either by etching or by forming stable compounds (especially silicides and borides). This paper deals with the possibility of forming cobalt–aluminium–compounds such as aluminides or aluminates during substrate pre-treatment with metallic aluminium or solutions, respectively, suspensions of aluminium-salts. Diamond deposition was carried out in a hot-filament-CVD-reactor. The diamond layers were examined with respect to growth rate, morphology and adhesion. Some improvements concerning the reduction of cobalt migration and diffusion were detected leading to increased layer adhesion, as compared to untreated hardmetal substrates.

A Low Temperature Synthesized NbC Grain Growth Inhibitor in WC-Co Hardmetal Alloy

A Low Temperature Synthesized NbC Grain Growth Inhibitor in WC-Co Hardmetal Alloy