WC Hardmetals Research Articles

Effect of C content on microstructure and mechanical properties of Cr-based hard composites obtained by different sintering methods

Nanocrystalline Cr-based-WC powders with different C contents (5.59, 6.03, and 6.69 wt% C) were sintered by four different sintering methods, including liquid phase sintering (LPS), hot pressing (HP), field-assisted hot pressing (FAHP), and spark plasma sintering (SPS). The densification, average WC grain size, phase composition, microstructure, and mechanical properties of Cr-based-WC hardmetals were dependent on the C contents and the applied sintering methods. The spark plasma sintering technique was superior to the other processing methods, giving fully dense samples (99.7%) at a lower sintering temperature and a shorter sintering time. The average WC grain sizes of samples with 5.59 wt% C sintered by LPS, HP, FAHP, and SPS were 0.9 μm, 1.0 μm, 0.7 μm, and 0.1 μm, respectively, which demonstrated that Cr-based binder had a strong inhibitory effect on the growth of WC grains. Brittle carbides (M23C6/M6C) were formed and further grew in LPSed and HPed samples, while Cr2O3 phase was detected in all FAHPed and SPSed samples. C element could react with the oxides and remove them from the samples by forming gaseous species such as CO and CO2 in LPSed and HPed samples sintered for hours. On the contrary, Cr2O3 phase was formed due to the strong affinity between Cr and O during FAHP and SPS sintering process, where the densification was finished in minutes. Moreover, the C content played a key role in determining the microstructure and phase formation. Generally, M6C, M23C6, Cr7C3, and graphite phases were subsequently induced with the increase of C content from 5.59 to 6.69 wt%, which was examined by the thermodynamic simulation of W–C–Cr–Fe–O system. The mechanical properties, including hardness, fracture toughness, transverse rupture strength, and compressive strength, of Cr-based-WC hardmetals were fully investigated. All sintered Cr-based-WC hardmetals showed an obvious advantage in hardness, considering that the WC volume fraction (only 70%) was much lower than that (≥90%) of commercial Co-based WC hardmetals. Especially, SPS-1 sample had achieved amazing compressive mechanical properties (hardness of 2219 HV30, fracture toughness of 8.2 MPa m−1/2, transverse rupture strength of 531 MPa, and compressive strength of 3296 MPa), which were superior to nanosized Co-based-WC hardmetals. The microstructural evolution mechanisms during LPS and SPS were further inferred.

Corrosion resistance of WC hardmetals with different Co and Ni-based binders

In this work, industrially relevant WC composites with alternative binders (Ni, CoNi, NiCr, CoCr, CoNiCr and NiCrMo) were produced and their corrosion resistance in 0.5 M NaCl was compared to conventional WC-Co system. The main objective is to develop hardmetals with higher corrosion resistance to extend the lifetime of tools and equipment, reducing the demand for new parts and maintenance, thus decreasing the total cost of the operation. The study includes different electrochemical techniques, namely corrosion potential monitoring, Electrochemical Impedance Spectroscopy (EIS) and polarization curves. The purpose is to evaluate the influence of each element on the corrosion resistance of the composites produced.

Properties of boron carbide thin films deposited by pulsed laser deposition

Boron carbide (B4C) is the third hardest material known after diamond or ta-C and cubic boron nitride (c-BN). Due to its super-hardness and, moreover, its stability up to high temperatures and chemical stability against ferrous materials , it is an interesting material for use as wear resistant coatings , particularly in applications where diamond or ta-C films are not possible to use. Super-hard boron carbide coatings were produced using Pulsed Laser Deposition (PLD) and their mechanical properties were investigated in dependence of the laser fluence and the substrate temperature during deposition. While substrate temperature was found to have a significant influence on the hardness of the coatings, the influence of the ablation fluence was insignificant. At all fluences used, the indentation hardness increases from some 31 GPa at room temperature up to 47 GPa at 500 °C. The Young's modulus was measured to be in the range of 470 to 600 GPa. The films show good adhesion on tungsten carbide but a poorer adhesion on cold working steel. The surface quality, however, was poor at film thicknesses sufficient for practical applications due to the incorporation of many droplets. In order to improve this, an approach was made using an alternative boron carbide target the results of which also are presented. The target was produced by depositing a several ten microns thick boron carbide film on a steel substrate using PLD. The films deposited using this target were found to have much less and much smaller droplets. • Boron carbide coatings with up to 47 GPa hardness can be prepared by Pulsed Laser Deposition. • The substrate temperature during deposition is crucial for the resulting film hardness. • Super-hard boron carbide coatings show a decrease in wear rate by a factor of 11 compared to uncoated WC hard metal. • The use of denser and more homogeneous targets provides less droplet incorporation.

Creep deformation of WC hardmetals with Fe-based binders

Fe is a candidate to replace Co in WC hardmetals, due to its lower cost and toxicity. Here, a WC-FeCr hardmetal was compression tested at 900–1200 °C. Particular attention was paid to the steady-state creep rates and stress-exponents (n) during isostress treatments. Three regimes of n are observed. Two of these were previously reported for WC-Co: power law creep (n ≈ 3) at stresses below ~100 MPa; and grain boundary sliding (n ≈ 1) at higher stresses. A previously unreported regime at very low stresses (<10 MPa), with an exponent of n ≈ 2, is also observed. By combining electron microscopy with X-ray diffraction texture measurements, the low stress regime is attributed to viscous flow of the binder, which is accommodated by diffusional creep in the WC skeleton. The mechanism may be applicable to other hardmetals. Compared to analogous WC-Co materials, WC-FeCr shows improved creep resistance below 1000 °C, which can be explained by its lower self-diffusivity, and a lower solubility for WC than Co. However, at temperatures corresponding to liquid eutectic formation (~1140 °C), its creep resistance becomes inferior. These results indicate FeCr may be a suitable replacement for Co provided the eutectic temperature is not exceeded.

WITHDRAWN: Wear resistance of nanostructured Cr-based WC hardmetals sintered by spark plasma sintering

WITHDRAWN: Wear resistance of nanostructured Cr-based WC hardmetals sintered by spark plasma sintering

Wear resistance of nanostructured Cr-based WC hardmetals sintered by spark plasma sintering

Nanostructured Cr-based WC hardmetals are successfully sintered by spark plasma sintering. The wear behaviour of these Cr-based WC hardmetals with different C contents ranging from 5.57 wt% to 6.91 wt%, is evaluated performing sliding wear tests under two different wear conditions. This work analyses the influence of the C content on the wear performance through the study of the phase formation and WC grain size. The Cr-based hardmetal with 5.57 wt% C content exhibits a lower wear rate than Co-based WC hardmetals tested under similar dry ball-on-plate wear conditions, even considering that these Co-based WC hardmetals have higher WC content (90 wt%) than Cr-based WC hardmetals (83.2 wt%). The combination of a nanosized WC grain and the avoidance of brittle (Cr,Fe)7C3 or soft graphite phases leads to a superior wear performance. Thus, the use of Cr-based binders in the hardmetal industry, alternatively to Co-based binders, is promising in applications in which high wear resistance is needed.

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.

The Effect of Spark Plasma Sintering Thermal Cycle on Behaviour of Fe-Based Hardfacings Reinforced with WC and WC-Based Hardmetal

The effect of composite materials reinforcement by hardmetal with various size of particles (WC 710-1000 μm, recycled WC-Co 710-800 μm and pure WC 10 μm) with Fe-based matrix thermally affected by spark plasma sintering (SPS) method at temperatures 1100°C, 1200°C and 1280°C was studied. The analysis of samples cross-section was performed with SEM and EDS to illustrate distribution of elements after thermal influence. The aim of this research was an investigation of diffusion process behaviour between Fe-based matrix and WC and WC based hardmetal particles at various temperatures. Results help to identify and understand melting and solidification of grains affected at high temperature. Similar temperatures are used for surfacing and repair of worn parts and recycled tungsten carbide particles (as reinforcement) could be used during manual or plasma transferred arc welding.

Corrosion behaviour of WC hardmetals with nickel-based binders

Cobalt is the standard binder in tungsten carbide (WC) hardmetals due to its compatibility with the WC phase, resulting in composites with exceptional hardness and wear resistance. However, their corrosion resistance is not satisfactory in many applications, leading to the early deterioration and failure of tools and equipment.In this work, the corrosion of WC hardmetals with three alternative binders (FeCoNi, NiCrCoMo and NiCrMo) is compared with a benchmark WC-Co composite, using electrochemical techniques such as open circuit potential (OCP) monitoring, polarisation curves and electrochemical impedance spectroscopy (EIS), assisted by scanning electron microscopy (SEM).

Sintering of WC hardmetals with Ni-Co-Cr-Ti-Al multi-component alloys

Cemented carbides with Ni-Co-Cr-Al-W binder phases have been obtained from WC-Ni-Co-Cr3C2-TiAl3 powder mixtures by using standard hardmetal processing routes. Dilatometric and DSC experiments confirm that melting events and shrinkage in these materials are shifted towards higher temperatures as the Al content increases. It is also shown that sintering in Argon is enhanced at very low Al levels (0.63 wt%), which is likely due to the efficient reduction of powder oxides via direct reaction with TiAl3. For higher Al additions, HIP treatments are needed in order to remove the residual porosity left after vacuum sintering. Microstructural characterization confirms that Al additions induce two types of precipitation phenomena in the metallic binder phase of these hardmetals: one consisting of very fine Al-rich oxides (product of the aforementioned powder oxide reduction) and the other comprising even finer γ’ type cuboids (similar to those found in Ni superalloys). As confirmed by TEM-EDS analyses, the binder phase of these cemented carbides is a multicomponent alloy incluiding Co, Ni, Cr, W and increasing amounts of Al as TiAl3 addition increases. About 0.5 wt% Al is lost to the powder oxides reduction process. TiAl3 additions have a significant hardening effect on WC-Ni-Co-Cr3C2 alloys. This is due to its effective role as WC grain growth inhibitor. Although, solution-aging treatments induce a finer and more homogeneous precipitation of the γ’ phase in these materials, their hardness is slighlty reduced if compared to that of as-HIPed samples.

Defect Control of the WC Hardmetal by Mixing Recycled WC Nano Powder and Tungsten Powder

Tungsten metal powder was added to recycled WC nano powder to control the macro and micro defects of WC hardmetal. The macro and micro defects caused by the excess carbon in the recycled WC powder were markedly removed after the addition of tungsten metal powder ranging from 2 to 6 wt%. The density and hardness of the WC hardmetals also increased due to the removal of defects after adding the tungsten metal powder. The density and hardness of WC hardmetals with the addition of W metal powder ranged from 8 to 12 wt% increased linearly as the W metal powder content increased due to the formation of a new (Co- and W-rich WC) composition. The surface morphology of the WC hardmetals was observed via field emission scanning electron microscopy, and a quantitative elemental analysis was conducted via X-ray fluorescence spectrometry and energy dispersive X-ray analysis. The density and hardness of the WC hardmetals were respectively measured using an analytical balance and a Vikers hardness tester. The effect on the defects in the recycled WC hardmetals through the addition of the tungsten metal powder was discussed in detail.

The preparation, preparation mechanism and properties of extra coarse-grained WC–Co hardmetals

Extra coarse-grained hardmetals with WC linear-intercept grain size of more than 6.5 μm were prepared by adding activated sub-micrometer WC powders during ball milling. Metallographic examination shows that the grain boundaries of WC with narrow size distribution are straight. Thermal conductivity, fracture toughness, oxidation resistance and corrosion resistance of these hardmetals were measured also. The results indicate the thermal conductivities and fracture toughness are more than two times of 0.8 μm WC hardmetals’ with the same cobalt content. The excellence of the fracture toughness is thanks to the plastic deformation of extra coarse-grained hardmetals. The oxidation resistance of the hardmetal with addition of nickel is not significantly improved, but adding nickel can improve the corrosion resistance.

A micromechanical constitutive modeling of WC hardmetals using finite-element and uniform field models

For constitutive modeling of WC hardmetals we used a full field finite element simulations of accurately reproduced scanning electron microscope images and a simple uniform field two-phase model. Both models combine isotropic Drucker–Prager elastic–plastic behavior of WC grains and isotropic von Mises elastic–plastic behavior of the binder, and include non-linear hardening. We performed simulations for representative volume elements using the generalized 2.5D formulation and demonstrated a good agreement between the two models in terms of the effective mechanical behavior. Effective elastic properties and coefficients of thermal expansion were obtained. Effective yield stresses evaluated at 0.1% of effective plastic strain were also computed for six different loading paths. Probability and joint probability histograms obtained in FE simulations are presented. We also studied the effect of residual thermal stresses, which appear in WC hardmetals due to cooling from sintering temperatures. Finally, we obtained a realistic yield surface for a three-dimensional microstructure using a uniform field model with spherical inclusions. This surface combines a Drucker–Prager region for moderate pressures and a von Mises region for high pressures, with a sharp transition between these two regions. Four different WC hardmetal grades were considered. In total, nine microstructures were reproduced in finite element models with the binder content ranging from 10% to 19%. A sensitivity study on the binder plastic properties was carried out, thus the obtained results for the yield surface are applicable to real hardmetals with different binder materials and various binder content.

Fundamentals of sintering nanoscaled binderless hardmetals

By reducing the metallic binder in cemented carbides, so-called binderless hardmetals or pure tungsten carbide ceramics can ultimately be achieved. Nanoscaled tungsten carbide has a very high hardness of above 3000HV1 and still offers quite a good fracture toughness of up to 7.5MPa∗m1/2. Since only solid state sintering can be used, sintering temperatures as high as 2000°C or higher are normally needed to completely densify these materials. At these temperatures abnormal grain growth which leads to a decrease in hardness is an often reported problem. However, using pressure assisted techniques such as SPS, ROC or HIP, WC ceramics can be produced at lower temperatures, although productivity and freedom in size and form are limited.The aim of this work was to investigate the drivers for densification, grain growth as well as the microstructure and chemical phases occurring during solid state sintering in a conventional SinterHIP furnace. A WC starting powder with DBET=115nm was milled to nanoscaled size (50 to 60nm), sintered at different temperatures (interrupted sintering experiments) and then analysed by using FESEM, XRD, Rietveld analysis, dilatometry and other methods.It could be shown that three main processes during sintering of slightly under-stoichiometric binderless WC hardmetals occur: first, the reduction of surface oxides at 800°C after which densification starts due to cleaned surfaces and the existence of W in a nascent state; second, the secondary carburisation of WC and W at around 1400°C; and, third, grain growth and pore annealing at around 1600°C when a closed porosity exists. By adding Cr3C2 as grain growth inhibitor the secondary carburisation step was lowered to around 1000°C and a solid solution of (W,Cr)2 instead of pure W2C was formed.

Grain growth during sintering of tungsten carbide ceramics

Grain growth and abnormal grain growth in tungsten carbide cobalt composites (cemented carbides, hardmetals) are usually discussed with respect to liquid phase sintering (Ostwald ripening). Densification and grain growth during solid state sintering are not as thoroughly studied but do play an important role in sintering hardmetals and, particularly tungsten carbide ceramics (binderless hardmetals). In this work the influences of sintering temperature, carbon content, additions of grain growth inhibitors, defects and dislocations (microstrain) introduced by milling on the densification and microstructure of WC ceramics were studied including density, micro structural, thermal and X-ray analysis. Microstrain promotes densification and results in lowering the sintering temperature, whereas free carbon seems to hinder densification at low temperatures and to promote it slightly at higher temperatures. Depending on sintering regime, free carbon and microstrain may drastically boost abnormal grain growth. By adding grain growth inhibitors, densification is shifted to higher temperatures. However, the addition prevents abnormal grain growth regardless of C-content and microstrain. Like in hardmetals grain growth inhibitors also inhibit normal grain growth. The findings are relevant for sintering of WC ceramics and hardmetals alike.

Synthesis mechanism and sintering behavior of tungsten carbide powder produced by a novel solid state reaction of W2N

Nanosized WC powder was synthesized by solid state reaction between tungsten nitride (W2N) and carbon black. The reaction path and the phase evolution during the synthesis were studied. It was found that W2N firstly decomposed to tungsten metal in the middle stage of synthesis, and then it reacted with carbon black to form W2C and WC in sequence. Using this WC powder as the starting material, binderless WC hardmetals were prepared by hot pressing, and the microstructure and mechanical properties of the material were investigated.

WC-8Co-2Al (wt%) Cemented Carbides Prepared by Mechanical Milling and Spark Plasma Sintering

Ultrafine grained WC cemented carbides were manufactured successfully by reaction sintering with Co-Al intermetallic compounds as binder from mixed powders of a nominal composition of WC-8Co-2Al (wt%). The purpose of adding aluminum to the composition is to promote the mechanical properties of hardmetals by the in situ formation of Co-Al intermetallic phase. To manufacture bulk ultrafine WC hardmetals, ultrafine and even nanocrystalline WC powders were prepared as precursors from the mixed commercial powders by mechanical milling. Experimental results show that the transverse rupture strength and hardness of sintered hardmetal reached the maxima, being 1747.8 MPa and 90.2 HRA respectively, when the milling time expands to 55 h. It is affirmed that mechanical milling weakens the thermostability of WC-8Co-2Al powders, and facilitates the formation of Co-Al intermetallic compounds. However, overlong milling results in decreasing of properties and densities of sintered hardmetals, because the powder particles are severely refined and markedly agglomerated.

Strength and failure of TiC based cermets

The strength and failure of TiC based cermets (designed for metalforming) under monotonously increasing and cyclic loading conditions were investigated. Comparative trials with an ordinary WC hardmetal (widely used in metalforming) complemented with XRD studies were performed. It is shown that the failure of the carbide composite is preceded by local plastic strain taking place in its both phases in the ductile binder and in the brittle carbide phase. Under cyclic loading conditions the plasticity of carbide composites decreases. The intensity in decrease depends on alloy composition.

LIQUID PHASE SINTERING OF SUB MICRON WC COMPOSITES CONTAINING NEW BINDERS BASED ON BORON DOPED ALUMINIDES

Densification behavior of sub micron tungsten carbide composites based on boron doped Fe 60 Al 40 and Ni 3 Al binders were investigated. Soft cobalt binder phase in WC hard metals was replaced by harder and tougher Fe 60 Al 40 and Ni 3 Al intermetallics with boron level up to 0.1wt%. Intermetallic binders were prepared in ultrafine form under controlled atmosphere using ring grinding, blended with sub micron WC powder and then uniaxilly hot pressed under 20 MPa pressure in argon atmosphere for 4 minutes duration at temperatures of 1450 °C, 1500 °C and 1550 °C. It was found that the density of composites increased significantly and WC grain size increased slightly with increase in sintering temperature. The composites exhibited highest density (about 99.5 %TD) and minimal grain growth in WC at sintering temperature of 1500 °C. The presence of boron was found to facilitate densification during liquid phase sintering and boron also resulted in achieving uniform microstructure with continuous binder phase, nearly full density and high faceted features in the structure.

Fabrication of WC-6Co-1.5Al (wt%) Hardmetal by High Energy Milling and Electrical Current Sintering

Ultrafine grain WC hardmetals were manufactured successfully from mixed powders of nominal composition of WC-6Co-1.5Al(wt%). To manufacture bulk ultrafine grain WC alloys, nanocrystalline WC powders as precursor were prepared for sintering from the mixed commercially available powders by high energy milling of 15 h. Then the electrical current sintering process of the milled powders was further studied. Experimental results showed that the density, hardness and transverse rupture strength of the sintered alloys reached 14.224 g/cm3, HRA 94 and 1660 MPa, respectively, by a appropriate coupling of pulse- and subsequent constant-current sintering and with the total sintering time of 6 min. It seemed difficult to obtain desirable sintered hardmetal by a single type of electric current sintering. However, the application of intensive pulse electric current was favorable to sintering, although it was difficult to achieve good WC alloys only by the pulse electric current sintering. A higher density WC alloy could be prepared by enlarging the electric current when only constant electric current was applied to sinter the milled powders. Unfortunately, the WC grains substantially coarsened concurrently so that the mechanical properties of the as sintered alloy decreased obviously.