Fine-grained WC Research Articles

Tensile and Fatigue Properties of WC-20 wt%Co Cemented Carbides under Microstructural Variations

WC-Co cemented carbide has excellent mechanical properties and is widely used in many industrial applications including cold forging dies and cutting tools. The tensile and fatigue properties of WC-20wt%Co cemented carbides with microstructural variations were investigated. Microstructure parameters such as Co binder content, WC particle size, binder mean free path, and contiguity were obtained by linear intercept method using BSE microstructure images. Standard specimens of WC-20wt%Co cemented carbides were prepared for tensile and fatigue testing. Uniaxial tensile stress-strain curves and tensile-compression fatigue S-N curves were obtained. The 22Co-Cr alloy with higher Co content showed the largest binder mean free path and the lowest continuity. The 20Co-d<sub>wc</sub> alloy with fine WC grains of submicron size showed the lowest binder mean free path due to fine WC grain distribution. The 20Co-d<sub>wc</sub> alloy with fine WC grains showed the highest tensile strength and fatigue strength, compared to other alloys. The 22Co-Cr alloy with a higher FCC Co phase content, which has excellent plastic deformability, showed higher fatigue properties. The fatigue life of the 22Co-Cr alloy increased with increasing compressive mean stress level. Based on the axial tensile and fatigue properties, a reasonable fatigue life prediction of WC-20wt%Co cemented carbide dies for cold forging can be estimated.

Preparation of low binder WC-Co-Ni cemented carbides with fine WC grains and homogeneous distribution of Co/Ni

Fine grained WC-Co-Ni cemented carbides with a low binder content have been successful manufactured via the powder metallurgy method. The WC-Co-Ni composite powder with homogeneous distributions of Co and Ni was synthesized by chemical co-precipitation with the raw materials of WC powder, oxalic acid and chlorides of Co and Ni. In this study, the effects of cobalt and nickel contents on the properties of cemented carbides were investigated systematically. The experimental results demonstrated that the alloy prepared by vacuum sintering had uniform grain size distribution and no abnormal growth of WC grains. When the cobalt content was 4 wt% and 5 wt%, the obtained composites almost completely consolidated with excellent hardness and fracture toughness. Among them, for the sample WC-4 wt%Co, the relative density could reach 99.37%, and the Vickers hardness and fracture toughness were 2010 HV30 and 8.55 MPa·m1/2, respectively. When Ni was added to replace part of Co, the relative density of WC-Co-Ni decreased and the fracture toughness was improved without basically sacrificing the hardness. For the sample WC-2 wt%Co-2 wt%Ni, the hardness decreased to 1921 HV30, while the fracture toughness increased to 8.97 MPa·m1/2.

Refined WC grain size and improved mechanical properties in a hardmetal WC-8Co processed via short-time semi-solid hot pressing

To minimize WC grain growth and avoid WC-WC non-metallurgical contacts in WC-Co hardmetals, we modified conventional solid-state hot pressing into short-time semi-solid hot pressing (STSSHP) for consolidation of WC-Co powders in the present study. STSSHP can produce fully dense WC-Co hardmetals free of WC-WC non-metallurgical contacts at semi-solid temperatures lower than the ideal liquid-phase sintering temperatures within several to several ten seconds, thus minimizing WC grain growth. Herein WC-8 wt%Co processed by STSSHP at 1340 ℃ (the WC-Co eutectic temperature) for only ~15 s achieved a relative density of 99.92% without WC-WC non-metallurgical contacts. Only slight WC grain growth occurred during STSSHP relative to grain sizes in powders. The STSSHP-ed WC-8 wt%Co exhibited much smaller WC grains and thus higher hardness, fracture toughness and bend strength than those in its liquid-phase sintered counterparts. STSSHP provides a novel avenue to retain fine WC grains and hence to improve both hardness and ductility of WC-Co hardmetals.

Mechanical and microstructural characterization of WC-Co consolidated by binder jetting additive manufacturing

A study was carried out to investigate the influence of the initial powder features on the microstructure, the phase composition, and on the mechanical properties of WC-12%Co samples produced by binder jetting and densified by sintering in vacuum at 1400 °C and by sinter-HIPing. The initial powder consisted of a mixture of fine-grained and coarse-grained WC-based particles, and W2C-based ones. The density of the printed samples was 97.4% and 99.3% after sintering and sinter-HIPing, respectively. Layer-oriented porosity was observed only in sintered samples. The microstructure of the samples consisted of a mixture of fine and coarse WC grains, which can be the result of the coalescence of grains from anomalous coarse grains in the powder particles. Vickers hardness and transverse rupture strength of sinter-HIPed samples are 1205 HV and 2257 MPa, respectively, which is coherent with the microstructural analysis and close to coarse-grained commercial products.

Effect of carbon content on the properties of inhomogeneous cemented carbides with fine-grained structures produced via one-step transformation

Inhomogeneous cemented carbides with fine-grained structure produced via one-step transformation were prepared using coarse WC, WO3, C and Co as raw materials. The effect of carbon content on the properties of inhomogeneous cemented carbides was studied. The results indicated that the peak intensity of Co (1 1 1) had a positive relationship with carbon content. The coarse WC grains were irregularly-shaped with some growth steps and the fine grains were near-spherical shape in the alloy with low carbon content. Upon increasing the carbon content, the coarse WC grains tended to be planar and the number of growth steps decreased gradually. The number of fine WC grains increased at higher carbon contents in the WC-Co3W3C-Co alloys, resulting in a decrease in the average WC grain size. Then, the number of fine WC grains decreased and the coarse WC grains grew upon increasing the carbon content in the alloys without the Co3W3C phase, which increased the average WC grain size. The mechanical property variation of the alloys was derived from the phase composition, morphologies and the size of WC grains. Thus, the alloy with 7.24 wt% carbon displayed the maximum hardness and transverse rupture strength due to its high number of prismatic WC grains and lack of free carbon.

Microhomogeneous WC-TiC-Co composite powders with enhanced sinterability via a two-step carburization method

WC-TiC-Co composite powders with fine WC grains and homogeneously distributed Co and inhibitors were prepared via a two-step carburization method. η phase powders were synthesized via the carburization of the crushed mixture of W, Co, C and TiC; WC-TiC-Co composite powders were then fabricated by the carburization of the η phase powders. The effects of TiC on the carburization process and the microstructure of composite powders, sinter and mechanical properties of WC-TiC-Co cemented carbides were investigated. Co2W4C type η phase powders were confirmed via the first carburization at 1150 °C for 2 h, which were further carburized into WC-TiC-Co composite powders at 1100 °C for 1 h under the flowing hydrogen. During the carburization of η phase powders, TiC accelerated the nucleation of WC through a heterogeneous nucleation mechanism, and effectively inhibited the intrinsic grain growth, which both led to fine and non-equilibrium WC grains with a near-sphere shape. Dense WC-(Ti,W)C-Co was confirmed via low-temperature vacuum sintering of the as-synthesized composite powders, owing to the homogeneously dispersed Co and (Ti, W)C grains. This work showed a way to fabricate micro-homogeneous composite powders of cemented carbides via carburization of the η phase powders.

Spark plasma sintering of fine-grained WC hard alloys with ultra-low cobalt content

The results of investigations of spark plasma sintering (SPS) kinetics of tungsten carbide nanopowders with different Co content (0.3, 0.6, and 1.0 wt%) and graphite (0.3 and 0.5 wt%) are described. The α-WC nanopowders were obtained by direct current (DC) arc plasma synthesis followed by annealing in hydrogen. Ultrathin Co layers were deposited onto the nanoparticles by a chemical-metallurgical method from a salt solution. The excess carbon was introduced by mixing the WC-Co powders with graphite that resulted in the increase of the oxygen concentration. The carbonization of the tungsten carbide hard alloy specimens was demonstrated to take place in SPS. It resulted in different phase compositions, along with hardness within surface and central layers of sintered ceramic specimens. The effects of the initial particle size, of the Co concentration, and of the graphite one on the stages of the SPS of the ultra-low Co hard alloys were studied. The specimens with uniform fine-grained structure, increased density, and improved mechanical characteristics were obtained. The hardness Hv for WC-0.6%Co-0.3%C hard alloy with averaged grain size 1.0–1.5 μm was 20.2–20.5 GPa at the minimum fracture toughness coefficient KIC = 9.2–10.4 MPa m1/2.

Microstructure and mechanical property of high velocity oxy-fuel sprayed WC-Cr3C2-Ni coatings

WC-Cr3C2-12Ni coatings, with Cr3C2 weight content of 10% and 30%, were fabricated on steel substrate by high velocity oxy-fuel (HVOF) flame spraying technique, and microstructure, phase composition and mechanical properties of WC-Cr3C2-12Ni coatings prepared by different oxygen flows were investigated. The results reveal that phase compositions of WC-Cr3C2-12Ni coatings were mainly influenced by the Cr3C2 content rather than the oxygen flow. WC-Cr3C2-12Ni coatings demonstrated low decarburization and decomposition of WC due to coarse WC gains and high in-flight particle velocities during HVOF. WC-Cr3C2-12Ni coatings exhibited low porosities and typical microstructures, where a mixture of fine and coarse WC grains was uniformly embedded in the Ni/Cr3C2 matrix. The microhardness, indentation fracture toughness and elastic modulus of WC-Cr3C2-12Ni coatings were improved by coarse WC grains, Cr3C2 addition and dense microstructure, which resulted in high wear resistances of WC-Cr3C2-12Ni coatings. The abrasive wear features with groove, peel-off and exposed/fractured WC grains were observed in WC-Cr3C2-12Ni coatings after dry-sliding wear. The relationship between microstructure and mechanical property of WC-Cr3C2-12Ni coatings was discussed from the viewpoints of powder properties and process parameters.

Kinetics of Spark Plasma Sintering of WC–10% Co Ultrafine-Grained Hard Alloy

The effect of the carbon content on the kinetics of spark plasma sintering of nanopowder and submicron powder of WC–10% Co compositions was studied. Free carbon in the graphite form was introduced into nanopowders and submicron powders by mixing. The activation energy of solid-phase sintering at isothermal holding and continuous heating was determined. It is shown that increase in the carbon content leads to decrease in the volume fraction of the η phase and shift of the shrinkage curve into the region of low temperatures. It is established that an increase in the carbon content does not have a significant effect on the sintering activation energy of nanopowders and submicron powders in the region of “average” heating temperatures, the value of which is close to the activation energy of grain boundary diffusion in cobalt. It is shown that an increase in the carbon content leads to a significant decrease in the activation energy of consolidation of WC–Co powders in the region of “high” sintering temperatures owing to a decrease in the concentration of tungsten atoms in the γ phase based on cobalt. The sintering kinetics of fine-grained WC–Co alloys is limited by the rate of Coble diffusion creep of cobalt.

Effects of fine WC particle size on the microstructure and mechanical properties of WC-8Co cemented carbides with dual-scale and dual-morphology WC grains

Dual-scale and dual-morphology WC grained WC-8Co cemented carbides comprising triangular or hexagonal fine WC grains and plate-like coarse WC grains were synthesized by vacuum sintering using Co, flaky graphite, WC, and coarse W as the starting materials. The effects of fine WC particle sizes on microstructure, relative densities, and mechanical properties of the dual-scale and dual-morphology WC grained cemented carbides were investigated. The results revealed that the growth of plate-like coarse WC grains was further promoted with the decrease in the particle size of the added fine WC; hence, their aspect ratio increased. In addition, added fine WC led to the separation of plate-like coarse WC grains so as to break their oriented arrangement and prevent their face contact; hence, plate-like coarse WC grains were completely covered by the Co binder phase. Moreover, the addition of smaller particle size of fine WC contributed to more uniform Co binder phase. When 0.4-μm WC powders was added, the aspect ratio of plate-like coarse WC grains was greater than that of plate-like WC grained cemented carbides without the addition of fine WC. The dual-scale and dual-morphology WC grained cemented carbides by adding 0.4-μm fine WC exhibited good comprehensive mechanical properties, with a transverse rupture strength of 3645 MPa, a Rockwell hardness of 91.5 HRA, and a fracture toughness of 12.3 MPa∙m1/2.

A New Approach of Wear Mechanism Map in Turning Inconel718 with PVD-Coated Inserts Using Advanced Techniques

In this paper, the relationship between the cutting conditions and the wear mechanisms in turning Inconel718 from both modeling and experiment points of view has been studied. The tool chosen consists of a hard fine-grained WC with 6% Co with TiAlN layer. As a result, the recommended machining conditions with minimal wear would be the selection of a tool with a radius of 1.6 mm and the cutting velocity in the range of 45 to 55 m/min. The optimal variables obtained from artificial neural networks and genetic algorithm are found to be in good agreement with the results of laboratory findings on the wear mechanism map. Also, the results showed that at lower cutting velocities and feed rates, the TiAlN layer acts to prevent the transfer of elements between the tool and workpiece (mild and transient wear zones), causing the turning forces to stabilize over time. However, with an increase in the cutting velocity and feed rate (severe zone), the TiAlN layer breaks off the tool surface resulting in a considerable increase in the friction coefficient, cutting forces, and the adhesive wear. The main reason for this phenomenon is the transfer of elements such as nickel, chrome, and iron to the flank face.

Microstructure and mechanical properties of dual-grain structured WC-Co cemented carbides

In this study, the dual-grain structured WC-Co cemented carbides were fabricated by in situ reaction using carbon, fine W powders, Co powders, and coarse WC powders as starting materials. The phase formation, microstructure, and mechanical properties of the prepared samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that the cemented carbides with both coarse and fine WC grains which were evenly distributed within the continuous Co binder phase could be obtained. A WC1-x phase of 5–6 atomic layers was observed between fine WC grain and Co binder phase. The hardness of the present cemented carbides was nearly similar to the ones prepared with the mixture of coarse WC, fine WC and cobalt powders. The fracture toughness of the present cemented carbides was greatly improved owing to crack deflection and transgranular fracture. The present dual-grain structured cemented carbides presented good comprehensive mechanical properties with a Vickers hardness of 1360 ± 37 HV30, a transverse rupture strength of 3304 ± 105 MPa, and a fracture toughness of 14.78 ± 0.45 MPa m1/2, respectively.

Features of Influence of Sintering Conditions of Fine-Grained WC—10Co Cemented Carbide on Its Structure, Physical-Mechanical and Operational Characteristics

The results of a study of the influence of the environment during sintering of fine-grained WC—10Co cemented carbide on its structural characteristics and physico-mechanical properties, as well as the operational durability and wear mechanisms of the cutting plates of this alloy in impact cyclic turning process of 40Kh steel parts are presented. It is shown that the sintering of a cemented carbide in vacuum as compared with sintering in a methane-hydrogen environment reduces the residual microporosity of its structure, improves the physico-mechanical properties and operational durability of the cutting plates made from it. A new phenomenon was found to be the formation of continuous chip in the conditions of discontinuous turning of 40Kh steel parts and the welding it to a working part of a cutting plate from WC-10Co cemented carbide sintered in vacuum, and the absence of such a phenomenon for an alloy sintered in a methane-hydrogen environment.

Effects of metal binder content and carbide grain size on the microstructure and properties of SPS manufactured WC–Fe composites

In the research, an investigation has been carried out on the microstructure and mechanical properties of WC–Fe composites, with various Fe content and either fine- or coarse-grained WC. The WC/Fe composite powders were processed by powder metallurgy method and consolidated to high density by spark plasma sintering (SPS). According to the thermodynamic calculations, the sintering temperature was selected to be 1050 °C. The experimental results indicated that there existed two main phases of WC, α-Fe and one minor Fe3W3C phase in the WC–Fe composites, which was consistent with the thermodynamic calculations. Besides, the relative densities of the composites increased with the Fe binder content, while exhibited different trends for the fine- and coarse-grained composites. Among the WC–Fe hard alloys, the composite with fine-grained WC and 30 wt % Fe binder possessed the highest hardness of 60.5 HRC. The maximum bending strength of 1851 MPa was attained for the composite with coarse-grained WC and 70 wt % Fe binder. The fracture mechanism of the composites was a combination of transgranular failure, intergranular failure, and Fe binder plastic tearing.

Effects of Post-spray Heat Treatment on Hardness and Wear Properties of Ti-WC High-Pressure Cold Spray Coatings

In this research, the effects of post-spray heat treatment at 550 and 650 °C for 1 h on a cermet Ti-WC nanostructured coating deposited onto AISI 304 stainless steel substrates by high-pressure cold spray was observed. A metallic Ti interlayer was further used to compensate for stresses resulting from subsequent heat treatment on the developed coating. Microstructural analysis of the as-deposited coating by scanning electron microscopy (SEM) showed mostly fine WC grain (below 1 µm) present in the coating with a few larger 4 µm grains dispersed homogeneously throughout. X-ray diffraction analysis of the as-sprayed coating showed no noticeable evidence of WC decarburization. Heat treatment of the coating caused porosity to decrease from above 1.7% to below 0.5%, traced by SEM image analysis. Post-spray heat treatment promotes the formation of new carbide phases caused by the reactions between the Ti binder and WC grains, resulting in significant increases to Vickers microhardness. Evidence of an SHS reaction that produces TiC with heat treatment is confirmed with SEM image analysis as well as (S)TEM area mapping techniques, further supported by selected area electron diffraction analysis. Three-body sliding wear/abrasion tests have shown that wear resistance of Ti-WC cold spray coatings increases with heat treatment as well. In all, the effect of post-spray heat treatment behavior of nanostructured Ti-WC coating will be compared with that of as-sprayed behavior and WC-Co cold spray coatings.

Densification Dynamics of Fine-Grained WC+25 wt.% Co Cermet During Low-Temperature Impact Sintering in Vacuum

The densification of fine-grained tungsten carbide-based cermet with 25 wt.% cobalt binder during impact sintering at 1150, 1200, 1250, and 1300°C with the initial impact velocity of 5.8 and 6.2 m/s, with the initial solid-phase state of the binder, is investigated. Based on the obtained experimental data and the calculated elastic properties of the samples and the impact machine, the simulation of densification dynamics using third-order dynamic system is carried out by hit-and-miss method and the value of the shear viscosity of cermet matrix determining the energy dissipation in the system and the irreversibility of the material densification are obtained. In addition, the data on the phase trajectory of the dynamic system motion, the duration of impact loading, the time variations of the force, compression, velocity, and acceleration of the system, densification work, and the mechanical-thermal effect resulting from the energy dissipation and causing a significant increase in the temperature of the porous cermet samples are obtained. At the starting temperature of the samples close to that of eutectic formation and low starting porosity of the samples, this temperature increase can cause the liquid phase squeezing from the sample volume into the porous graphite shell that protects the samples against adhesion to the metal die. The estimated activation energy of the viscous flow of the matrix forming the porous cermet is 1.1 eV or 103 kJ/mol.

Initial surface failure and wear of cemented carbides in sliding contact with different rock types

The initial wear, deformation and degradation of cemented carbide in contact with different rock types are studied using a crossed cylinder sliding test. The sliding distance is limited to centimetres at a time, interrupted by successive SEM analysis. This allows for careful studies of the gradually changing microstructure of the cemented carbide during the test. Five different rock types are included; granite, metal sulphide ore, mica schist, quartzite and marble. All rock types are very different in microstructure, composition and properties. The cemented carbide grade used for the evaluation contains 6 wt% Co and fine (~ 1 µm) WC grains, a grade commonly used in rock drilling. The results show that the cemented carbide microstructure becomes altered already during the very first contact with rock. The initial wear rate and wear character is highly influenced by the rock type. The initial wear of the cemented carbide is highest against quartzite and lowest against marble.

Microstructure and mechanical properties of WC–Co-based cemented carbide with bimodal WC grain size distribution

In order to improve the wear resistance of coarse-grained WC–Co cemented carbides, the fine WC powder were used to reinforce the metallic binder. These WC–Co-based cemented carbides having bimodal WC grain size distributions were synthesized by liquid phase sintering. For comparison, the cemented carbides having unimodal WC grain size distributions were synthesized. The microstructure, hardness, fracture toughness and wear resistance of these cemented carbides were investigated. The results show that adding fine WC powder is an effective method to improve the wear resistance of coarse-grained WC–Co cemented carbides. The WC size, mean free path and fracture toughness decrease with the addition of fine WC powder, while the hardness exhibits an opposite trend. The impact-wear coefficient of bimodal distribution cemented carbides is noticeably lower than that of the unimodal one with the same hardness, which means that the cemented carbides with bimodal grain structure have better combination of hardness and impact-abrasive wear resistance. The impact-abrasive wear mechanism of the bimodal cemented carbides is that the fine WC grains prevent abrasive wear and the coarse WC grains prevent impact wear.

A novel route for the synthesis of ultrafine WC-15 wt %Co cemented carbides

In this study, a solution chemical route for the synthesis of ultrafine WC-15 wt %Co cemented carbides was proposed with WC-6Co aggregates as raw materials. The chemical Co was uniformly distributed around the WC-6Co aggregates and the Co layers in the raw materials had been well preserved. During sintering, the chemical Co could diffuse along the Co layers and thus avoid the aggregation and growth of WC grains. Additionally, the homogeneous distribution of Co phase and the existent of Co layers promoted the WC reprecipitations on the same crystal during the solution/re-precipitation process. The fine WC grains and the homogeneous of WC and Co phases resulted in the high hardness and transverse rupture strength.

Interfaces between Model Co-W-C Alloys with Various Carbon Contents and Tungsten Carbide.

Interfaces between alloys simulating binders in WC-Co cemented carbides and tungsten carbide were examined on the micro-, nano-, and atomic-scale. The precipitation of fine WC grains and η-phase occurs at the interface of the alloy with the low carbon content. The precipitation of such grains almost does not occur in the alloy with the medium-low carbon content and does not take place in the alloy with the high carbon content. The formation of Co nanoparticles in the binder alloy with the medium-low carbon content was established. Interfaces in the alloy with the medium-low carbon content characterized by complete wetting with respect to WC and with the high carbon content characterized by incomplete wetting were examined at an atomic scale. The absence of any additional phases or carbon segregations at both of the interfaces was established. Thus, the phenomenon of incomplete wetting of WC by liquid binders with high carbon contents is presumably related to special features of the Co-based binder alloys oversaturated with carbon at sintering temperatures.