Microstructure Of WC Research Articles

Role of Co Content on Densification and Microstructure of WC–Co Cemented Carbides Prepared by Selective Laser Melting

WC–Co cemented carbides, well-known as the conventional tooling materials, have not been successfully produced by one step additive manufacturing processes such as selective laser melting (SLM) yet. The microstructure evolution as well as WC grain growth behavior has rarely been investigated in detail during SLM process. In this study, the WC–Co cemented carbides with different Co contents (12–32 wt%) were prepared by optimized SLM processes for comparative investigation of densification behavior, microstructure characterization and mechanical property. The increase in Co content in feedstock carbide granules can improve the densification behavior during SLM process. The SLM processed WC-12Co shows larger average WC grain size and higher percentage of coarser WC grains as compared with both WC-20Co and WC-32Co. The microstructure characterization, combined with finite element simulation, shows the WC grain growth mechanisms include agglomeration and dissolution-deposition of WC during SLM process and agglomeration of WC is an important mechanism especially for WC–Co cemented carbides with Co content as low as 12 wt%. The comparison between horizontal (perpendicular to the SLM laser beam) and vertical (parallel to the SLM laser beam) cross sections of carbides shows that SLM process introduces a certain degree of microstructure and mechanical behavior anisotropy for WC-12Co, WC-20Co, and WC-32Co.

Microstructure of WC–Co–VN carbide catrix for drilling tools diamond-containing materials

The results of studies aimed at improving the structure and performance characteristics of carbide matrices WC-Co samples, formed by cold pressing and thereafter hot pressing, for drilling tools diamond-containing materials. It is shown that the introduction of vanadium nitride in an amount of 3% into the composition of the starting materials 94WC-6Co provides an increase in the hardness of the sample from 22.8 to 34.2 GPa, the strength limit in compression from 4800 to 5340 MPa and bending from 2200 to 2280 MPa, as well as reduction of wear intensity from 1710×10-6 to 5200×10-6 g/m. The revealed effect of the formation of a fine-grained structure of a carbide matrix WC-Co-VN with increased mechanical and operational characteristics indicates the need for their use in the development of effective tools for drilling oil and gas wells.

An observation of the binder microstructure in WC-(Co+Ru) cemented carbides using transmission Kikuchi diffraction

The microstructure of WC-(Co+Ru) cemented carbides was studied using transmission Kikuchi diffraction (TKD). We report that the CoFCC grains contain Σ3-annealing twins which undergo an incomplete martensitic transformation to form CoHCP laths on cooling below TMs. The parent CoFCC and product CoHCP phases are related via the Shoji-Nishiyama orientation relationship. Four variants of the CoHCP plates exhibit a special misorientation of 70.5 ± 0.2°〈112¯0〉. A higher transformed fraction was observed in the sample possessing the lower binder content and finer WC grains. We suggest that this is due to the availability of more WC-binder interfaces, which facilitate CoHCP plate nucleation.

Microstructure of WC–Co–VN hard-alloy matrix for diamond-bearing details of drilling tools

The paper presents the research results on the structure and performance characteristics improvement of WC-Co hard-alloy matrix samples formed via the methods of cold and further hot moulding for the diamond-bearing details of drilling tools. It is shown that applying vanadium-nitride in 3 % in the composition of 94WC-6Co source materials provides the increase of sample hardness from 22.8 to 34.2 hPa, the compressive and bending strength from 4800 to 5340 MPa and from 2200 to 2280 MPa correspondingly, as well as the decrease of wear intensity from 1710×10-6 to 5200×10-6 g/m. The effect of formation of fine-grained structure for WC−CoNbN hard-alloy matrix with improved mechanical and performance characteristics revealed in the study points to the necessity of their usage in the development of efficient tools for drilling of oil and gas wells.

Impact of laser irradiation on microstructure and phase development of tungsten carbide - cobalt

Previous studies on Laser Powder Bed Fusion of tungsten carbide-cobalt (WC-Co) revealed several defects in the laser molten microstructures. To analyze the defect formation mechanisms and the fundamental impact of laser energy input on phase development and microstructure of WC- Co, single-pulse and single-track analogy experiments are conducted. Conventional WC-Co is used as substrate. Variations in cobalt content and processing conditions are investigated. It is found that a single laser exposure can be enough to induce decomposition of microstructure, WC grain growth, formation of non-equilibrium phases and thermal cracking. Based on the results, defect formation mechanisms and countermeasures are discussed.

Phase composition and microstructure of WC–Co alloys obtained by selective laser melting

Phase composition and microstructure of initial WC, BK8 (powder alloy 92 wt.% WC–8 wt.% Co), Co powders, ball-milled powders with four different compositions (1) 25 wt.% WC–75 wt.% Co, (2) 30 wt.% BK8–70 wt.% Co, (3) 50 wt.% WC–50 wt.% Co, (4) 94 wt.% WC–6 wt.% Co, and bulk alloys obtained by selective laser melting (SLM) from as-milled powders in as-melted state and after heat treatment were investigated by scanning electron microscopy and X-ray diffraction analysis. Initial and ball-milled powders consist of WC, hexagonal α-Co and face-centered cubic β-Co. The SLM leads to the formation of major new phases W3Co3C, W4Co2C and face-centered cubic β-Co-based solid solution. During the heat treatment, there occurs partial decomposition of the face-centered cubic β-Co-based solid solution with the formation of W2C and hexagonal α-Co solid solution. The microstructure of obtained bulk samples, in general, corresponds to the observed phase composition.

Rapid consolidation of nanostructured WC-FeAl hard composites by high-frequency induction heating and its mechanical properties

In the case of cemented WC, Co or Ni is added as a binder for the formation of composite structures. However, the toxic behavior of both Co and Ni and their cancerogenic classification in Europe and the US, the high cost of Co or Ni, the low hardness and the low corrosion resistance of the WC-Ni and WC-Co cermets have generated interest in recent years for alternative binder phases. In this study, FeAl was used as a novel binder and consolidated by the high-frequency induction sintering (HFIS) method. The advantage of this process is not only rapid densification to near theoretical density but also the prohibition of grain growth in nano-structured materials. Dense WC-FeAl with a relative density of up to 98% was obtained within two minutes by HFIS under a pressure of 80MPa. The effect of FeAl addition on mechanical properties, consolidation and microstructure of WC was evaluated.

Effects of TaC on microstructure and mechanical properties of coarse grained WC–9Co cemented carbides

Coarse grained WC–9Co cemented carbides with 0–1.0% TaC (mass fraction) were fabricated by HIP-sintering and gas quenching. The effects of TaC on the microstructures and mechanical properties were investigated using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), X-ray diffractometry (XRD) and mechanical properties tests. The results show that the maximum values of hardness and strength are HV 1124 and 2466 MPa respectively when 0.4% TaC is added. When the content of TaC is more than 0.6%, the grain size of WC is no longer affected by the amount of TaC, and (W,Ta)C occurs as well. Moreover, the strength and fracture toughness increase and the (Ta+W) content decreases with the increase of TaC content. The dependence of (Ta+W) content on the mechanical properties indicates that (Ta+W) content in Co should be decreased as low as possible to improve the mechanical properties of coarse grained WC–TaC–9Co cemented carbides with the microstructure of WC+γ two phase regions.

Microstructure of WC–Co hard alloy surface after laser treatment

The microstructure of WC/Co hard alloys after surface laser treatment is studied. It is established that during the laser treatment of the hard alloys, the chemical interaction between the components of the WC/Co powder mixture occurs in the samples’ surface according to the reaction 3Co+3WC→(Co; W)3C+2C. High cooling rate during the laser treatment allows preserving the products of this reaction, i.e. graphite inclusions and complex carbide (Co; W)3C to room temperature.

Microstructure level modelling for properties prediction of WC–Co cemented carbides

A random distribution micromodel was developed in order to predict the properties of WC–Co cemented carbides. MATLAB and VC++ were hybrid programmed to extract the necessary information for modelling the microstructure from scanning electron microscopy images. By analysing the distribution regularity of the microstructural parameters, the randomness characterisation of the microstructure of WC–Co cemented carbides was accomplished using probability density function. The parameterised model based on ‘random method’ was developed, in which the random distribution characteristics of microstructural topology parameters, such as average grain diameter, major axis, minor axis, centroid and grain orientation, were considered, with Co volume fraction freely controlled. The representative volume element (RVE) size was determined using the moving window method, with average grain diameter distribution as the evaluation criterion. It was found that the RVE should contain ∼120 grains. The RVE was directly imported in the finite element software ABAQUS, and the finite element simulation of microindentation tests and uniaxial tensile tests were accomplished to calculate the hardness and elastic properties of cemented carbides. The predicted results are in good agreement with the experimental dates, which reveals that the microstructure level finite element model can effectively predict the properties of WC–Co cemented carbides.

Analysis of the electrochemical behaviors of WC–Co alloy for micro ECM

Micro electrochemical machining (ECM) of tungsten carbide with cobalt binder (WC–Co) was studied using ultrashort pulses. In ECM, the machining characteristics were investigated according to machining conditions such as electrolyte, workpiece potential, and applied voltage pulse. Using a mixture of sulfuric acid and nitric acid, microstructures with a sharp edge and good surface quality were machined on tungsten carbide alloy. The potentials of workpiece electrode and tool electrode were determined by considering the machining rate, machining stability, and surface quality of products. With the negative potential of the workpiece electrode, oxide formation was successfully prevented and shape with good surface quality in the range from Ra 0.069μm to 0.075μm were obtained by electrochemical machining. Moreover, the performance of ECM, which includes machining gap, tapering, surface roughness, and machining time, without tool wear was compared with that of electrical discharge machining (EDM). Microstructures of WC–Co with a sharp edge and good surface quality were obtained by electrochemical milling and electrochemical drilling. Micro electrochemical turning was also introduced to fabricate micro shafts.

Microsliding wear behaviour of HVOF sprayed conventional and nanostructured WC–12Co coatings at elevated temperatures

Conventional and nanostructured WC–12Co coatings were deposited on 1Cr18Ni9Ti substrate using high velocity oxy-fuel spraying. The surface roughness and microhardness of as sprayed coatings were measured, while the phase compositions and microstructures of WC–12Co feedstock powders and corresponding coatings were respectively analysed by means of X-ray diffraction and scanning electron microscopy. Moreover, the microsliding wear behaviour of both coatings sliding against Si3N4 ball from room temperature to 500°C was evaluated under unlubricated condition with an oscillating SRV tribotester. Results reveal that nanostructured WC–12Co coating not only had better wear-resistance than the conventional WC–12Co coating but also led to reduced wear of the Si3N4 counterpart under the same test condition, which could be attributed to the fine microstructure and improved mechanical properties of the nanostructured coating. The detailed wear mechanisms for both coatings have also been discussed.

Influence of machining with defined cutting edge on the subsurface microstructure of WC–Co parts

Abstract To machine WC–Co cemented carbides, discontinuous cutting (milling, shaping, etc.) with diamond tools can be an economical alternative to grinding or electrical discharge machining operations for finishing mechanical parts after sintering. However, the machining with cutting depths within the range of sintering allowance (≈0.2 mm) leads to high cutting forces which may influence the quality of the workpiece. Single cuts of a shaping process of WC–Co with different rake angles and cutting depths were examined. SEM micrographs of the machined subsurface zone allow the measurement of microstructure parameters by an automated method programmed in MATLAB. The result is the determination of grain size, mean free path of the binder, contiguity and cobalt content of a selected region of interest within the subsurface compared to the according bulk properties. Regarding the influence of the cutting conditions, it can be assumed that the damage of the subsurface zone correlates with the acting forces. Larger cutting depths and an extremely-negative rake angle of the tool, which both lead to higher loads, generate larger and further reaching damage in the subsurface. Within deeper influenced subsurface zones, parameters vary depending on the distance to the surface. Close to the surface, small fragmented grains can be found while in the deeper regions of the damaged subsurface zone only few single larger cracks were observed.

Effect of V, Cr and Mn additions on the microstructure of WC–Co

Abstract In this work, the impact of 0.5 at% additions of V, Cr and Mn on the microstructure of WC–Co is studied. Mn has negligible effect on the hardness, WC grain size, contiguity and fraction of Σ2 grain boundaries compared to the reference. Cr causes the material to become harder, have a higher contiguity and fraction of Σ2 grain boundaries. V results in the hardest material, the highest contiguity, the smallest grain size and the highest fraction of Σ2 grain boundaries.

Microstructure of WC in WC–Co cemented carbides consolidated by electric discharge

The microstructure of WC in WC–Co cemented carbides fabricated by electric discharge compaction was investigated. A high density of defect, such as stacking faults, was found in WC grains, and lattice distortion also occurred. It indicated that the high strain rate and instantaneous stress induced the formation of defect during densification.

Influence of starting powder on the microstructure of WC–Co hardmetals obtained by spark plasma sintering

WC–Co hardmetals using different starting powders were densified by spark plasma sintering (SPS). Sintering cycles were performed at 1240 °C for the WC–12 wt.% Co–0.45 wt.% VC grade and 1280 °C for the WC–6 wt.% Co grade, applying a pressure of 60 MPa in combination with a heating rate of 100 °C/min. The influence of the carbon content and homogeneity of the starting powders on the final microstructure and mechanical properties was investigated. A thin graded surface layer is observed in case of slightly carbon deficient starting powder. Co agglomerates were observed in the SPS materials obtained from lower energetically multi-directionally mixed WC–Co powder, whereas a homogeneous microstructure was obtained when using attritor-milled or chemically synthesized WC–Co starting powder with adjusted carbon balance. The observed microstructures are related to the measured mechanical properties.

Microstructural characterization and microscopy analysis of laser cladding Stellite12 and tungsten carbide

Stellite12 cobalt base alloys with different WC content were deposited on SK3-carbon tool steel by laser cladding. The behavior of WC particulates, including the dissolution, distribution, and microstructures of WC–Co–Cr–C composite coatings with rapidly solidifying were investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Several significantly different solidified microstructures were characterized by dendrites, interdendritic eutectics, faceted dendrites (third phase) and the retaining of WC particles in the laser clads of Stellite12 + WC, under different laser energy densities. When WC was completely melted and fully dissolved into the Stellite12 melt pool, the basic solidification, characterized by the matrix and faceted dendrites (third phase) in various shapes, and the compositional evolution of the clad layers remained nearly identical, no matter the added WC was increased to 10%, 20% or 40% (wt.%). The faceted dendrites (third phase) contained the majority of W as well as some Cr, Co while more Cr and Co were located in the matrix. The X-ray diffraction analyses indicated the existence of σ-Co, M 23C 6, M 6C and M 7C 3 (M = W, Cr, Co) in the Stellite12 alloys with different WC contents when deposited on substrates by laser cladding.

Mechanical properties, phases and microstructure of ultrafine hardmetals prepared by WC–6.29Co nanocrystalline composite powder

WC–6.29Co nanocrystalline composite powder prepared by spray thermal decomposition–continuous reduction and carburization technology was consolidated by spark plasma sintering (SPS), spark plasma sintering with hot isostatic pressing (HIP), vacuum sintering and vacuum sintering with hot isostatic pressing (HIP). The mechanical properties, phases and microstructure of WC–6.29Co hardmetals prepared by different processes were researched. The results show that WC–6.29Co ultrafine cemented carbide consolidated by spark plasma sintering with hot isostatic pressing can reach 99.0% relative density, and transverse rupture strength (TRS) is more than 2740 MPa, Rockwell A hardness (HRA) is more than 93.8, the average grain size is less than 400 nm, and WC–6.29Co ultrafine cemented carbide with excellent properties is achieved. Although specimens consolidated by spark plasma sintering can reach 99.1% relative density, Rockwell A hardness is more than 94, but transverse rupture strength (TRS) is very low. Compared with specimens prepared by spark plasma sintering, there are no eta phases in specimens consolidated by spark plasma sintering with hot isostatic pressing. Furthermore the average grains size of specimens prepared by vacuum sintering or vacuum sintering with hot isostatic pressing is coarser than that of specimens prepared by spark plasma sintering with hot isostatic pressing, but mechanical properties are lower than that of specimens prepared by spark plasma sintering with hot isostatic pressing. Spark plasma sintering with hot isostatic pressing can not only decrease the average grain size of sintered specimens, but also increase mechanical properties of WC–6.29Co ultrafine cemented carbide. The specimens prepared by spark plasma sintering with hot isostatic pressing have better properties than those prepared by spark plasma sintering, vacuum sintering or vacuum sintering with hot isostatic pressing.

Cr-rich layer at the WC/Co interface in Cr-doped WC–Co cermets: segregation or metastable carbide?

The effect of Cr on the microstructure of WC–Co alloys after liquid-phase sintering is studied as a function of the C content of the alloy and for two Cr for Co substitution ratio. The distribution of Cr is investigated using X-ray energy dispersive spectrometry. A Cr enrichment at WC/Co interfaces is detected for both C- and W-rich alloys. The observations by high-resolution electron microscopy point out a thin face centred cubic layer at the interface between WC and Co. This layer has grown epitaxially on the surface of the WC grains with two orientation relationships depending on the WC plane. The lattice parameter mismatch is close to 0.2% for the basal plane and less than 3% for the prismatic plane of WC. The composition and crystal structure of this compound correspond to the metastable (Cr,W)C phase that is stabilized by the low energy of the WC/(Cr,W)C interface.

Thermo-chemical treatment of PM materials with carbon and nitrogen: V.Iancau et al. (Technical University of Cluj-Napoca, Cluj-Napoca, Romania.)

Coarse grained WC–9Co cemented carbides with 0–1.0% TaC (mass fraction) were fabricated by HIP-sintering and gas quenching. The effects of TaC on the microstructures and mechanical properties were investigated using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), X-ray diffractometry (XRD) and mechanical properties tests. The results show that the maximum values of hardness and strength are HV 1124 and 2466 MPa respectively when 0.4% TaC is added. When the content of TaC is more than 0.6%, the grain size of WC is no longer affected by the amount of TaC, and (W,Ta)C occurs as well. Moreover, the strength and fracture toughness increase and the (Ta+W) content decreases with the increase of TaC content. The dependence of (Ta+W) content on the mechanical properties indicates that (Ta+W) content in Co should be decreased as low as possible to improve the mechanical properties of coarse grained WC–TaC–9Co cemented carbides with the microstructure of WC+γ two phase regions.