WC-Co Powder Research Articles

Comparison of Plasma Transferred Arc and Submerged Arc Welded Abrasive Wear Resistant Composite Hardfacings

EComposite hardfacings produced by Plasma Transferred Arc Welding (PTAW) and Submerged Arc Welding (SAW) possess a good combination of hardness, wear resistance and fracture toughness, thus providing high wear resistance. Although they cannot substitute and be compared with conventional WC-Co based hardmetals, still they can be used in many applications where high wear resistance, hardness and toughness are in great demand. In this study two different hardfacing production technologies PTAW and SAW, were used to produce the hardfacings for abrasive wear conditions. In both cases hardfacings were welded on the top of low alloy steel using different proportions of disintegrator milled hardmetal WC-Co powder of different fractions as a reinforcement and self-fluxing alloy as a matrix. They were analysed in regard to Rockwell and Vickers hardness, wear behaviour, and microstructural analysis. SAW hardfacings were subjected to Rockwell hardness test after process and after two cycles of tempering; secondary hardness effect was detected as increment of hardness values from 39 HRC to 58 HRC after first cycle of tempering. High Vickers hardness values did not correlate with wear results, as it commonly shows hardness of hardmetal particles. Dissolution of hardmetal particles in the matrix was observed in both PTAW and SAW hardfacings with higher amount in the later. This amount correlated with heat input during welding process. Wear test results in abrasive emery wear (AEMW) and abrasive wheel wear test (AWW) showed almost analogous tendency, with slightly lower wear in later. Both types of hardfacings have shown promising results in intensive wear conditions. DOI: http://dx.doi.org/10.5755/j01.ms.24.2.19121

Influence of using electroless Ni-P coated WC-Co powder on laser cladding of stainless steel

In this investigation, nickel (Ni)-coated WC-12Co powder was produced using an electroless nickel‑phosphorus (Ni-P) plating process and then used as a feedstock material for laser cladding of AISI 321 stainless steel substrate. The correlations that exist between key operational parameters of the process (i.e., laser power, scanning speed, powder feeding rate) and geometrical characteristics for single claddings (i.e. height, width, dilution, and wetting angle) were predicted and analyzed by regression analysis method (RA). A processing window for laser cladding of Ni-coated WC-12Co powder on AISI 321 stainless steel was developed based on an empirical-statistical model and the processing window was compared with that for laser cladding of commercial WC-12Co powder without nickel coating. Furthermore, the influence of using Ni-coated WC-12Co powder on the microstructure and microhardness was investigated and compared with those of laser cladded commercial WC-12Co claddings. The results showed that the use of Ni-coated WC-12Co powder could significantly reduce the porosity percentage and lead to high-quality claddings.

Metallographic Analysis of the Suitability of a WC-Co Powder Blend for Selective Laser Melting Technology

The main objective of this article is to familiarize readers with the first outputs from PhD research by David Bricín, especially with the metallographic analysis, which was carried out on the first series of printed samples. The PhD thesis deals with the processing of powder mixtures based on WC-Co using selective laser melting (SLM) and other technologies. This article specifically deals with the use of SLM for the processing of a WC-Co powder mix. The grains of this powder mixture are not granular, but separate grains of carbides and binders. This powder blend was processed on a 3D SLM printer using various printing parameters. Variable parameters included laser power and scanning speed. Other print parameters were kept constant. The properties of the powder mixture and the printed samples were evaluated by metallographic analysis using light and scanning electron microscopy. These analyses were further supplemented by X-ray diffraction phase analysis, chemical analysis by EDX, and analysis of mechanical properties by compression strength testing. The evaluation of the analyses determined how the printing parameters and the type of powder used affect the development and distribution of the structure in the printed samples and how the mechanical properties of the print are then affected. For example, it has been found that increasing the scanning speed results in a more pronounced mixing between the carbide grains and the binder, which then has a positive influence on the mechanical properties of the print. In addition, the experiments found the energy at which the porosity in the printed samples was significantly reduced, and the direction in which further experiments are to take.

Synthesis of WC composite powder with nano-cobalt coatings and its application in WC-4Co cemented carbide

Composite powder of tungsten carbide with nano-cobalt coatings was successful manufactured via a novel sol-gel process, and coarse grained WC-4 wt% Co cemented carbide was produced from the cobalt-coated WC powder. Phase composition and microstructure evaluation were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results revealed that the WC particles were completely encased in the CoC2O4 layers by the sol-gel process. Core-shell structured WC-4 wt% Co composite powder was subsequently prepared by reducing the WC-CoC2O4 precursor powder in hydrogen atmosphere. The morphology and crystal structure of the cobalt-coating were found to depend on the reduction temperature. Chemical analyses revealed that the WC-Co composite powders were non-contaminated and composition-controllable. WC-4 wt% Co cemented carbide fabricated from the cobalt-coated WC composite powder showed a refined and uniform microstructure, and therefore improved mechanical properties, compared to the cemented carbide made by ball-milled WC-Co powder. The transverse rupture strength was 2249 MPa in the cemented carbide from the cobalt-coated WC powder, compared to 1871 MPa of the carbide made by the ball-milling powder.

Fabrication of WC-Co cemented carbides with gradient distribution of WC grain size and Co composition by lamination pressing and microwave sintering

WC-Co composite powders with different particle sizes and Co contents were prepared by ball milling WC and Co powder mixtures for different durations. Functionally graded WC-Co cemented carbides with both Co content and WC grain size gradient were prepared by lamination pressing different WC-xCo (x = 10, 15, 20) powder mixtures and microwave sintering the layered compacts. The WC-xCo powder mixtures were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), and the phase composition and microstructure of the functionally graded cemented carbides (FGCCs) were investigated by XRD, and FE-SEM coupled with energy dispersive spectroscopy (EDS). Mechanical behaviors of the layered WC-Co materials were measured and compared with those of WC-Co cemented carbides with single composition. The results showed that increasing the milling time from 6 to 24 h, results in the decrease of the particle size of WC-Co composite powders from 0.31 to 0.11 µm. After lamination pressing and microwave sintering, the WC-Co samples show nearly complete densification with a relative density higher than 99.7% and no ƞ-phase was detected in the FGCCs. The Co content and WC grain size in FGCCs decrease from the core to the surface. Homogenization of Co has hardly occurred and no cracks have formed between the layers in the sintered samples. In the inner layer, the mean WC grain size is 529 nm, while in the outer layer it is only 274 nm. Because of the difference in Co content and WC grain size, FGCCs have a Rockwell hardness of 90.75 HRA at the surface, which decreases to 86.75 HRA in the core. However the fracture toughness increases from 11.53 at the surface to 18.12 MPa m−1/2 in the core. The present results show that FGCCs with high outer layer hardness and high inner layer toughness were successfully prepared.

Impact of crystal defects on the grain growth of cemented carbides

Two different WC-Co powder mixtures were produced through a 200h milling process. One of these mixtures were heat treated with the aim to remove the crystal defects from the WC grains. Image analysis showed that the two powders had the same average WC grain size and grain size distributions. X-ray diffraction showed much more narrow peaks for the material produced from the heat treated powder thus interpreted to have much less crystal defects.The powder mixtures were used to produce cemented carbides. Image analyses showed a clear difference for the WC grain sizes where the material produced from the heat treated powder had a significantly smaller mean WC grain size. It is therefore concluded that the crystal defects are of importance and facilitate WC grain growth during sintering.

Nanocrystalline WC-Co HVAF Coatings by Utilizing Novel Powder Manufacturing Route Using Water-Soluble Raw Materials

In this study, nanostructured WC-Co coatings were produced using experimental nanocrystalline WC-12Co and WC-24Co powders produced by a novel chemical synthesis route. Test coatings were produced using HVAF spraying keeping the temperature as low as possible during the deposition in order to avoid decomposition of the nanocarbides. In experimental powders, two different Co incorporation methods were used: a conventional way in which cobalt was incorporated as a metallic Co powder and a chemical synthesis way in which cobalt acetate was used as a cobalt source. When using cobalt acetate, it decomposes to metallic cobalt during the process. Experimental powders in which cobalt acetate has been used as cobalt source resulted poor deposition efficiency. With warmer parameters, powders resulted better DE, but significant WC decarburization and the dissolution into the matrix phase occurred. Powders in which Co has been introduced as Co powder showed enhanced DE enabling spraying with decreased temperature and higher particle velocity, resulting in coatings with less WC decomposition. Especially, an experimental powder in which Co has been incorporated both as Co powder and as Co-Ac results very fine nanocarbide structure with significantly less WC decomposition having a hardness value of 1201 HV0.3, even with 24% Co.

Acceleration effect of cobalt on carburization of tungsten at low temperature

The acceleration effect of cobalt on carburization of tungsten at low temperatures was studied by a series of experiments of W, C and Co reactions and theoretical evaluations. Due to Co addition, the formation temperature of WC was greatly reduced (e.g. a decrease from 1300 °C to 850 °C by addition of 1.0 wt% Co). Below the formation temperature of the ternary phase, the converted fraction of W carburization increases with increased Co. Modeling of reaction kinetics revealed that the process of W carburization is dominated by carbon diffusion, and the reaction rate constant is increased much with the increase of Co. The mechanism was proposed that Co acts as the medium for rapid diffusion of C atoms and promotes W carburization by accelerating reaction diffusion kinetics. The present findings will facilitate to develop methods for synthesis of WC-Co composite powders at low temperatures.

An empirical-statistical model for laser cladding of WC-12Co powder on AISI 321 stainless steel

In this study, laser cladding of WC-12Co powder on AISI 321 steel substrate was optimized using an empirical-statistical viewpoint. The correlations that exist between key parameters of the process (i.e., laser power, scanning speed, powder feeding rate) and geometrical characteristics for single clads (i.e. height, width, dilution, and wetting angle) were predicted and analyzed by regression method (RA). The preliminary geometrical considerations allowed to choose the processing parameters which led to high quality clads with minimum porosity. All considerations finally resulted in the development of a processing map that shows the optimum parameters for laser cladding process of WC-12Co powder on AISI 321 stainless steel.

Structure, Mechanical and Tribological Properties of HVOF Sprayed (WC-Co+Al) Composite Coating on Ductile Cast Iron

The paper presents the results of examinations of WC-Co coating sprayed on ductile cast iron by high velocity oxygen fuel spray process (HVOF) with powder containing Al particles in an amount of 10%. The impact of Al particles added to the tungsten carbide coating on the structure, mechanical and tribological properties in the system of (WC-C)/ductile cast iron was examined. The microstructure of the thermal sprayed WC-Co+Al coating was characterized by light, scanning electron (SEM) and transmission electron (TEM) microscopes as well as the analysis of chemical and phase composition in micro areas (EDS, XRD). It was found that by supersonic thermal spraying with WC-Co powders with the addition of Al particles, the coatings of low porosity, high hardness, a very good adhesion to the substrate, compact structure with molten Al particles and finely fragmented WC particles embedded in a cobalt matrix, reaching the nanocrystalline sizes were obtained. Moreover, the results were discussed in reference to examination of bending strength considering cracking and delamination in the system of (WC-Co+Al)/ductile cast iron as well as hardness and wear resistance of the coating. It was found that the addition of Al particles was significantly increase resistance to cracking and wear behaviour in the studied system

Microstructure-property relations in WC-Co coatings sprayed from combinatorial Ni-plated and nanostructured powders

In this study, we addressed the effects of Ni-plating and nanostructuring of WC-Co powders on the microstructure-property relations and WC decarburization of WC-Co composite coatings. By high velocity oxygen fuel (HVOF) spraying, the coating materials were produced from four different powders, referred to as microstructured WC-Co (mc-WC), nanostructured WC-Co (nc-WC), Ni-plated microstructured WC-Co (Ni/mc-WC) and Ni-plated nanostructured WC-Co (Ni/nc-WC). We found that the coatings, deposited from Ni-free powders, undergo a high level of decarburization, thereby yielding W2C phase formation in brittle amorphous W-rich Co matrix that could be preferential site for crack propagation. On the contrary, the Ni-plating treatment could reduce the amount of decarburization of the coatings, resulting in the uniform distribution of WC phase in the W-depleted Co matrix. Consequently, we revealed the underlying mechanisms responsible for WC decarburization in the coatings, and further suggested that the coating deposited from Ni-plated and nanostructured powders exhibits a superior combination of hardness and fracture toughness as compared to other coatings.

Microstructures and Properties of Nano-structural WC-12Co Coatings Deposited by AC-HVAF

WC-12Co powder with nano WC grains was used to deposit wear resistant coatings by AC-HVAF (acukote high velocity air-fuel) spray system. The phase compositions and microstructures of the coatings were examined. The microhardness, fracture toughness and wear resistance were investigated. According to the X-ray diffraction (XRD) analysis, the principal phase is WC, and its decomposition products are not found. The sprayed coating has a porosity lower than 1% and the grain sizes of coatings are among 80∼100 nm which contribute to an average microhardness (HV 0.3 ) as high as 19 403 MPa on the surface and 17 410 MPa on the cross-section. At the load of 15 N and rotational speed of 1198 r/min of WC counter body, the nano-sized coatings show a decreased average mass loss by 40% compared with micro-sized coatings, and a stable friction coefficient of 0.26∼0.28 (micron-sized coatings: 0.25∼0.4) in the dry wear conditions. It is concluded that the sprayed nano structural WC-12Co coatings have a better wear resistance.

Spark Plasma Sintering of a Hard Metal Powder Obtained from Hard Metal Scrap

Spark plasma sintering is used to consolidate a WC-Co powder produced from milled discharged hard metal cutting inserts. The powder was mixed to 10%wt Co powder and spark plasma sintered (SPS) under 80Mpa pressure, at 1200oC with heating rate of 160oC/min. A reference alloy was prepared with a commercial WC powder and 10%wt. Co under the same conditions. The structures of both alloys were investigated by means of scanning electron microscopy and Vickers hardness. The results show structures with densities and hardness at the same level. SPS can be used to produce hard metal pieces directly from scrap powder without any prior chemical and thermal treatment.

Effect of nano-sized TiC powder on microstructure and the corrosion resistance of WC-Co thermal spray coatings

In this article, the electrochemical behavior of HVAF WC-17Co coatings modified with TiC deposited on low alloyed steel substrates was investigated. As a reference for coating material a commercial WC-17Co powder was used. The WC-17Co feedstock powder modification process was based on the addition of nano-sized TiC (weight ratio from 1 to 7%) by mechanical mixing. The aim of the research is comparing the microstructure and some selected properties of HVAF coatings with different amounts of TiC. The preliminary characterizations concerned the coating porosity, phase composition and microstructure. Then a corrosion resistance for all variant of coatings was evaluated by electrochemical polarization tests at room temperature in 3.5% NaCl and 0.5M H2SO4 solutions. The results showed an evident influence of nano-sized TiC addition on both on the corrosion resistance and the electrochemical properties of WC-17Co coatings. In the case of investigation in 3.5% NaCl, addition of nano-sized TiC caused shifting polarization curves in more noble range, however in 0.5M H2SO4 solution no significant changes in corrosion behavior were observed. Addition of nano-sized TiC significantly decreases corrosion resistance of WC-17Co+TiC coatings.

STEM analysis of WC-Co coatings modified by nano-sized TiC and nano-sized WC addition

Thermally sprayed hardmetal WC-Co-based coatings are widely applied in order to provide good wear resistance to mechanical components working in demanding environment. The WC-Co coatings are obtained by various spraying methods such as High Velocity Oxygen Fuel (HVOF) or High Velocity Air Fuel (HVAF) techniques. Additional improvement of coatings' properties can be achieved by adding very fine carbides to the WC-Co powder feedstock. The usual approach is the agglomeration and sintering of submicron-sized carbides without or with micro-sized carbides (bimodal distribution). In this work, WC-Co powders were modified by blending them with 5wt% nano-sized TiC (<100nm) or 5wt% nano-sized WC (<100nm). The coatings were deposited by High Velocity Air Fuel (HVAF) on a steel substrate. As reference, pure WC-Co coating was also sprayed. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) were employed for the microstructural characterization of the deposited coatings. The formation of solid solution of tungsten in cobalt with the composition Co0.9W0.1 by selected area electronic diffraction, was detected. W dissolution effect was observed to be lower in the case of nano-sized TiC-added coatings. Microstructural investigation by STEM indirectly indicates that nano-sized WC completely dissolved in the cobalt matrix, inducing solid solution of tungsten in cobalt formation in the 5wt% WC-added coating. The formation of new phases containing Ti, except TiC, was not found in the coating. Obtained results suggest that nano-sized TiC may increase the stability of metallic matrix microstructure in a WC-Co-based coating.

Mechanical Properties and Corrosion Resistance of HVOF Sprayed Coatings Using Nanostructured Carbide Powders

Abstract Nanostructured and composite WC-12Co coatings were prepared by means of the supersonic spray process (HVOF). The microstructure and composition of WC-12Co nanostructured powder were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). Investigations revealed nano grains of WC with the size in the range of 50-500 nm. The nanostructured sprayed coating was analysed by SEM and phase composition was investigated by X-ray diffractometer (XRD). A denser coating structure with higher hardness was observed compared to conventional coating with a small amount of W2C, WC1−x, W and some amorphous phase. Young’s modulus and hardness were determined by depth sensing indentation in HVOF sprayed WC-12Co nanostructured coatings. Results were compared to conventional coatings and the relevance of the nanostructure was analyzed. An indentation size effect was observed on the polished surface and cross-section of both coatings. Data provided by indentation tests at maximum load allow to estimate hardness and elastic modulus. Enhanced nanomechanical properties of conventional coating in comparison to nanostructured one were observed. Nanostructured coatings WC-12Co (N) revealed significantly better corrosion resistance.

Recovery of tungsten carbides to prepare the ultrafine WC-Co composite powder by two-step reduction process

A relatively low-cost, controllable particle size and eco-environmental recycling process for the oxidation and reduction of waste YG8 cemented carbides was achieved. The WC-Co composite powder has been synthesized by two-step reduction process. Reduction of WO3/CoWO4 in the hydrogen gas at 1073K is a feasible way for the production of W18O49 phase. Then the carbothermal reduction of the as-synthesized powder was carried out in Argon atmosphere mixed with ethanol gas, which was used as reductant as well as carbon source. Both the temperature and time in the present method of combing the higher reactivity of the precursor material and the better contact between the gas-solid reactants are apparently reduced as compared with the conventional methods. Based on the thermodynamic calculation and XRD analysis, the possible reactions involved in the carbothermal reduction were proposed. The influence of the holding time and carbon content were investigated. The two-step reduction process is a useful technique to prepare homogeneous WC-Co composite powder with particle size below 200nm at low temperature.

A Comparison between Hardness, Corrosion and Wear Performance of APS Sprayed WC-CoMo and WC-Co Coatings

In order to be competitive, it is demanded to have thin, tough and long lasting coatings. An important aspect is to use stable deposition technologies. As Cr assures wear, corrosion and high temperature resistance, the most employed coatings in industry generally contain Cr. Nevertheless, Cr is a hazardous element for the humans’ health, therefore, sustainable alternatives are needed to be implemented. The aim of this work is to investigate the microstructure, hardness, corrosion resistance and wear behavior of the novel WC-CoMo compared to conventional WC-Co coatings. So far, WC-CoMo coatings are not part of state of the art regarding the Atmospheric Plasma Sprayed (APS) coatings. WC-Co powder in plain form and mechanically mixed with Mo was deposited using the APS method on standardized Type A Almen Strips (C67 steel). The size of the powder grains varies between 5 µm and 30 µm. The obtained samples were investigated by means of Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy, X-Ray Diffraction, and hardness, wear and corrosion behavior were also evaluated. Results revealed formation of different intermetallic phases around the WC particles, which have a benefic influence on the coating characteristics and microstructure.

Synthesis and characterization of WC-Co nanosized composite powders with in situ carbon and gas carbon sources

This study presented nanosized WC-Co composite powders synthesized using a one-step reduction-carbonization process with a combination of CH4/H2 as a gas carbon source and soluble starch as an in situ carbon source. The results of carbon analysis and X-ray diffraction revealed that WC-Co nanocomposite powders with a pure WC and Co phase could be obtained at 1100 °C after 0.5 h. A higher gas flow ratio of CH4/H2 during the reduction-carbonization process led to a higher total carbon content of the sample. A field emission scanning electron microscope confirmed that the particles in the WC-6 wt% Co composite powders had the lowest average size of 43 nm with equiaxed shapes. A sintering neck was observed in the WC-3 wt% Co composite powders whereas faceted particles were found in the WC-12 wt% Co composite powders. Moreover, this method has advantages of simple processing, rapid synthesis and good applicability in potential industry application.

Microstructure and wear performance of heat treated WC-12Co microwave clad

In this study, WC-12Co powder was deposited onto stainless steel substrate using microwave hybrid heating technique. The deposits were then heat treated within a temperature range of 400 °C–900 °C in controlled (argon) atmosphere. In microwave cladding C and W get liberated due to decarburisation of WC and combine with Co to form amorphous phases. Changes in the constituting phases and microstructure due to post heat treatment were investigated using X-ray diffraction and scanning electron microscopy, respectively. Microstructural observations and phase analysis indicated the formation of eta-phases during heat treatment up to 400 °C, which lead to increase in the hardness and wear resistance of the clads. Microhardness examination revealed significant improvement in hardness post heat treatment at 400 °C. The abrasive wear behaviour of the WC-12Co has been correlated with microstructural and phase changes due to post clad heat treatment at various temperatures.