The Seminar is currently running in Reutte, Austria.

Special interest: Hardmetal powder production and recycling

Review of the hardmetal recycling market and the role of the zinc process as a recycling option, T. Karhumaa, Tikomet Oy, Finland 

Recycling in the hardmetal industry has greatly gained in popularity since 2005, due to the increase in tungsten prices. Recent developments in hardmetal recycling, the various recycling options and their market shares in Europe are reviewed in this paper and the future outlook is discussed. According to the author, a significant new development in recent years has been the emergence (surely the re-emergence – KJAB) of the zinc process as a recycling option. The basics of the zinc process are reviewed. 

Both lab and production-scale experiments were carried out. WC/Co zinc reclaim powders were blended with virgin raw materials and the effect of such additions on powder processing and the properties of sintered hardmetal were analysed. Results of both laboratory and production scale tests are shown and discussed. 

The results are claimed to show that WC/Co zinc reclaim powders when added in the right amounts and properly processed had no negative effect on the properties of sintered hardmetal. 

Based on the results various applications for zinc reclaim powders are discussed. (It will be interesting to see the complete paper. The zinc process has been around for a long time and its main drawbacks – poor grain-size distribution and build-up of impurities – still militate against zinc reclaim being used for high-grade cutting tools and inserts, the most important product of the hardmetals industry – KJAB). 

Numerical simulation of organic binder decomposition during thermal debinding, T. Kraft, Fraunhofer Institute for Mechanics of Materials IWM, Germany

In some powder processing routes, debinding of green parts can be critical due to possible damage. Especially large parts or very dense compacts combined with higher heating rates are vulnerable. This paper presents a model which describes the chemical decomposition of organic binders and consequential development of internal gas pressure. It includes combined Maxwell-Stefan and Knudsen diffusion and the seepage flow of multiple gas reaction products through a porous body. The model has been implemented into the finite element program ABAQUS. The internal stresses developed by gaseous overpressure during decomposition are shown for some simple examples. When predicted, these stresses indicate critical temperature regions and can help to optimise debinding conditions. 

Effect of WC material properties and milling parameters on the grain size and sintering behaviour of coarse WC powder, A. Johansson, Seco Tools AB, Sweden 

Wet-milling and sintering behaviour of two coarse WC powders was studied experimentally. Microscopy, laser light scattering, BET adsorption isotherms, X-ray powder diffraction and magnetic coercivity measurements were used to quantify the effect of material and process parameters on the evolution of WC grain-size distribution throughout the process. Analysis of results showed that WC grain-size reduction with increasing milling time was characterised by a power law. Evaluation of the effect of several process parameters on milling efficiency was facilitated by the use of simple scaling factors. In addition, the correlation between WC grain size after ball-milling and after sintering was investigated. 

Property changes induced in submicron WC through various final milling techniques, R. Cook, Global Tungsten & Powders Corp, USA 

Milling techniques vary in both energy and mechanism applied to powder samples. Powder properties and resulting sintered properties change accordingly. This study looked at these by comparing a wide variety of commonly employed milling operations, including conventional ball milling, attritor milling, pin milling and rod milling, as applied to sub-micron WC. A wide range of physical, chemical and sintered properties were examined to evaluate the results of milling in sub-micron WC and WC/Co. From these studies, certain attributes such as density, particle size distribution and sintered shrinkage could be directly modelled according to milling time and type. However, each mill style (combination of impact and shear) has a different phenomenological response. 

Rim-nitrided WC powder: preparation, properties and use for hardmetals, M. Wetzel, TU Bergakademie Freiberg, Germany

Nitrogen uptake of commercial WC powders of different grain size was studied by high-pressure nitridation (≤ 200bar N2). The nitrided powders were characterised by XRD, chemical analysis, SEM, TEM and for their grain-size distribution. Interestingly, it was found that a perceptible amount of nitrogen dissolved in WC. As the diffusion rate of N in WC was extremely slow, nitridation occurred in the outermost rim of the WC particles. Under appropriate nitridation conditions the powders were even purified with respect to the oxygen level and showed no particle growth. 

Sintering experiments were performed with these powders for possible use in the preparation of hardmetals. It was found that nitrogen could retard low-temperature grain growth and thus hardmetals of reduced grain size could be obtained. Mass-spectrometric delubing experiments showed that the powders lost nitrogen during sintering. If the powder particles had a steep N gradient (essentially a nitrogen-rich rim) no porosity remained in the hardmetals, contrasting with powders of uniform nitrogen distribution with similar overall nitrogen content. 

Analysis of WC with increased Ta doping, J. Weidow, Chalmers University of Technology, Sweden

Tungsten and tantalum metal powders were co-carburised to yield a mixture of cubic and hexagonal carbide. The carburisation was made through a two-step carburisation process with (W,Ta)2C powder as an intermediate product. X-ray diffraction analysis showed that the lattice parameters of the hexagonal phase in the fully carburised powder were larger than those of pure WC, indicating the formation of a mixed crystal carbide (W,Ta)C. The powder with the largest lattice parameters was investigated in detail. A method to produce atom-probe tomography specimens of this powder was developed and measurement showed Ta solubility expressed as Ta/(Ta+W) to be as high as 0.086 (ie about 8.6 at%). In addition, it was found by electron backscatter diffraction that the (W,Ta)C grains had a large fraction of Σ2 grain boundaries as well as a small fraction of what was suggested as Σ4 grain boundaries. 

Special Interest: Hardmetal trends in processing

Thermal spraying – a technology for hardmetal coating solutions, L.-M. Berger, Fraunhofer Institut IWS, Germany 

Thermal spray processes represent an important and rapidly growing group of surface modification technologies, it is the only surface technology allowing the production of hardmetal coatings and is thus complementary to sintering technology and applicable to large parts. 

This contribution reviews the development of these coatings, including different processes and feedstock materials, and examines the technical possibilities of state-of- art coating. Hardmetal coatings arere based mostly on WC, but also on Cr3C2. Cubic hard phases based on TiC offer an alternative. Hardmetal compositions for coatings (large Cr3C2 addition, high binder content) are considered. The need for fundamental research is outlined for understanding metallurgical reactions during spray processes, eg the formation and properties of (W,Cr)2C. Microstructure/property relationships are discussed in relation to feedstock powder properties and the spray processes applied. Besides traditional use as wear-resistant coatings, new applications, such as service under contact loading, are envisaged. 

Development of a HVOF WC based thick coating, J. He, Global Tungsten & Powders Corp., USA 

To avoid severe wear, gas, oil and other mining industries need very thick wear-resistant layers, currently produced by PTA techniques. It is known that HVOF thermal spray coatings have advantages over PTA and plasma coatings in wear resistance, based on the microstructure formed at lower temperatures and higher particle velocities. This work examines the microstructure and mechanical properties of WC/10Ni HVOF thick coatings of up to 1000μm. The coatings were produced using both densified and non-densified thermal spray powders, the thick coatings being sprayed on both rotating and stationary substrates. The resulting microstructure was comparable to those of typical HVOF coatings with no cracks or large voids in either coating or near coating/substrate interface. Thick coatings have the same level of porosity (less than 0.5%), microhardness (~1030 HV300) and bond strength (~10 ksi) as the corresponding normal HVOF coatings in the 100 to 300µm range. 

Rapid synthesis of ultrafine WC-Co cemented carbides, X. Song, Beijing University of Technology, China 

The University claims to have developed novel routes for rapid preparation of ultrafine WC/Co cermets, with potential industrial applications.  One is the rapid synthesis of WC/Co composite powder with controllable particle size by in situ reduction/carbonisation reactions using metal oxides and carbon black as raw powders. The powder products, which have advantages in the purity, uniform particle-size distribution and homogeneous binder phase distribution, are available as high-performance coating materials, as well as powders for sintering cermet compacts. The second is a one-step route for synthesising cermet compacts by reaction sintering in a spark plasma sintering system. This route is applicable to preparing dense nanocrystalline cermet pressings and is suggested for development as a net-forming fabrication technique for certain cermet products. Besides the development of preparation methods, mechanisms for rapid synthesis have been studied, and the prospects for applying the techniques are discussed. 

Characterisation of gas reactions during ceramic and hardmetal production, T. Gestrich, Fraunhofer Institute IKTS Dresden, Germany 

Gas reactions are critical steps in the elevated-temperature production of hard materials like cemented carbides (eg WC/Co) and ceramics (eg liquid phase sintered SiC). Thermogravimetry (mass changes), differential scanning calorimetry (caloric effects) and mass spectrometry (gas analysis) were employed to study gas reactions. Thermodilatometry (length changes) also gave valuable information. The paper shows how gas reactions are influenced by grain size, grain-growth inhibitors and amount or composition of binder phase. Through kinetic analysis of thermoanalytical results it was found possible to predict thermal behaviour for any temperature/time profile or special mass change behaviour. This knowledge could be used to avoid problems like porosity, which influenced important properties of the sintered material, such as strength or carbon balance. The acquired knowledge may lead to a better cost efficiency, stabilisation of properties and technological improvement. 

On the interaction of alumina during sintering of cemented carbides, A. Bicherl, Wolfram Bergbau und Hütten AG, Austria

Even today sintering of certain cemented carbide parts is carried out in pusher-type furnaces under hydrogen atmosphere. These items are embedded in a coarse-grained alumina/carbon fill. Other grades are sintered in vacuo on a graphite tray coated by a fine suspension of alumina. 

During the sintering sequence, oxygen partial pressure within the heating zone is significantly reduced by the presence of carbon. Partial carbothermic decomposition of alumina takes place, resulting in uptake of aluminium in the binder phase during sintering.

This investigation demonstrated that both temperature and atmosphere determined the amount of aluminium in solution. Under extreme sintering conditions, the formation of intermetallic phases could be observed. The amount of aluminium was measured as a function of temperature and respective binder alloy system (Co, Ni, or Fe base). The influence of dissolved aluminium on subsequent cooling and the properties of the sintered parts is discussed. 

Selective laser sintering of tungsten carbide inlays for local wear protection of injection moulding tools, H. Koehler, BIAS - Bremer Institut für angewandte Strahltechnik, Germany 

Materials for injection moulding tools like steel 1.2343 need to withstand different types of wear. Tool wear typically varies locally, increasing dramatically at injection sites. Inserts of wear-resistant material are used in such areas. The selective laser sintering/melting process was chosen to produce near-net-shape parts from a variety of materials. The highly wear-resistant alloys 83WC/17Co  and 88WC/12Co were chosen to be generated on steel substrates. Major challenge was the determination of process parameters to meet desired densities and avoid cracking. The influence of process parameters and weld trajectories on density was investigated. It was found that the key to achieving densities greater than 95% throughout the specimens was the combination of comparatively low scanning speed, large beam diameter and checkerboard-shaped weld trajectories. Inserts fulfilled the requirements for brazing to tool steel 1.2343. Success was attained with the complete process chain of generating and joining wear-resistant inlays.