Special interest sessions differ from normal conference sessions in that the papers presented are more tightly related to a particular theme. The participants are free, not only to discuss individual papers but also to make unconnected contributions to the subject. As with papers already previewed, edited abstracts of papers are given in their programmed order of presentation.

The Plansee Seminar takes place from  3–7 June, 2013 in Reutte, Austria.

Special Interest Session 1: 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 gained greatly in popularity since 2005, due to the increase in tungsten prices. Recent developments in hardmetal recycling, various recycling options and their market shares in Europe are reviewed and the future outlook is discussed. 

According to the author, a significant new development in recent years has been the emergence of the zinc process as a new recycling option. The basics of the zinc process will be reviewed and its current position in the recycling market discussed. (The “new” appellation seems a little odd, since the zinc recycling process has been established for many years and its drawbacks – notably the retention of impurities and difficult-to-control grain-size distribution - make its use for the highest-quality hardmetals dubious at least – KJAB)

Both laboratory and production-scale experiments were carried out, where WC-Co zinc reclaim powders were blended with virgin raw materials and the effect of such additions on powder processing and properties of sintered hardmetal investigated. Results of both sets of tests will be shown and discussed. 

The results are claimed to show that WC-Co zinc reclaim powders, when added in the right amounts and processed properly, have no negative effect on the properties of sintered hardmetal. Based on these results, various applications for zinc reclaim powders are discussed 

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. Large parts or very dense compacts, in combination with higher thermal heating rates, are vulnerable. This paper presents a model which describes chemical decomposition of organic binders and consequential development of internal gas pressure. It includes combined Maxwell-Stefan and Knudsen diffusion and seepage flow of multiple gaseous reaction products through a porous body. The model is implemented into the finite element program ABAQUS. Internal stresses developed by this gaseous overpressure are discussed, their prediction indicating critical temperature regions and helping 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 were 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 chain. Analysis of the results showed that WC grain size reduction with increasing milling time was characterised by a power-law behaviour. Evaluation of the effect of several process parameters on milling efficiency could be facilitated by simple scaling factors. The correlation between WC grain size after ball-milling and after sintering was also investigated. 

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

Milling techniques applied to powder samples vary in both energy and mechanical design, changing both powder and sintered compact properties accordingly. This study looked at these by comparing a wide variety of commonly employed milling operations, including ball milling, attritor milling, pin milling and rod milling, as applied to submicron WC. (Disappontinglyly, my personal preference, vibratory ball-milling, was not included – KJAB.) A wide range of physical, chemical and sintered properties were monitored for changes due to milling of submicron WC and WC-Co. From these studies, certain attributes such as densities, particle size distributions and sintered shrinkage could be directly related to milling time and type. Each mill style (combination of impact and shear) had 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 grain-size distribution. Interestingly, it was found that a perceptible amount of nitrogen dissolves in WC. As the diffusion rate of N in WC is extremely slow, nitridation occurs in the outermost rim of WC particles. With appropriate nitridation conditions the powders are even purified with respect to oxygen level and do not show particle growth. 

Sintering experiments investigated these powders for possible use in the preparation of hardmetals. It was found that nitrogen could retard low-temperature grain growth, to obtain hardmetals of reduced grain size. Mass-spectrometric de-lubing 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 with uniform nitrogen distribution and 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 in a two-step process to yield a mixture of cubic and hexagonal carbide. (W,Ta)2C powder was 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. Atom-probe tomography showed Ta solubility expressed as Ta/(Ta+W) to be as high as 0.086 (about 8.6 at%). In addition, it was found with 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 to be Σ4 grain boundaries. 

Special Interest Session 2: 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. Claimed in this paper to be “the only surface technology” permitting the production “of hardmetal coating solutions,” it would thus be complementary to sintering technology and applicable to large parts. 

The author’s survey reviews the development of these coatings, including processes, feedstock materials and technical features of state-of-art coatings. Hardmetal coatings are mostly based on WC, but also on TiC cubic hard phases and Cr3C2. Specifics of hardmetal coating compositions (large Cr3C2 additions, high binder contents) are considered. The need is outlined for fundamental research on metallurgical reactions during the spray process, for example the formation and properties of (W,Cr)2C. Microstructure/property relationships are discussed in relation to feedstock powder properties and the applied spray processes. 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 minimise wear, gas/oil and other mining industries employ very thick wear-resistant layers, currently produced by PTA (plasma transferred arc) techniques. The author explains that HVOF (high-velocity oxy-fuel) thermal spray coatings have advantages over PTA and other plasma coatings in wear resistance, based on microstructure formed at lower temperatures and higher particle velocities. This contribution discusses the microstructure and mechanical properties of WC/10%Ni HVOF coatings of up to 1000μm (1mm) thickness, using both densified and non-densified powders sprayed on rotating and stationary substrates. The resulting microstructures were comparable to typical HVOF coatings, with no cracks or large voids either in the coating or near the coating/substrate interface. Thick coatings had the same level of porosity (less than 0.5%), microhardness (~1030 HV300) and bond strength (~10ksi) as typical HVOF coatings in the 100 to 300µm range. 

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

The author will describe recently developed novel routes for the rapid preparation of ultrafine WC-Co hardmetals, with potential applications in industry. One route is the rapid synthesis of WC-Co composite powder with controllable particle size by in-situ reduction and carburisation using metal oxides and carbon black as raw materials. The powder products, said to have advantages in pure phase constitution, uniform particle size and homogeneous distribution of the binder phase, are available as high-performance coating materials, as well as powder intermediates for sintered compacts. The second route is a one-step method for bulk synthesis and reaction sintering in a spark plasma sintering system. This route can be employed to prepare dense nanocrystalline compacts, for example as a net fabrication technique for selected products. Preparation methods and mechanisms of rapid synthesis have been studied, and the prospects for commercial exploitation are discussed. 

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

Gas reactions are critical steps during the thermal treatment for the production of hard materials like WC-Co hardmetals or ceramics like liquid-phase-sintered SiC. Methods to study gas reactions include thermogravimetry, differential scanning calorimetry and mass spectrometry. Thermodilatometry, analysing length changes, can also give 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 results, it was found possible to predict thermal behaviour for any temperature/time profile or special mass change. This knowledge could be used to avoid problems like porosity, which critically influences such properties of the sintered material as strength and carbon balance. It was hoped that the newly developed process knowledge would lead to better cost efficiency, stabilisation of properties and improvements in technology. 

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

Even today, the author will explain, sintering of certain cemented carbide parts is carried out in push-type furnaces under hydrogen atmosphere. In this case the parts are embedded in a coarse-grained alumina/carbon fill. Some other grades are sintered in vacuum on a graphite tray coated by a fine suspension of alumina. 

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

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

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

Injection moulding tool materials like steel 1.2343 need to withstand different types of wear. Tool wear typically varies locally, increasing dramatically in areas of injection. Hence inserts of wear-resistant materials 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 wear-resistant alloys WC/Co 83/17 and WC/Co 88/12 were chosen for generation on steel substrates. A major challenge was the evaluation of process parameters to meet desired densities and avoid cracking. The influence of such parameters and weld trajectories on density was investigated. It was found that the key to achieving densities of >95% throughout the specimens was the combination of comparatively low scanning speed, large beam diameter and checker-board-shaped weld trajectories where single boards were made with overlap. Inserts fulfilled the requirements for brazing to tool steel 1.2343 and the complete process chain of generating and joining wear-protective inlays was realised.