W.D. Schubert

Hardness to toughness relationship of fine-grained WC-Co hardmetals

The hardness to toughness relationship of fine-grained WC-Co hardmetals was studied based on Palmqvist indentation toughness measurements. Sixty-five commercial and lab-sintered hardmetals of different composition, microstructure and manufacturing history were investigated to build up a representative hardness/toughness measurement band. This band is then used to discuss the influence of the various alloy- and process-related parameters on the hardness to toughness relationship of WC-Co composites. Beyond that, optimal hardness/toughness combinations can be assessed for the hardness range of 1400–2200 HV30. In general, the higher the hardness of the alloys, the longer were the indentation cracks, indicating a decrease in fracture toughness with increasing hardness. However, at a certain hardness, the toughness of individual alloys varied significantly. For example, at HV30:1670, the sum of crack lengths varied between 287 μm (high toughness) and 449 μm (low toughness), which corresponds to fracture toughness values of 11.5 and 9.2 MNm−32, respectively. Very fine-grained hardmetals (ultrafine grades) were shown to be not necessarily tougher than coarser grained alloys (submicron grades), in particular in the hardness range of 1450–2000 HV30, although they exhibit significantly more binder at a given hardness. Only in the high hardness range of > 2000 HV30 might they be of advantage. Samples, exclusively doped with Cr3C2 as growth inhibitor exhibit more favorable hardness/toughness combinations than comparable VC-doped alloys. However, other parameters, such as sintering temperature, sintering time, or the gross carbon content of the respective alloys must be taken into consideration for obtaining optimal hardness/toughness combinations.

General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production

Abstract Ultrafine WC/Co hard metals (average WC grain sizes ≤ 0.5 μm) can be successfully and reliably obtained by conventional hard metal manufacturing techniques. In this paper, some of the crucial aspects of conventional powder manufacture, powder milling and liquid phase sintering are discussed. Conventional ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization can be carried out. However, inherent to the powder processing techniques used and the particle growth mechanisms involved (oxide precursors used, reduction and carburization history), there exists a lower limit beyond which-finer WC powders cannot be produced. This limit lies in the particle size range of 50–150 nm (0.05–0.15 μm). Powder milling is carried out to obtain an even dispersion of the Co binder in the ultrafine WC matrix. The more uniform the phase distribution (WC, Co, grain growth inhibitor) within the green powder compact, the more uniform will be the material transport during sintering, and hence the uniformity of the WC grain growth/growth inhibition during sintering. Enhanced WC grain growth occurs early in the sintering cycle, even below the temperature at which the liquid phase is formed. This growth can be largely restricted by the addition of VC. However, effective grain growth inhibition has to take place already during this early period of solid-state sintering. The ‘early’ availability of the grain growth inhibitor at the WC/Co interface can, therefore, determine the degree of growth inhibition. Ultrafine hard metals are in particular prone to discontinuous grain growth of the WC. Different reasons for this local growth mode are propounded relating to both the chemical as well as the geometrical departures from uniformity in the green powder compact. While it is still not possible to predict exactly an ultimate WC grain size limit, below which WC grain growth can no longer be restricted, even with proper inhibitor additions, experimental evidence indicates that this average WC grain size limit lies in the range of 200–300 nm. This limit is inherent to the existing conventional processing techniques (powder manufacture, milling, liquid phase sintering) and the WC growth mechanisms involved and can be overcome only by establishing a completely new route in hard metal manufacture.

On the reduction of NS-doped tungsten blue oxide—Part II: Mechanistic considerations

Abstract Part I of this paper gave a review of the literature. A detailed description of experimental observations on the influence of the NS-dopants (K, Al, Si) on the hydrogen reduction of tungsten blue oxide is given in Part II. Alterations in the suboxide sequence as compared to undoped oxides, intermediate formation of β-W, and W separation from dopant–T-O compounds (bronzes, tungstates) during the transition WO2→W were observed. Incorporation of the dopants, as necessary for the development of non-sag properties in the final wire, occurs via CVT overgrowth of dopant phases in the WO2→3W step. Particle size of those phases increases with increasing reduction temperature.

Dopant incorporation during technical reduction of K,Al,Si-doped tungsten blue oxide

Abstract The basic ability to form a long-grained interlocked non-sag structure in tungsten wire is intrinsic to the tungsten-metal powder used for the wire production. The tungsten-powder properties are, however, to a large extent a function of the reduction parameters. This paper discusses how the various parameters involved in the NS-doped blue oxide reduction affect specific metal-powder properties: grain size, the amount of incorporated dopants, and composition and size of the incorporated dopant inclusions. A model of the dopant incorporation is proposed that agrees with experimental observations and explains the individual actions of the doping elements K, Al, and Si.

On the reduction of N.S.-doped tungsten blue oxide—Part III: Technological considerations

Abstract Part I of this paper gave a review of the literature, Part II presented a mechanism for the doping element incorporation during reduction and Part III now discusses technological relationships between the reduction parameters and the N.S.-W powder properties. Under isothermal reduction conditions, dopant incorporation is affected predominantly by the reduction temperature (decreasing with increasing temperature), while grain size is influenced by both temperature and boatload. During temperature profile reduction (industrial practice) all parameters influencing the relationship between actual reduction temperature and reduction stage (WO X ) affect both grain size and dopant incorporation. Besides the absolute amount of incorporated dopants, the sizes and compositions of the incorporated dopant phases are affected as well, an important factor with respect to the further processing to high-quality non-sag wire.

Archaeometallurgical Simulations of the Processes in Bloomery Furnaces from the Hallstatt and Medieval Period

Bloomery furnaces were the first units for iron smelting. In the Hallstatt period small bowl-type furnaces were used and until the medieval period the size of such furnaces was increasing continuously. Experimental archaeologists reconstruct bloomery furnaces to study the processes of bloom production. In a small bowl-type furnace (Hallstatt period) at Asparn and in a larger shaft-type furnace (medieval period) at Ybbsitz smelting experiments were performed, The samples contained metallic iron and slag. Various amounts of iron in different stages of conglomeration up to larger iron pieces were found. The slag belongs to a fayalitic-type, consisting of wustite (FeO), fayalite (Fe2SiO4) and glass-phase (amorphous Ca-, Al-silicates) in various concentrations. The yield of metallic iron was highly different for the various experiments. In general, more metallic iron was formed in the larger shaft-type furnaces. A large bloom was not obtained.

Control of the Cobalt Surface Layer Formation on Cemented Carbides

The formation of a surface layer of cobalt on cemented carbides which occurs on cooling during sintering is an often observed phenomenon which has been discussed in the recent literature. The presented work shows different factors which influence the formation of the layer and proposes factor-related mechanisms. For this purpose cemented carbide samples with different compositions, WC grain sizes and carbon contents were produced and studied. The results reveal that besides the cooling conditions also the variations in composition and microstructure of the material play an essential role for the formation of a surface layer.

Direct phase identification with SIMS by evaluation of atomic and cluster ion intensities: A Study of calcium sulfide formation in hard metals during sintering

A SIMS method for the direct identification of micro and nanophases based on the quantitative evaluation of atomic and cluster ion intensities is described and applied to the study of the formation of CaS phases in hard metals.