Development of materials with high resistance against abrasion wear has been the target of extensive investigation and scientific research work. The mining industry is one of the fields that use abrasion resistant materials. These materials usually consist of a high volume fraction of hard ceramic phases embedded in a tough metallic matrix and can only be produced by the powder metallurgy route via hot isostatic pressing (HIP). In this work, the products of alternative manufacturing procedures are investigated. Powder sintering and direct hot extrusion of encapsulated mixtures of iron base tool steel and hard ceramic phase are presented. Hot direct extrusion enabled the production of particulate reinforced tool steel metal matrix composites (PRM) with up to 30% of fused tungsten carbide (FTC, WC/W2C) or titanium carbide (TiC). The selected matrices are commonly used hot (AISI-H13) and cold (AISI-D7) work tool steels. Plasma transferred arc welding (PTAW) is also used as a route for producing wear resistant coatings with FTC particles and H13 matrix on a substrate. The behaviour of tungsten carbide is studied in the mixture with carbonyl iron powder (CN), graphite and elemental boron during sintering. The manufacturing processes are explained and resulting microstructures are presented.
Particle distribution and their reaction with tool steel matrices are studied by optical microscopy, scanning and transmission electron microscopy. X-ray diffraction (XRD) is used to determine the phases in the initial powder and the extruded samples. Wear resistance of the extrudate and the welded coating is studied according to ASTM G-65 dry sand rubber wheel abrasion test in laboratory air at room temperature.
Optical microscopy reveals a nearly homogeneous distribution of FTC and TiC within the core of the extrudates. Alignment of FTC and TiC hard phases is seen in the extrusion direction. High heat input in the PTAW results in massive dissolution of FTC particles in the matrix and decreases the volume fraction of hard particles. Quantitative analyses of the phase volume fractions in the extrudates are done based on Rietveld refinements of the synchrotron X-ray diffractograms. Both extrudates with cold work and hot work tool steel matrices showed 13-30 vol.-% retained austenite together with different carbides like chromium rich M7C3, M23C6 and, in the case of cold work tool steel, vanadium rich MC phases. Extrudates with FTC hard phases besides WC and W2C also contain 4 to 17 vol% of Eta-carbide Fe3W3C. The Eta-carbide forms around the FTC hard phases, because of interdiffusion with the metal matrix at elevated temperatures. The thickness of the diffusion zone around the FTC particles depends on the temperature and time of processing. In composites containing TiC particles, no diffusion zones around the TiC particles are detected. In the PTA welded coating, this rim is significantly smaller than on the extrudates. No diffusion zone is visible around the tungsten carbide particles in the sintered mixture with carbonyl iron powder where Eta-carbides are formed at the grain boundaries as eutectic from melting with austenite. Boron acts as a liquid phase sintering agent and helps in densification of samples and decreasing the porosity. Comparing wear mass losses of the extrudates with the PTA welded coating showed higher wear resistance of the extrudates. The reason is explained by SEM images of worn surface, which shows thicker Eta-carbide rims around particles in the extrudates acting as a binder to keep the hard particles in the matrix. Furthermore, the relevant volume fraction of hard particles consists of the remaining FTC particles plus the Eta-carbide interface reaction product. Both are smaller in the PTAW coating