Comprehensive Investigations of the Supersolidus Liquid-Phase Sintering of Two Plastic Mold Steels
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PLASTICS are subjected to combined loading by wear and corrosion during the extrusion process. Wear is mainly caused by additives, such as fibers, being added to reinforce the resulting material, whereas corrosion is due to the addition of acids or reaction products occurring during processing. The load spectrum is intensified by processing temperatures of up to 673.15 K (400 °C). Therefore, these materials must have adequate wear and corrosion resistance at elevated temperatures. For this specific application, the so-called plastic mold steels are used, which can be produced either by conventional means or by powder metallurgy (PM). Hot isostatic pressing (HIP) is a commonly used method of compaction in the powder metallurgical route. To provide corrosion resistance, plastic mold steels contain high amounts (>12 pct) of chromium dissolved in the iron lattice. A sufficient amount of carbon permits martensitic hardenability. The wear properties can be improved by the presence of hard phases, which significantly increase the wear resistance provided that H. HILL, S. SIEBERT and W. THEISEN, Chair Management, and S. HUTH, Group Leader, are with the Ruhr-University Bochum, Institute for Materials, Chair of Materials Technology, D-44780 Bochum, Germany. Contact e-mail: [email protected] S. WEBER, Group Leader, Ruhr-University Bochum, Institute for Materials, Chair of Materials Technology, is with Helmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH, D-14109 Berlin, Germany. Manuscript submitted May 4, 2009. Article published online January 30, 2010 686—VOLUME 41A, MARCH 2010
they are harder than the affecting abrasive particles and of similar size.[1] Hard phases typically found in these steels are iron-chromium carbides of the M7C3 type and vanadium-rich cubic monocarbides.[2] Generally, PM steels and wear-resistant metal-matrix composites (MMCs) can be processed to semifinished products either by sintering or by HIP. The powders of plastic mold steels are usually prealloyed and exhibit a low sintering activity, which hampers full densification by pressureless solid-state sintering only. In recent years, several articles have been published on the liquid-phase sintering of prealloyed tool steels by means of so-called supersolidus liquid-phase sintering (SLPS).[3–7] Compared to PM-HIP, which requires encapsulation of the powder, the benefits of sintering by SLPS are reduced costs, a greater flexibility with respect to the geometry, and the possibility of producing near-net-shape parts. The SLPS takes advantage of the fusion interval between the solidus and the liquidus and is carried out at a temperature that ensures a liquid-phase fraction of 20 to 30 pct arising from a partial melting of the material.[8] The SLPS can produce dense microstructures with marginal coarsening effects in tool steels.[9] The densification process starts with the formation of a liquid phase along the grain boundaries and at contact points between powder particles. As the amount of liquid phase increases, wetting of the interfaces occurs and the polyc
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