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Aciers rapides produits par métallurgie des poudres. État de l’art et perspectives
Processing through powder metallurgy becomes the only feasible way for the most highly alloyed of high speed steels (HSS), with carbon contents between 0.7 wt % and 2.5 wt% containing substantial amounts of carbide formers W and/or Mo, V, Cr.
The basis of the project of the European collaborative study to arrive at the definition of new grades of HSS that meet the sintering temperature through “computer assisted design of sinter able tool steels”, is presented..
AISI M2 powders were produced from the elements by mechanomaking, a room temperature process driven by high energy ball milling. The as-mechanomade powders with relative densities 0.5, were consolidated by cold and then hot isostatic pressing. Fully dense solids with carbide sizes around 20 nm are obtained after Hot Isostatic Pressing (HIP) at 950°C, while the matrix is constituted of a very fine array of crystallites below 10 nm. An as-quenched hardness of 69 HRC was obtained and a Hall-Petch behavior was observed for Vickers hardness. No evidence of residual austenite was found in either as-HIPed or quenched samples.
High speed steel AISI M2 was obtained by high energy milling from the elemental powders. The kinetics of synthesis were followed by Mössbauer spectroscopy and X-ray diffraction. About 14 h are required to complete alloying (12 at.% solute content) of the matrix and formation of carbides (M6C and Fe3C) all having grain sizes below 20 nm. The mean particle size was 50 μm and the individual particles were almost fully dense. Limited grain growth to <100 nm at temperatures up to 1300°C was observed. This new concept for the fabrication of alloys and materials is suggested to be termed mechanomaking.
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