Production of a Powder Metallurgical Hot Work Tool Steel with Harmonic Structure by Mechanical Milling and Spark Plasma
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GRAIN refinement is a well-accepted method to improve the mechanical properties of metallic materials. However, grain refinement to nanosized (NC) regime generally results in poor ductility because of early plastic instability.[1,2] In samples fabricated from powders for instance, processing related defects such as residual porosity, impurities, and incomplete consolidation even if the full densification is achieved will also cause early plastic instability.[1,3] Moreover, according to a very limited data in the literature, an overall decrease in fracture toughness by refining the microstructure down to ultra-fine grained (UFG) or nanosized regime is observed.[4,5] Therefore, efforts, including the fabrication of bimodal grained microstructures, have been made to fulfill the requirements for ductility and strength in structural parts.[6,7] One of the most promising methods, providing the achievement of the so-called ‘‘harmonic microstructure,’’ has been introduced by sekiguchi et al.[8] This Powder Metallurgy (PM) route involves the severe plastic deformation on the surface of powder particles to a certain depth by controlled mechanical milling and subsequent fast consolidation. The method thus allows the development of an interconnected network of UFG microstructure (Shell) surrounding a coarse grained
FARAZ DEIRMINA, MASSIMO PELLIZZARI, and MATTEO FEDERICI are with the Department of Industrial Engineering, University of Trento, Via sommarive 9, 38123 Trento, Italy. Contact e-mail: [email protected] Manuscript submitted September 15, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
(Core) matrix. The mechanical properties strongly depend on the volume fraction and distribution of UFG (Shell). While the UFG shell provides high strength, the core is responsible for good ductility, so that a controlled shell to core ratio provides an excellent properties combination.[9] In most cases, the data reported in the literature is limited to pure metals (i.e. Ni, Cu and Ti) and relatively low hardness alloys, most of these alloys show easy to deform FCC crystal structure (e.g. stainless steel).[9–11] Dislocation interactions in materials with a BCC structure is accompanied with the formation of immobile dislocations, crack nucleation and finally brittle fracture of the powders.[12] So, in the case of BCC structured materials, controlling the volume fractions in harmonic structures can alternatively be achieved by mixing of the severely deformed high energy ball milled UFG particles and the as received powders. Hot work tool steels with tempered martensitic microstructure strengthened by the dispersion of fine secondary carbides show high hardness and good wear resistance, and are frequently being used as structural parts. Therefore, a lot of effort has gone into elaborating on the relationship of microstructural and chemical parameters and their subsequent effects on mechanical properties such as strength, toughness and thermal fatigue resistance of the tool material.[13–15] PM production route generally facilitates the achie
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