R -Curve Approach to Describe the Fracture Resistance of Tool Steels
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TOOL steels for cold forming and shearing applications are typically processed to develop microstructures with large volume fractions of primary carbides embedded in a tempered martensite matrix. They are produced by either ingot cast or powder metallurgy routes followed by hot working, and such microstructure is attained after heat treatment. Alloy carbides are produced by solidification and coexist with austenite during hot working and austenitizing. Hot working breaks up the segregated solidification structure. However, alloy carbides are stable during this processing stage; thus, they are elongated and dispersed in bands oriented along the forging direction. The slow cooling of the conventional static cast ingot drives formation of coarse eutectic carbide structures. As these are difficult to break down during hot working, non-uniform microstructures with marked anisotropy are the final result of the ingot cast metallurgy route (IM). Contrarily, powder metallurgy (PM) tool steels show fine and uniform distribution of carbides as a consequence of rapid solidification during atomization. Hence, manufacturing process INGRID PICAS, formerly Researcher with Fundacio´ CTM Centre Tecnolo`gic, Plac¸a de la Cie´ncia 2, 08243 Manresa, Spain, is now R&D Manager with ICL-Iberia, Afores s/n, 08260 Su´ria, Spain. DANIEL CASELLAS, Director of Materials Technology Area, is with Fundacio´ CTM - Centre Tecnolo`gic. Contact e-mail: daniel. [email protected] LUIS LLANES, Professor, is with CIEFMA Departament de Cie`ncia dels Materials i Enginyeria MetalÆlu´rgica, Universitat Polite`cnica de Catalunya, ETSEIB, Av. Diagonal 647, 08028 Barcelona, Spain. Manuscript submitted September 9, 2015. Article published online April 1, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
dictates important aspects of the microstructure of tool steels, which in turn controls their mechanical properties and tool performance characteristics. Microstructural effects on fracture, fatigue, and wear behavior in tool steels have been extensively addressed in the literature during the last three decades.[1–12] The role of carbide size, volume fraction, and carbide distribution on fracture properties of tool steel was first studied by Horton and Child.[1] They show that primary carbide nucleates the initial crack and that distribution of carbide clusters affects the crack path through metallic matrix areas, besides determining the quantity of plastic work expended in fracturing the metallic matrix. According to these findings, the fairly superior fracture toughness observed in ingot cast tool steels compared to PM steels was explained by the relatively smaller mean free path between carbides in the latter. Owing to the smaller ligament sizes in PM steels, microcracks formed at or in carbides link up with a minimum of plastic deformation of the matrix.[1] Aimed to understand the different fracture events in tool steels, numerous efforts have been made to determine the stress field in the tempered martensite matrix at the neighboring of alloy carbides, and to determine
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