Influences of the Graphite Growth Direction on the Thermal Conductivity of Cast Iron
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THE ability to transfer heat is important for many cast components, e.g., permanent molds and brake discs, where the heat has to dissipate quickly and the temperatures should be kept low to sustain dimensional stability and reduce internal stresses. Recently, the thermal conductivity of cast iron was reviewed.[1] The thermal conductivity of the cast iron matrix is considerably lower than the thermal conductivity of the precipitated graphite, and the thermal conductivity of gray iron is superior to that of spheroidal graphite iron due to the extended graphite flakes and the short matrix bridges between the eutectic cells. Furthermore, in gray irons, the dominating growth of graphite occurs in the a direction of the hexagonal crystal, which has considerably higher thermal conductivity than along the c direction. For the interested reader, a large amount of literature data regarding the thermal conductivity of graphite have been assembled in Reference 2. The common opinion is that the growth of lamellar graphite preferentially occurs along the basal planes, i.e., along the a direction, while spheroidal graphite grows in a radial manner in the c direction of the hexagonal graphite crystal. Chunky graphite is a degenerate graphite form, sometimes appearing in heavysection spheroidal graphite iron castings. Chunky DANIEL HOLMGREN and INGVAR L. SVENSSON, Professors, are with the Department of Mechanical Engineering/Component Technology, Jo¨nko¨ping University, Jo¨nko¨ping, Sweden. Contact e-mail: [email protected] RIKARD KA¨LLBOM, Postdoctoral Student, is with the Swedish Foundry Association, Jo¨nko¨ping, Sweden. Manuscript submitted May 16, 2006. 268—VOLUME 38A, FEBRUARY 2007
graphite is interconnected within eutectic cells of sizes similar to the eutectic cells of gray iron.[3] Some authors consider that chunky graphite is similar to compacted graphite with some typical distinctions: (1) local appearance and (2) finer structure.[4] Models describing the growth of compacted graphite have been presented and the growth direction of compacted graphite was believed to alter between the c and the a directions.[5,6] In contrast, other authors consider that the growth of compacted graphite occurs mainly along the c direction.[7] Different theories concerning the formation and growth of chunky graphite exist.[3] Liu et al. found that intermediate graphite forms occurred in the boundaries between regions consisting of spheroidal graphite and chunky graphite.[8] Chunky graphite was therefore regarded as a deteriorated form of spheroidal graphite, and similar to a theory presented by Gagne´ and Argo,[9] the growth mechanism of chunky graphite was regarded as occurring in a spiral manner around the central axis of the basal planes. According to Itofuji et al., the crystallographic substructures of spheroidal, compacted, and chunky graphite can be regarded as basically the same, consisting of stacked graphite layers, which individually have the a-axis of the hexagonal graphite crystal as the dominant growth direction.[10,11] More
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