Thermal Conductivity Analysis of Compacted Graphite Cast Iron After a Creep Test

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COMPACTED graphite cast iron (CGI) has excellent tensile strength and heat-transferring properties.[1,2] Because of the outstanding mechanical and physical properties, CGI is suitable for complex-shaped components[3,4] and has gradually become an essential manufacturing material for advanced engine combustor parts, such as the cylinder head.[5–8] With an increase in the power density in engine combustors, the cylinder head endures a high temperature, which requires high-temperature strength and thermal conductivity for thermal fatigue during service.[9–11] Recently, numerous studies have evaluated the mechanical properties and thermal conductivity of cast iron as a material for diesel heads. Selin[12] and Qiu[13] investigated the tensile property of CGI at elevated temperatures. They showed that the tensile strength initially decreases slowly and then rapidly with an increase in temperature. Microcracks easily initiate at the graphite/matrix interface and then propagate along the graphite, which results in failure. Because of its

WU YUE, LI JIANPING, YANG ZHONG, GUO YONGCHUN, MA ZHIJUN, LIANG MINXIAN, YANG TONG, and TAO DONG are with the Shaanxi Province Engineering Research Centre of Aluminium/Magnesium Light Alloy and Composites, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, P.R. China. Contact e-mail: [email protected]. Manuscript submitted November 2, 2018. Article published online May 22, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A

brittleness, graphite often contains natural microcracks in the microstructure, negatively affecting the mechanical properties. However, graphite provides an outstanding heat conduction path because of its excellent thermal conductivity. The thermal conductivity of graphite depends on its growth direction, sizes, and shape. Holmgren et al.[14] estimated the relationship between the graphite growth direction and thermal conductivity of cast iron. They stated that the thermal conductivity of flake graphite is better than that of vermicular graphite followed by nodular graphite, because heat diffuses faster in the a-axis direction of graphite than in the c axis. Hecht et al.[15] also showed that the thermal diffusivity of cast iron increases with the size of A type flake graphite. In addition, Ma et al.[16] and Liu et al.[17] analyzed the effect of the graphite 3D structure on the thermal conductivity. Ma stated that the thermal conductivity of CGI increases with the vermicularity by an FE model. Liu determined that interconnected graphite in the 3D structure improves the heat conduction of the cast iron when the graphite is less spheroidal. Moreover, the casting processes and alloying element level can also affect the thermal conductivity. Holmgren et al.[18] showed linear relationships between the thermal conductivity and carbon content, carbon equivalent, and fraction of pearlite transformed from the former primary solidified austenite. The best thermal conduction property corresponds to a medium cooling rate. Maluf et al.[19] demon

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