19 International Journal of Metalcasting/Fall 09 ON THE MECHANISM OF CASTING SKIN FORMATION IN COMPACTED GRAPHITE CAST IRON S. Boonmee and D. Stefanescu The Ohio State University, Columbus, OH, USA Copyright © 2009 American Foundry Society Abstract The mechanical properties of ductile iron (DI) and compacted graphite iron (CGI) iron are measured and reported on standard machined specimens (as per ASTM). However, most castings retain most of the as-cast surface. This surface layer (the casting skin) includes both surface and subsurface features. Because of the casting skin, the mechanical properties of the part are in most cases significantly lower than those found on the standard ASTM machined specimens. After reviewing the current understanding of the current understanding of the mechanism of formation of the casting skin in cast iron, the authors propose a magnesium diffusion theory of skin formation in compacted graphite cast iron that includes the role of natural convection during solidification. The theory is supported by a computational model and experimental measurements of compositional changes at the casting surface. Keywords: compacted graphite, casting skin, graphite degradation, ferritic rim, pearlitic rim Introduction Mechanical properties of most metallic materials are test- ed on standard machined specimens. This is true for duc- tile iron (DI) and compacted graphite iron (CGI). How- ever, DI and CGI castings are typically used with as-cast surface and minimum machining. It should be noted that this as-cast surface contains both surface and subsurface features such as surface roughness, graphite degradation, ferritic rim and pearlitic rim. The term casting skin is used to refer to these features. The presence of the cast- ing skin commonly deteriorates the mechanical properties of DI and CGI. As a result, lower mechanical properties than prescribed by the ASTM standard are expected when significant casting skin is retained on the part. Therefore, the effect of casting skin on mechanical properties of DI and CGI should be investigated, the mechanism of cast- ing skin understood, and methods to prevent or minimize casting skin developed. The occurrence of a decarburized skin on gray iron castings was documented as early as 1962. Reisener 1 attributed this phenomenon to the interaction of the casting surface layer with the moisture in the mold, which results in the formation of carbon dioxide and hydrocarbon gasses at the metal/mold interface. These gasses then escape through the molding material. These chemical reactions could lead to the deple- tion of carbon in the casting skin. The effect of moisture can be minimized using pyrophyllite (an aluminum silicate hydroxide - Al 2 O 3 ·SiO 2 ·H 2 O) as a mold coating. Pyrophyllite expands at high temperature and produces a dense interfacial barrier layer. It is widely accepted that the depth of the graphite-free layer on the casting surface is generally dependent on section size. 1,2 Several processing parameters affect the depth of graphite- free layers. According to Rickards 1 the depth of graphite-free layer depends linearly on the square root of time in the tem- perature range 1050 - 1150°C (1922 - 2102°F). This implies that diffusion is the control mechanism for this phenomenon. Apparently, carbon equivalent has a small effect on the depth of graphite-free layers. Use of coal dust in the molding sand had been suggested for minimizing the depth of graphite-free layer. The coal generates a carbonaceous (reducing) atmo- sphere which counteracts the decarburizing effect. Narasimha and Wallace 3 showed that a ferritic layer is to be expected on the surface of commercial gray iron castings. Increasing the Si content may result in a higher amount of ferrite in the casting skin. Again, seacoal in the green sand was found to reduce the ferritic rim in gray iron casting. While the formation of a ferritic rim and graphite depletion in gray iron have been extensively investigated as described, less information is available in the literature on the nature, effect and mechanism of casting skin formation in DI and CGI. Some recent findings by a number of researchers have been summarized by Stefanescu et al. 4 Furthermore, the pa- per presented a methodology for the quantification of the casting skin and proposed mechanisms for graphite degrada- tion and depletion in the casting skin. In addition, depletion of Mg near the surface of CGI was proposed as a possible cause for graphite degradation. A diffusion based model that supports the proposed mechanism was also introduced. It is reviewed in some detail in the following section.