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Metal Casting Technologies : September 2007
62 www.metals.rala.com.au ABSTRACT ompacted Graphite Iron (CGI) has become the alloy of choice for automotive diesel engine blocks and other specialty castings. The increased strength and ductility combined with thermal conductivity, vibrational damping properties, and machinability provides a unique set of properties suited for demanding applications. However, CGI requires a level of process control different than that needed for either gray (lamellar graphite) or ductile (spheroidal graphite) irons. One concern with CGI is the potential to form a "flake skin" with lamellar graphite at the mold/metal interface. This can decrease strength and ductility to an extent far greater than the relative thickness of the skin because the flakes can act as stress concentrators and crack initiation sites. Methods are needed to control flake skin formation in CGI, either to minimize its formation or to control the depth in critical areas. However, the development of control methods depends on understanding the mechanism and factors affecting skin formation. This in turn requires a reliable measurement method for evaluating the depth and severity of the flake skin. Experiments were conducted with a standard test casting to evaluate flake skin formation. Measurement and evaluation are more problematic than originally expected. This paper reports the test methods, initial results and difficulties in developing a reliable methodology for measuring and controlling flake skin formation in CGI. INTRODUCTION Known for more than half a century, compacted graphite iron, a member of the cast iron family, has had little presence in the world of engineered metals. This was because of the difficulty in microstructure control and stability. However, with advancements in process control technology, CGI can now be reliably produced in high volume manufacturing environments. Many foundries worldwide are producers of CGI castings. The automotive industry places increasing demand on CGI, it has become the preferred material in diesel engine cylinder blocks for passenger vehicles and light trucks because of combined high strength at reduced weight. Other applications include brake discs and drums, exhaust manifolds, cylinder heads, flywheels and gear covers. CGI is an interesting material with physical properties between those of gray and ductile irons. The shape of the graphite particles distributed in the iron matrix adds complexity to the system. Contrary to the elongated and randomly oriented flake graphite particles of gray iron (Figure 1a) and the spheroidal geometry of the graphite in ductile iron (Figure 1c), the graphite in compacted iron (Figure 1b) when viewed in 2-D appears as worm-like or vermicular shape, shorter and thicker with rounded edges. SEM images show indeed that the vermicular graphite particles are highly interconnected and form a coral-like morphology. This intricate graphite growth is responsible for the CGI properties. Under stress conditions gray iron is described as a brittle metallic material with virtually no plastic deformation and CGI as a quasi-brittle material since it exhibits little ductility and very little elongation (about 2-3%). Hsu et al. published fracture toughness, KIC, data for both types of cast irons. Experimental measurements indicate that KIC values for as-cast samples are about 3-4 times greater for CGI.2 Fracture surfaces are described as large cleavage planes and river patterns for flake graphite, while more fibrous with observable dimples and ductile tearing for compacted graphite. This explains the differences in mechanical characteristics. Table 1 shows average mechanical and physical properties for the gray and ductile irons relative to CGI. Beyond the aspects already discussed another one of importance is the slight differences in chemistry. Sulfur and magnesium contents are critical for the microstructure stability of this family. Sulfur levels for gray irons typically range between TECHNICAL FEATURE Figure 1. SEM images of a) gray iron, b) CGI and c) ductile iron1 C By Flávia C. Duncan* and Jörg Kroker, Ashland Inc"., Dublin, OH, USA Difficulties in controlling and measuring flake skin in CGI Property FG CG SG Ultimate Tensile Strength 55% 100% 155% Yield Strength - 100% 155% Elastic Modulus 75% 100% 110% Fatigue Strength 55% 100% 125% Hardness 85% 100% 115% Damping Capacity 285% 100% 65% Thermal Conductivity 130% 100% 75% Fracture Toughness 30% 100% - Table 1. Properties of cast irons3 FG: flake graphite, CG: compacted graphite, SG: spheroidal graphite