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Metal Casting Technologies : September 2008
TECHNICAL FEATURE Compacted Graphite Irons on the Move By Dr. John Pearce The need for continual improvements in fuel effi ciency and environmental performance has led to accelerating production of Compacted Graphite Irons (CGI) for cylinder blocks and heads for advanced automotive diesel engines . A lthough at present no foundries in Thailand are currently producing CGI they have many questions about how it is produced, and on its properties and other potential applications. Of special interest to motor racing fans here is that all cars racing in the NASCAR series have CGI blocks  and that, with a CGI engine, the Audi R10 TDI has scored a hat trick of wins in the Le Mans 24 hour race . This short review therefore looks back at the reasons why, after a very slow start, CGI is now moving at high speed. Characteristics of compacted graphite irons Compacted Graphite Irons contain eutectic graphite whose morphology is intermediate between that of the flake and nodular forms. Compacted graphite structures were first studied some sixty year ago as a result of their unintentional production during the initial development of nodularizing treatments for Ductile Iron production. It was found that compacted graphite was obtained when there was insufficient residual Magnesium or Cerium in the liquid iron to completely nodularize the graphite during solidification. Since then a number of compacting treatments have been developed based on the use of Magnesium, Rare Earths and Magnesium + Titanium, etc. Compacted graphite can also be produced when grey irons contain high Nitrogen contents. The typical microstructure of CGI is shown in Figure 1. Because of its appearance in two dimensions compacted graphite is often described as “vermicular”, which means worm-like. During early work on cerium treated irons the term “quasi-flake” was also used to describe compacted graphite. SEM examination of deep etched specimens reveals that compacted (vermicular) graphite grows in eutectic cells in which there is a continually branched graphite structure as in normal flake graphite irons. However the flakes in the compacted structure are shorter and thicker than those formed in normal grey irons and significantly their ends are not sharp but are rounded. In three dimensions the compacted graphite structure is more like coral rather than worm-like but the term vermicular is still used to describe this form of graphite. Because of their rounded but still interconnected graphite structures compacted graphite irons have mechanical and physical properties in between those of flake graphite (FC) irons and ductile (FCD) irons. The tensile strength of CGI is less dependent on carbon equivalent than FC irons. As for UTS the fatigue strengths of CGI are also better than those of FC grades but inferior to those of FCD. Mechanical property requirements from the most recent ISO 16112:2006 Standard cover grades with minimum tensile strength levels from 300 to 500MPa. CGI is also covered by ASTM standard A842-85 and SAE standard J 1887. As for FC and FCD irons most of the major vehicle producers now have internal company standards for CGI. In spite of their inferior mechanical properties FC irons are superior to FCD irons with respect to their damping capacity, thermal conductivity and machinability. FC irons also have better founding characteristics as FCD presents greater problems in running and feeding complex shaped castings. Damping capacity, the ability to dampen vibration and noise, and thermal “THE TENSILE STRENGTH OF CGI IS LESS DEPENDENT ON CARBON EQUIVALENT THAN FC IRONS.” 60 www.metals.rala.com.au