gray cast irons

Alloys of iron, carbon and silicon in which more carbon is present than can be retained in austenite. The carbon in excess of austenite solubility in iron precipitates as graphite flakes. Approximate composition of gray irons is: 2.5 to 4.0 percent carbon, 0.5 to 1.0 percent manganese, 1.0 to 3.0 percent silicon, 0.05 to 0.15 percent sulfur and 0.05 to 0.8 percent phosphorus. Some Society of Automotive Engineer grades are G-1800, G-2500, G-3000, G-3500 and G-4000.