...About Steel and Iron

In gray and Ductile Irons, carbon in excess of its solubility in solid iron is present as finely dispersed graphite shapes, rather than as Fe3C (cementite, iron carbide). In GRAY CAST IRONS the graphite flakes act as stress raisers and under stress help crack propagation (See illustration A). As a result, gray cast irons are weak, with ultimate tensile strength is the order of 20,000 to 58,000 p.s.i. (150 to 400 mPa) and with practically no elongation. The size of graphite flakes varies with production conditions and, also, with casting thickness. Normally flake lengths are from 0.1 to 1.0 mm.

CAST IRON WITH SPHEROIDAL GRAPHITE (i.e. Ductile Iron) became an industrial reality in 1948. A suitable treatment of the molten iron causes the graphite to precipitate as spheroids, rather than flakes. The nearly spherical shape of the graphite removes the "crack" effect and, in fact, the graphite spheroids act as "crack-arresters", as shown impressively on the page (See illustration B).