According to the European standard DIN EN 1561, grey cast iron (EN-GJL), also known more simply as “gray iron”) is a carbon-containing iron material; the carbon present in the material is almost entirely present as flake graphite (in lamellar form) in the microstructure. The letters “GJL” in the German designation stand for G = Gusswerkstoff (= cast material); J = (I) iron; and L = lamellar. In the past, the denominations GG or Grauguss (lit: gray-cast) were commonly used in the German-speaking world. The graphite is present in a 3-dimensional structure which can be imagined as looking something like a bundle of salad leaves. When microsectioned, these “leaves” (flakes) then appear as plates or lamellae.
Gray iron with lamellar graphite has properties which make it an excellent choice for casting, the key term here being: freedom of design. This suitability for casting allows extremely economical production of components, as grey cast iron is the least expensive cast iron material. However, it must be admitted that it also possesses – apart from its unmatched damping ability – the comparatively worst mechanical properties among the cast irons.
It has both fairly low strength (between 150 and 300 N/mm2), and is extremely brittle – a property which many an experienced Master Caster has made use of in memorable demonstrations for the benefit of new apprentices. A thin-walled gray iron casting is picked up, the master takes a sniff, and says, “Yup, smells like gray iron” – before tossing the casting high up in the air and watching it shatter on the concrete floor of the workshop in front of the wide eyes of the startled apprentice. Grey cast iron it is – SG cast iron would have survived such treatment looking a bit “beat up”, at worst. Proof of the keenly trained nose of a Master Caster, who can smell the difference after so many years! (Especially after glancing at the tag on the material beforehand)…
The comparatively low strength of cast iron with flake graphite can be explained by its heterogeneous microstructure. The graphite precipitates during solidification and forms thin veins throughout a pearlitic or ferritic matrix (depending on how quickly the material is cooled). These thin veins act as internal notches, which weaken the matrix, and at the tips of which internal stresses peak.
The combination of these effects produces a marked decrease in strength compared to SG cast iron. However, the (for all practical purposes) incompressible flakes of graphite can support higher compressive loads – the ultimate compressive strength is some 3 to 4 times higher than the tensile strength.