Cast iron structure and property variation between pearlitic and spheroidal (nodular) grey cast iron.

Background

Most grey cast irons consist of graphite, the free carbon form, in a steel-like (ferrite and some pearlite) matrix. In ordinary grey cast iron the carbon separates as graphite flakes which act as internal notches; they lower strength and ductility and give low impact resistance, although they give excellent machinability and damping.

Pearlitic grey cast iron

In this form the graphite is present as coarse flakes or lamellae dispersed in a pearlitic matrix (alternating lamellae of ferrite and iron carbide/cementite). The flake graphite interrupts the metal matrix so there is little plastic deformation; the material fractures in a brittle manner. Its tensile strength is low (about 100-150 MPa), ductility/elongation is very small, but it has excellent compressive strength, very good damping/vibration absorption, good machinability (graphite acts as a self-lubricating chip breaker), good abrasion resistance, low cost and good casting "fluidity" (graphite flakes promote good melt flow and reduce shrinkage). It is used for engine bed plates, cylinder blocks, liners (exposed to wear), brake drums, pumps and frames. The graphite gives self-lubrication and good thermal and frictional properties.

Spheroidal (nodular/dutile) grey cast iron

Here the graphite is precipitated as spheres (nodules) by inoculation, for example with magnesium or cerium, so the metal matrix is nearly continuous around the graphite. Because the graphite no longer acts as sharp internal notches, the matrix can deform plastically, giving much higher tensile strength (400-800 MPa), real ductility/elongation (10-20%), good fatigue resistance, impact toughness and shock resistance, while retaining the cheap castability of cast iron. It has lower damping than flake iron. It is used where shock and fatigue are a concern, e.g. crankshafts of small/large marine engines, camshafts, gearbox parts, and components subjected to impact and cyclic loading.

Theory of production / treatment

Pearlitic grey iron: made by casting a hypereutectic/ordinary grey iron melt slowly so the carbon separates as graphite flakes during cooling; a slow cooling rate through the eutectic range and a phosphorus-carbon eutectic permits the flakes to grow. No inoculant is added, so the flake structure develops naturally.

Spheroidal grey iron: obtained by inoculation and slight modification - adding small quantities of magnesium and/or cerium (spheroidising elements) to the melt just before pouring, and/or by magnesium nodularisation. The inoculant provides nucleating sites so the graphite precipitates as compact spheres instead of flakes. Careful cooling and control of silicon/sulphur content are also used. The matrix may be heat treated (normalised or annealed) to control ferrite/pearlite.

In both cases the "steel-like matrix" means the metal part between the graphite can be pearlite, and its properties combine with the graphite form to give the differing behaviour described.