Cast iron structure and the variation of properties between pearlitic and spheroidal grey cast iron.

Structure

Most grey cast irons consist of free carbon precipitating as graphite in a steel-like matrix (ferrite/pearlite). The form the graphite takes strongly governs the mechanical properties.

Pearlitic grey cast iron - graphite as flakes

In pearlitic grey cast iron the carbon appears as coarse graphite flakes in a pearlitic (ferrite and cementite lamellae) matrix. The flake graphite acts as internal cracks/notches which prevent the matrix from deforming plastically; hence the material fails in a brittle manner at low tensile stress.

Properties: low tensile strength (approx 100-150 MPa), very low ductility/impact resistance, but good compressive strength, excellent damping/vibration absorption, good thermal conductivity and self-lubricating machinability (graphite chip-breaker), good wear resistance, low cost and excellent casting fluidity/low mould shrinkage.

Uses: engine bed plates, cylinder blocks and liners, flywheels, brake drums, frame castings, pump bodies and foundations where vibration damping and machinability matter.

Spheroidal (nodular) grey cast iron - graphite as spheres

By inoculation (e.g. with magnesium/cerium) the graphite is made to precipitate as compact spheroids (nodules) rather than flakes. Because the rounded graphite no longer acts as sharp stress raisers, the metal matrix is nearly continuous and can yield plastically. Result: much higher tensile strength (400-800 MPa), genuine ductility/elongation, good fatigue and impact/toughness/resistance to shock, retaining low-cost castability.

Uses: crankshafts, camshafts, connecting rod type components, gearbox cases and parts subjected to impact, fatigue or shock loads.

Treatment to produce the two types

• Pearlitic grey iron: ordinary grey iron melt is cast and allowed to cool slowly so that during solidification carbon separates as graphite flakes; no spheroidising inoculant is added, so the natural flake structure forms. Slow cooling through the eutectic temperatures promotes flake growth.
• Spheroidal grey iron: a small addition of magnesium and/or cerium (spheroidising/inoculant elements) is added to the melt shortly before pouring, providing nuclei so the graphite grows as compact spheres; controlled cooling and correct sulphur/silicon control complete the process. Optional heat treatment of the matrix refines the pearlite.

Summary of differences: flake graphite = strong compression/damping, brittle in tension; nodular graphite = strong, ductile, tough, impact resistant.