Question 6 of 7: Carbon Form and Morphology in Gray and White Cast Iron

Question VI: Carbon Form and Morphology in Gray and White Cast Iron (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

6.1 — (i) Carbon form and morphology in gray cast iron

In conventional gray cast iron, essentially all of the carbon above the matrix's own solubility limit is present as FREE, elemental graphite — not combined as a carbide. Morphologically it forms as coarse, interconnected flakes: during eutectic solidification, graphite (which is thermodynamically favoured over cementite at the slow cooling rates and moderate-to-high silicon levels typical of gray iron) nucleates and grows preferentially along its own basal (0001) plane, so each graphite particle grows fast in two directions and only slowly in the third, producing thin, curved, interconnected flakes radiating out from a common nucleus rather than a compact (spheroidal) particle. It is this flake morphology — sharp-tipped, plate-like graphite acting as an internal stress-raiser/crack-initiator network running through an otherwise ductile ferrite or pearlite matrix — that gives gray iron its characteristic brittleness (essentially zero tensile ductility) despite the matrix itself being reasonably ductile; the flakes are also what gives gray iron its name (a fracture surface exposes the graphite and looks visually gray).

6.2 — (ii) Carbon form and morphology in white cast iron

In white cast iron essentially NONE of the carbon exists as free graphite: it is instead entirely COMBINED, chemically bonded as iron carbide, cementite (Fe₃C), following the metastable Fe–Fe₃C system rather than the stable Fe–graphite system. Morphologically, the eutectic cementite solidifies as a continuous, interconnected network (ledeburite: cementite plus a pearlite/austenite-derived matrix) rather than as isolated particles, so the microstructure is a hard, brittle, continuous carbide skeleton enclosing islands of the transformed matrix. Because cementite is colourless/white and reflects light strongly, a fracture surface through this carbide network looks bright white — the origin of the name "white" cast iron — and the same continuous hard-carbide network that gives the name is also what makes white iron extremely hard and wear-resistant but essentially non-machinable and very brittle.

6.3 — (iii) How to produce white cast iron

White iron formation means SUPPRESSING graphitization, i.e. keeping carbon combined as cementite rather than letting it precipitate as free graphite during solidification and cooling. The competition between the stable (graphite) and metastable (cementite) eutectics is governed by cooling rate and composition, so white iron is produced by pushing BOTH levers toward cementite:

1. Fast cooling / chilling. Casting against a metal chill (or into a thin/small-section mould) removes heat fast enough that there is no time for the slower graphite-forming reaction to proceed; the melt instead freezes along the faster-kinetics metastable Fe–Fe₃C path. This is the basis of "chilled iron" castings, which are deliberately white (hence chilled and wear-resistant) at the chilled surface while the slower-cooling core remains gray.
2. Low silicon content. Silicon is the classic graphitizing element (it raises the carbon activity and promotes the stable graphite eutectic); keeping Si low (and/or adding carbide-stabilizing elements such as Cr, Mo, V, Mn, which form their own stable carbides and further suppress graphite nucleation) shifts the competition decisively toward the cementite eutectic even at a moderate cooling rate.

In practice the two levers are combined: a low-silicon iron poured into a chilled mould reliably solidifies white, while the same composition poured into a thick sand mould (slow cooling) may solidify gray — which is also why the SAME casting can be deliberately chilled locally (e.g. mill rolls, jaw-crusher liners) to be white and wear-resistant at a working surface while staying gray (tougher, machinable) through the bulk.