Question 6 of 7: Ductility of Cast Irons

Question VI: Ductility of Cast Irons (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) Why gray cast iron is brittle

Gray cast iron solidifies with a large fraction of its carbon as free graphite flakes — thin, plate-like, essentially strengthless particles distributed through the metallic (ferritic/pearlitic) matrix. Graphite has negligible cohesive strength of its own and does not bond strongly to the surrounding matrix, so each flake behaves mechanically like a pre-existing internal crack. Under tensile or bending load, the SHARP TIPS of these interconnected flakes act as severe stress concentrators, and cracks nucleate there at low applied stress and propagate readily along/between adjacent flakes. The matrix itself may be capable of some plastic flow, but fracture initiates at the flake network long before that ductility can be mobilized, so the bulk material shows very low tensile elongation (typically well under 1%) despite reasonable compressive strength (compression does not open the flake-tip cracks the same way).

6.2 — (ii) A practical route to more ductile cast iron

Treating the molten iron with a small addition of magnesium (or cerium), typically as an Mg-ferrosilicon or Mg-nickel master alloy added just before pouring, converts the eutectic graphite morphology from interconnected FLAKES to isolated, rounded spheroidal (nodular) graphite — producing ductile (spheroidal-graphite, "SG") iron. Mechanistically, the Mg (or Ce) addition ties up sulfur and oxygen (which otherwise promote flake growth) and modifies the graphite growth crystallography so that graphite grows outward radially from a point nucleus rather than laterally as an interconnected plate. A rounded nodule has a far lower stress-concentration factor than a sharp flake tip, so cracks do not initiate as readily at low stress, and the continuous, largely unbroken metallic matrix can accommodate significant plastic strain before failure. The result is tensile elongations of tens of percent for ductile iron versus near-zero for gray iron, at broadly comparable strength and cost. (A malleabilizing anneal of white cast iron, which decomposes cementite into rounded "temper carbon" nodules by a different route, is an accepted alternative answer with the same underlying mechanism — rounding the graphite/carbon particle geometry to remove the stress-concentrating flake tips.)