What is ductile iron? Spheroidal graphite cast iron and the magnesium addition

Why graphite shape changes everything

In grey cast iron the graphite is present as thin flakes with sharp tips. Every flake tip behaves like a notch, concentrating stress and making it easy for a crack to propagate. The material therefore fractures with almost no elongation.

With spheroidal graphite the same carbon gathers into round nodules. A sphere is not a stress-concentrating geometry; the metal matrix stays continuous and carries the load. The result is a material giving 10–18% elongation, resistant to impact, with tensile strength that can reach the 400–900 MPa range.

The role of magnesium

Graphite does not solidify spheroidally on its own. The sulphur and oxygen present in molten iron are surface-active elements that encourage graphite to grow as flakes. Magnesium ties up both of them and so clears the way for graphite to grow as spheres.

At the end of treatment the aim is usually a residual magnesium of 0.03–0.06% in the cast iron. That range is narrow and critical:

• Too little magnesium: nodularisation is incomplete, vermicular or flake graphite remains, and the intended mechanical properties are not reached.
• Too much magnesium: slag formation, carbide precipitation and the risk of casting defects all rise, and the cost is wasted.
• Holding time: magnesium escapes the bath over time. Pouring must be completed within a set period after treatment, otherwise nodularisation regresses (fading).

How magnesium is added

Magnesium boils at 1090 °C; dropped in pure form into an iron bath at 1450 °C it vaporises in a violent reaction. It is therefore always added with a carrier, in a controlled way.

Nickel-magnesium master alloy

Alloyed with nickel, the vapour pressure of magnesium falls, the reaction softens and the yield rises. It is preferred for grades where nickel remaining in the alloy is wanted or harmless, and especially in heavy-section, high-strength ductile iron.

Ferrosilicon-magnesium (FeSiMg)

The most widespread method in the foundry industry. The magnesium is carried in a ferrosilicon matrix and applied by the sandwich, tundish or in-ladle methods.

Cored wire feeding

Magnesium-bearing wire is fed into the bath at a controlled rate. The yield is high and it is the most repeatable method.

Sulphur control

The yield of the magnesium addition is directly tied to the sulphur in the bath: the incoming sulphur has to be tied up first, and only the remaining magnesium can be used for nodularising. Sulphur is therefore brought as low as possible before treatment.

• Using low-sulphur charge material reduces the cost of the magnesium addition directly.
• The target before nodularising is generally below 0.015% sulphur.
• Sulphur can be lowered further with desulphurising materials such as calcium silicide.
• The sulphur content of coke and carbon raisers should be capped in the specification.

Inoculation

Nodularising alone is not enough. After treatment the bath is depleted of sites where graphite can nucleate; without inoculation, carbides (chill) form and machinability falls.

Ferrosilicon-based materials are generally used as inoculants, and may contain barium, calcium, strontium or rare earth elements. Inoculation is carried out just before pouring or during it (in-stream inoculation); like magnesium, its effect fades with time.

Buying ductile iron raw material

1. For the nodulariser, write the magnesium content, the carrier and the particle size.
2. If you prefer nickel-magnesium, state the nickel-to-magnesium ratio and the limits for residual elements.
3. Require a sulphur ceiling on carbon raisers and charge materials.
4. Put the active elements of the inoculant (Ba, Ca, Sr, rare earths) and its particle size into the specification.
5. Require a certificate of analysis for every lot; nodularising yield is very sensitive to composition.