Malleable Cast Iron

Malleable cast iron is a type of cast iron that displays properties similar to low-carbon steels, produced by subjecting white cast iron to certain heat treatments (put simply, heating white cast iron to 900–950°C and cooling it slowly). In this respect, it is more expensive than gray cast iron but cheaper than steel.

The new properties obtained by subjecting white cast iron to a special softening anneal after casting have made it possible to treat malleable cast iron as a separate group.

We mentioned above that malleable cast iron is produced from white cast iron. If white cast iron containing a high proportion of carbon (approximately 3–3.5%) in its internal structure is annealed for a long time at high temperature in an oxidizing atmosphere, it takes on the malleable cast form.

This condition appears more clearly especially near the surface (4 mm). This is because the amount of carbon near the surface is reduced by burning off under the effect of the oxidizing atmosphere and temperature, resulting in a non-homogeneous structure. The lower carbon content near the surface and higher carbon content in the interior prevent the cast iron from having a homogeneous structure. In white cast iron with a lower carbon ratio (2.5–3%), the result is obtained by holding it at high temperature in a neutral atmosphere. In this case, the internal structure is homogeneous.

Microstructure of malleable cast iron (above)

Mechanical Properties of Malleable Cast Iron

Since the carbon in malleable cast iron is present in a nodular rather than flake form, it has a certain degree of ductility. Malleable cast iron is tougher than white or gray cast iron and can bend a reasonable amount.

Gray cast iron breaks after a small amount of bending. White cast iron is like glass — it does not bend at all, it just breaks. In these cast irons, as with strength, ductility depends not only on the metallurgical structure but also on the distribution of free graphite.

When malleable cast iron is welded, the beneficial effect of the heat treatment is lost as the weld metal cools, and a brittle structure (chilling) occurs as a result of rapid cooling. For this reason, the weld zone must be re-heat-treated after welding to restore the structure of the malleable cast iron.

Obtained by Annealing

Malleable cast iron is obtained by subjecting white cast iron to a special annealing heat treatment.

High Ductility

Thanks to the structure it gains after annealing, malleable cast iron is much more ductile than white cast iron.

Use in Pipe Fittings

Thanks to its good machinability and strength, malleable cast iron is widely used in pipe fittings.

Applications of Malleable Cast Iron

Where is malleable cast iron used? Malleable cast iron has quite a wide range of applications. It is used in flanges, pipes, fittings, and valve parts. Many automobile parts, compressor crankshafts and hubs, transmission and differential parts, connecting rods, and universal joints are manufactured from malleable cast iron.

It is used in the automotive industry for mass-produced parts that require a ductile internal structure. Its attractive feature is that it costs less than steel material.

Related Questions

This material is obtained by subjecting white cast iron to a special, lengthy annealing process, and this extra step increases the cost. However, it still remains more economical than the more complex production process required for steel.

During annealing, the carbon near the surface burns off through contact with the oxidizing atmosphere and is reduced, while the carbon in the interior is less affected by this. This differing carbon loss causes the material to have a non-homogeneous structure from the surface toward the center.