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SOLUTIONS: Cast Iron

2.07

White cast iron

Malleable cast iron

White cast iron has a similar chemistry to gray iron, but is different in that the carbon is present in the form of iron carbide (Fe3C) instead of as free carbon (graphite). This is achieved by keeping the silicon content low and by rapid cooling, which does not permit the carbon to move to the crystal borders, but become trapped inside the crystals. This results in a martensitic structure, which is very brittle and hard. In a fracture the iron carbide will give the surface a silvery white appearance, hence the name white cast iron.

Malleable cast iron starts life as an ordinary white cast iron, very hard and brittle. It is then annealed (heat treated) in a furnace at a temperature above 870°C, at which point the silicon can break down the iron carbides into iron and carbon. During a period from 1 to 6 days, the carbon actually moves through the solid iron to collect in little gobs or nodules of graphite. Iron with this microstructure is called malleable and its properties are opposite to those of white iron, from which it is derived. Instead of being hard and brittle, it is easily machinable, and is malleable. Due to the cost of manufacturing it, malleable cast iron is limited in application to relatively small parts. The strength of malleable cast iron varies from 340 MPa to 520 MPa.

White cast irons are useful for wear resistant service but due to their crack sensitivity they are very difficult to weld and may be considered unweldable.

Characteristics:

Characteristics:

Hard Brittle Abrasion-resistant

Non-weldable

Will absorb mild to medium torsion forces.

Fair to good weldability. When strength goes above 410 MPa welding becomes difficult.

Iron Carbides

Etched white iron

Ferrite

80

Malleable iron with temper carbon aggregates