White Iron

White iron is so called because the carbon in it is present as iron carbide rather than graphite, so it has a whiter appearance. This is created by either reducing the silicone content or by adding chromium, which reacts with the carbon to form a more stable chromium carbide; this must be accompanied by a higher cooling rate to reduce the breakdown of the carbides. The resultant metal is extremely hard, part of this coming from the hardness of the carbides themselves; this in turn gives it a high degree of abrasion resistance but the large agglomerates of carbide lead to an increase in brittleness.

It is possible, by accelerating the cooling of the casting, to cause a higher degree of white iron to form on the outside and grey iron to form on the inside; this is called a "chilled" casting and benefits from a hard exterior but a generally tougher and less brittle interior. If it is not possible to cool a casting quickly because of the large size of it an increase in the proportion of chromium may be necessary, since the carbides of chromium are more stable than those of iron and by this method very large castings such as large ships' propellers can be manufactured. The presence of chromium also gives increased resistance to corrosion.

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