White Cast Iron
Definition - What does White Cast Iron mean?
White cast iron is a type of carbon-iron alloy that contains carbon content greater than 2% in the form of cementite.
The name white cast is derived from its white surface, which is caused by carbide impurities that allows cracks throughout the metal. When fractured it exhibits a silver-like (white) fracture.
A lower silicon content combined with faster cooling makes it possible for white cast iron to precipitate metastable phase cementite, Fe3C, as a product instead of graphite. The cementite that precipitates when it melts creates large particles in the form of a eutectic mixture, whereas the other phase is austenite that may transform to martensite upon cooling.
White cast iron is usually considered too brittle to be used for many structural components, but due to its hardness and abrasion resistance properties and low cost, it is an acceptable choice for those applications where wear resistance is desirable. White cast iron has high compressive strength and wear resistance. High-alloy white cast irons are primarily used for severe abrasion- and erosion-prone conditions because they demonstrate fair to excellent corrosion resistance when relatively high levels of chromium and other alloy elements are present.
Corrosionpedia explains White Cast Iron
White cast iron displays white colored cracks when it fractures due to carbide impurities. The carbon content within the cast iron precipitates large molecules that increase its hardness and durability. It is cost effective and readily available, making it highly sought after.
The eutectic carbides component of white cast iron is too large to provide the precipitation hardening seen in some steels, where plastic deformation may be inhibited in cementite. However, increasing the bulk hardness of the cast iron and its substantial volume fraction in a given mixture provides adequate precipitation. The large fraction of carbide in the material makes it reasonable to classify white cast iron as a cermet.
White cast iron is classified into three categories:
- Normal white cast iron, which contains the elements Si, C, Mn, S and P, with no other alloy elements
- High-alloy white cast iron, where the total mass fraction of alloying elements is more than 5%
- Low-alloy white cast iron, where the total mass fraction of alloying elements present is less than 5%
When white cast iron is used in thick castings it is difficult to cool it fast enough to solidify the melt through the entire casting. However, rapid cooling can be used on the shell of white cast iron, and thereafter the remainder can cool slowly to form grey cast iron at the core. The resulting casting is called a chilled casting, and its benefits include a hard surface and a tougher interior.
During the crystallization process and cooling and heat treatment, the final content, type and distribution of cast iron phases are formed along with the cast iron structure. The casting fracture appearance depends on the type of high-carbon phases. The appearance of the cast iron fracture determines whether the metal is classified as a grey cast iron or a white cast iron.