Editing Cast iron (section)

===White cast iron===
White cast iron displays white fractured surfaces due to the presence of an iron carbide precipitate called cementite. With a lower silicon content (graphitizing agent) and faster cooling rate, the carbon in white cast iron precipitates out of the melt as the [[metastable]] phase [[cementite]], Fe<sub>3</sub>C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles. As the iron carbide precipitates out, it withdraws carbon from the original melt, moving the mixture toward one that is closer to [[eutectic]], and the remaining phase is the lower iron-carbon [[austenite]] (which on cooling might transform to [[martensite]]). These eutectic carbides are much too large to provide the benefit of what is called precipitation hardening (as in some steels, where much smaller cementite precipitates might inhibit [plastic deformation] by impeding the movement of [[dislocation]]s through the pure iron ferrite matrix). Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures. In any case, they offer hardness at the expense of [[toughness]]. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a [[cermet]]. White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces ([[impeller]] and [[Volute (pump)|volute]]) of [[slurry pump]]s, shell liners and [[lifter bar]]s in [[ball mill]]s and [[autogenous grinding mill]]s, balls and rings in [[coal pulveriser]]s.

[[File:Querschnitt Schalenhartguss.jpg|thumb|right|Cross section of chilled cast-iron roll]]
It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron. The resulting casting, called a ''chilled casting'', has the benefits of a hard surface with a somewhat tougher interior.{{Citation needed|date=March 2021}}

High-chromium white iron alloys allow massive castings (for example, a 10-tonne impeller) to be sand cast, as the chromium reduces cooling rate required to produce carbides through the greater thicknesses of material. Chromium also produces carbides with impressive abrasion resistance.<ref>{{cite journal|last1=Kobernik| last2=Pankratov|date=11 March 2021|title="Chromium Carbides in Abrasion-Resistant Coatings"|url=https://link.springer.com/article/10.3103/S1068798X20120084|journal=Russian Engineering Research|volume=40| issue=12|pages=1013–1016| doi=10.3103/S1068798X20120084| s2cid=234545510|access-date=29 September 2022}}</ref> These high-chromium alloys attribute their superior hardness to the presence of chromium carbides. The main form of these carbides are the eutectic or primary M<sub>7</sub>C<sub>3</sub> carbides, where "M" represents iron or chromium and can vary depending on the alloy's composition. The eutectic carbides form as bundles of hollow hexagonal rods and grow perpendicular to the hexagonal basal plane. The hardness of these carbides are within the range of 1500-1800HV.<ref>{{cite journal|last=Zeytin|first=Havva|title=Effect of Boron and Heat Treatment on Mechanical Properties of White Cast Iron for Mining Application|journal=Journal of Iron and Steel Research, International|volume=18|issue=11|pages=31–39|doi=10.1016/S1006-706X(11)60114-3|year=2011|s2cid=137453839}}</ref>