Do you know White Cast Iron ?
White Iron is unalloyed cast iron with low carbon and silicon content such that the structure is hard brittle iron carbide with no free graphite. These irons are limited in application because of the lack of impact resistance and the difficulty in maintaining the structure in thicker sections. In some cases the castings are designed and produced to have a white structure in certain areas and a grey or flake structure elsewhere to improve toughness.
It is the iron that displays white fractured surface due to the presence of cementite. With a lower silicon content and faster cooling, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles, usually in a eutectic mixture, where the other phase is austenite (which on cooling might transform to martensite). These eutectic carbides are much too large to provide precipitation hardening (as in some steels, where cementite precipitates might inhibit plastic deformation by impeding the movement of dislocations through the ferrite matrix).
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