Cast Irons

According to various experts, cast iron is an iron alloy that typically contains more than 2% carbon along with other elements like silicon. The iron-carbon phase diagram illustrates the complexity of this material, showing how graphite can form in certain conditions. Cast irons are primarily classified into three types: grey, spheroidal, and white.

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Cast irons usually include alloying elements that aid in stabilizing the graphite phase, allowing for precipitation even when the weight percentage is below 4.3%, which is the eutectic composition on the Fe-C phase diagram.

Grey Cast Irons

Grey cast irons typically have higher C or Si content than white cast irons and require a lower cooling rate. Their name derives not from color but from the appearance of their fracture surfaces. They display good ductility and their fracture surfaces are non-reflective.

Steps on cooling:

1. The alloy begins to precipitate graphite when it falls below the liquidus. For a simple Fe-C system, this means it must be hypereutectic. However, the addition of Si alters the eutectic composition by stabilizing the graphite.
2. At eutectic temperature, a cementite and austenite eutectic forms from the remaining liquid phase known as ledeburite.
3. As temperature decreases, carbon diffuses out of solid solution to the graphite precipitates.
4. Upon reaching the eutectoid temperature, austenite transforms to pearlite. Some alloying elements may influence this final transformation.

The resulting microstructure exhibits graphite flakes within a transformed ledeburite matrix; more micrographs can be found in resources such as the DoITPoMS micrograph library.

Spheroidal Cast Irons

Spheroidal cast irons are similar to grey cast irons but contain inoculants—usually Mg or Ce—that change the structure of graphite precipitates from flakes to spheres.

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Two theories exist to explain the inoculants' effect: one suggests they 'poison' the graphite growth sites, slowing down their growth, while the other posits an increase in interfacial energy that minimizes the surface area per volume.

The cooling steps for spheroidal cast irons mirror those of grey cast irons, with graphite spheres forming in place of flakes.

White Cast Irons

White cast irons contain less Si or C than grey cast irons and cool more rapidly, which favors cementite over graphite formation. The term 'white' refers to the reflective nature of their fracture surfaces, indicative of brittleness.

The cooling path is determined by the composition of the melt, whether it is hypereutectic or hypoeutectic. A hypereutectic composition results in early cementite precipitation, while a hypoeutectic results in austenite precipitation.

It is critical to note that hypereutectic has a higher carbon content than eutectic composition, while hypoeutectic has a lower carbon content.

The first phase to precipitate forms dendrites due to non-equilibrium effects, with cooling melts not strictly following phase diagram predictions. When the eutectic point is crossed, the surviving melt solidifies as austenite-cementite eutectic (ledeburite). Carbon continues to diffuse from austenite to cementite as cooling proceeds, culminating in austenite's transformation into pearlite at low temperatures. Fast-cooled white cast irons can shift to martensite.

For further inquiries, feel free to explore cast iron prototypes.