What are Thermal Properties of Cast Iron - Definition

Thermal Properties of Cast Iron – Melting Point – Thermal Conductivity. Because of its high fluidity when molten, the liquid iron easily fills intricate molds and can form complex shapes.

Gray cast iron have also an excellent damping capacity, which is given by the graphite because it absorbs the energy and converts it into heat. A large damping capacity is desirable for materials used in structures where unwanted vibrations are induced during operation such as machine tool bases or crankshafts.

In materials engineering, cast irons are a class of ferrous alloys with carbon contents above 2.14 wt%. Typically, cast irons contain from 2.14 wt% to 4.0 wt% carbon and anywhere from 0.5 wt% to 3 wt% of silicon. Iron alloys with lower carbon content are known as steel. The difference is that cast irons can take advantage of eutectic solidification in the binary iron-carbon system. The term eutectic is Greek for “easy or well melting,” and the eutectic point represents the composition on the phase diagram where the lowest melting temperature is achieved. For the iron-carbon system the eutectic point occurs at a composition of 4.26 wt% C and a temperature of 1148°C.

Cast iron, therefore, has a lower melting point (between approximately 1150°C and 1300°C) than traditional steel, which makes it easier to cast than standard steels. Because of its high fluidity when molten, the liquid iron easily fills intricate molds and can form complex shapes.

Thermal Properties of Cast Iron

Thermal properties of materials refer to the response of materials to changes in their temperature and to the application of heat. As a solid absorbs energy in the form of heat, its temperature rises and its dimensions increase. But different materials react to the application of heat differently.

Melting Point of Cast Irons

Melting point of gray cast iron – ASTM A48 steel is around 1260°C.

Melting point of martensitic white cast iron (ASTM A532 Class 1 Type A) is around 1260°C.

Melting point of malleable cast iron – ASTM A220 is around 1260°C.

Melting point of ductile cast Iron – ASTM A536 – 60-40-18 steel is around 1150°C.

In general, melting is a phase change of a substance from the solid to the liquid phase. The melting point of a substance is the temperature at which this phase change occurs. The melting point also defines a condition in which the solid and liquid can exist in equilibrium.

Thermal Conductivity of Cast Irons

The thermal conductivity of gray cast iron – ASTM A48 is 53 W/(m.K).

The thermal conductivity of martensitic white cast iron (ASTM A532 Class 1 Type A) is 15 – 30 W/(m.K).

The thermal conductivity of malleable cast iron is approximately 40 W/(m.K).

The thermal conductivity of ductile cast iron is 36 W/(m.K).

The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity, k (or λ), measured in W/m.K. It is a measure of a substance’s ability to transfer heat through a material by conduction. Note that Fourier’s law applies for all matter, regardless of its state (solid, liquid, or gas), therefore, it is also defined for liquids and gases.

Cast Iron Solidification

Cast iron is one of the most complex alloys used in industry. Because of the higher carbon content, the structure of cast iron, as opposed to that of steel, exhibits a carbon-rich phase. Depending primarily on composition, cooling rate, and melt treatment, the carbon-rich phase can solidify with formation of either a stable (austenite-graphite) or a metastable (austenite-Fe₃C) eutectic. Cementite (Fe₃C) is a metastable compound, and under some circumstances it can be made to dissociate or decompose to form α-ferrite and graphite, according to the reaction:

Fe₃C → 3Fe (α) + C (graphite)

Thus, two types of eutectic solidification can occur. Moreover, various graphite shapes exist depending on chemical composition and cooling rate. Graphite formation is promoted by the presence of silicon in concentrations greater than about 1 wt%. Also, slower cooling rates during solidification favor graphitization (the formation of graphite).

References:

Materials Science:

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Cast Irons

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