Melting

  1. Transition of a substance from a solid to a liquid state.

  2. In foundry industry, general term for the production of liquid metals and alloys.

From a physical point of view, melting takes place by adding heat energy until the molten state is reached. This heat requirement depends on the chemical composition of the casting material and its physical properties. It is composed of the heat of heating, the heat of melting and the heat of superheating.

Heating heat is the portion required to heat the melt from room temperature to the melting temperature; this heating heat results from the average specific heat capacity and the heating temperature interval. For the transition from the solid to the molten state, an additional heat requirement - the heat of fusion, also called latent heat quantity - is required. The order of magnitude of the heat of fusion also depends on the material. In order to overheat the melt to casting temperature, a further heat supply is necessary. This superheat is determined by the specific heat capacity in the liquid state.

Here is an example: For pure copper with a melting point of 1083 °C and an average specific heat capacity of 0.42 kJ/kg - K for the solid state and 0.5 kJ/kg - K for the liquid state as well as a heat of fusion of 212 kJ/kg, the following energy calculation results:

1 kg of copper requires from 20 °C to a melting point of 1083 °C, i.e. for a temperature interval of 1063 K, a heating heat of 1063 - 0.42 = 446.5 kJ, furthermore a heat of fusion of 212 kJ and for a supply temperature of 1200 °C, i.e. for a temperature interval of 1200-1083 = 117 K, a superheat of 117 - 0.5 = 58.5 kJ. The total heat requirement is therefore 446.5 + 212 + 58.5 = 717 kJ/kg copper.

For other pure metals the caloric values are given in the table. The actual energy requirement is of course higher because heat losses occur in the melting furnace. Therefore, the supplied energy is not used 100 % for melting and superheating. Electric furnaces have a relatively high efficiency (65 to 80 %) and the fuel-heated crucible, trough and drum furnaces (20 to 35 %) have a much lower efficiency. This also depends on the design, the capacity, the thermal insulation and a possible waste heat recovery.
 

 

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