Thermo-optical Emission
During heat treating, a blacksmith causes phase changes in the iron-carbon system in order to control the material's mechanical properties, often using the annealing, quenching, and tempering processes. In this context, the color of light, or "blackbody radiation," emitted by the workpiece is an approximate gauge of temperature. Temperature is often gauged by watching the color temperature of the work, with the transition from a deep cherry-red to orange-red (815 °C (1,499 °F) to 871 °C (1,600 °F)) corresponding to the formation of austenite in medium and high-carbon steel.
Maximum carbon solubility in austenite is 2.03% C at 1,420 K (1,150 °C).
Read more about this topic: Austenite
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