tempering
Heat treatment for improving the properties of metals, often used for steel alloys. The metal is heated to a certain temperature and then quenched (cooled suddenly) in a water or oil bath to fix its state.
The temperature of steel during this process can be measured by changes in the colour of the metal as it gets hotter: it is light yellow at around 230°C/446°F, changes to dark yellow, red-brown, then purple, and finally becomes blue at around 295°C/563°F.
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