Bluing
Subjecting the scale-free surface of a ferrous alloy to the action of air, steam, to other agents at a suitable temperature, thus forming a thin blue film of oxide and improving the appearance and resistance to corrosion is done in a bluing furnace. Note: This term is ordinarily applied to sheet, strip, or finished parts. It is used also to denote the heating of springs after fabrication to improve their properties.
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