A thin, tightly adhering oxide skin that forms when steel is tempered at a low temperature, or for a short time, in air or a mildly oxidizing atmosphere. The color, which ranges from straw to blue depending on the thickness of the oxide skin, varies with both tempering time and temperature. Before the use of instruments such as pyrometers, colours were used to judge temperatures when hardening and tempering. For example, on carbon tool steel where the tempering range may typically be from 200oC to 350oC, the colours change with the rise in temperature giving Light Straw at around 210oC, Purple at 275oC, and Grey at 330oC. The practice still continues in workshops where controlled heat treatment facilities are not available
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