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Differentially tempered steel. The various colors produced indicate the temperature the steel was heated to. Light straw indicates 204 °C (399 °F) and light blue indicates 337 °C (639 °F). [1] [2] Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys.
The tempering colors can be used to judge the final properties of the tempered steel. Very hard tools are often tempered in the light to the dark straw range, whereas springs are often tempered to the blue. However, the final hardness of the tempered steel will vary, depending on the composition of the steel.
The steel is then heated in a localized area, such as the hammering-end of a chisel or the handle-end of an axe. The smith then carefully gauges the temperature by watching the tempering colors of the steel. As the steel is heated, these colors will form, ranging from yellow to brown, purple, and blue, and many shades in between, and will ...
The steel is gradually heated until the desired temper colours are drawn, generally at a temperature significantly lower than the alloy's critical temperature. Different colours in the temper spectrum reflect different balances of hardness to toughness, so different temper levels are appropriate for different applications.
The practice of using colours to determine the temperature of a piece of (usually) ferrous metal comes from blacksmithing. Long before thermometers were widely available, it was necessary to know what state the metal was in for heat treating it and the only way to do this was to heat it up to a colour which was known to be best for the work.
Abrasion resistant steel undergoes a two-step heat treatment process called quenching and tempering, which alters the steel's grain structure to increase hardness and toughness. [2] During the quenching phase, the steel is heated to an above-critical temperature and is then rapidly cooled with water.
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Tempering colors are produced when steel is heated and a thin film of iron oxide forms on the surface. The color indicates the temperature the steel reached, which made this one of the earliest practical uses of thin-film interference. Iridescent interference colors in an oil film