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发表于 2013-3-6 21:11:01
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Tempering colors
Tempering colors of steel
Steel that has been freshly ground or polished will form oxide layers when heated. At a very specific temperature, the iron oxide will form a layer with a very specific thickness, causing thin-film interference. This causes colors to appear on the surface of the steel. As temperature is increased, the iron oxide layer grows in thickness, changing the color.[18] These colors, called tempering colors, have been used for centuries to gauge the temperature of the metal. At around 350˚F (176˚C) the steel will start to take on a very light, yellowish hue. At 400˚F (204˚C), the steel will become a noticeable light-straw color, and at 440˚F (226˚C), the color will become dark-straw. At 500˚F (260˚C), steel will turn brown, while at 540˚F (282˚C) it will turn purple. At 590˚F (310˚C) the steel turns a very deep blue, but at 640˚F (337˚C) it becomes a rather light blue.[19]
The tempering colors can be used to judge the final properties of the tempered steel. Very hard tool steel is often tempered in the light to dark straw range, whereas spring steel is often tempered to the blue. However, the final hardness of the tempered steel will vary, depending on the composition of the steel. The oxide film will also increase in thickness over time. Therefore, steel that has been held at 400˚F for a very long time may turn brown or purple, even though the temperature never exceeded that needed to produce a light straw color. Other factors affecting the final outcome are oil films on the surface and the type of heat source used.[19]
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发表于 2013-3-5 20:29:49
楼主: upposition
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发表于 2013-3-5 22:22:37
发表于 2013-3-6 09:11:40
