Tempering Colors
If steel has been freshly ground, sanded, or polished, it will form an oxide layer on its surface when heated. As the temperature of the steel is increased, the thickness of the iron oxide will also increase. Although iron oxide is not normally transparent, such thin layers do allow light to pass through, reflecting off both the upper and lower surfaces of the layer. This causes a phenomenon called thin-film interference, which produces colors on the surface. As the thickness of this layer increases with temperature, it causes the colors to change from a very light yellow, to brown, then purple, then blue. These colors appear at very precise temperatures, and provide the blacksmith with a very accurate gauge for measuring the temperature. The various colors, their corresponding temperatures, and some of their uses are:
- Faint-yellow – 176 °C (349 °F) – engravers, razors, scrapers
- Light-straw – 205 °C (401 °F) – rock drills, reamers, metal-cutting saws
- Dark-straw – 226 °C (439 °F) – scribers, planer blades
- Brown – 260 °C (500 °F) – taps, dies, drill bits, hammers, cold chisels
- Purple – 282 °C (540 °F) – surgical tools, punches, stone carving tools
- Dark blue – 310 °C (590 °F) – screwdrivers, wrenches
- Light blue – 337 °C (639 °F) – springs, wood-cutting saws
- Grey-blue – 371 °C (700 °F) and higher – structural steel
Beyond the grey-blue color, the iron oxide loses its transparency, and the temperature can no longer be judged in this way. The layer will also increase in thickness as time passes, which is another reason overheating and immediate cooling is used. Steel in a tempering oven, held at 205 °C (401 °F) for a long time, will begin to turn brown, purple or blue, even though the temperature did not exceed that needed to produce a light-straw color. Oxidizing or carburizing heat sources may also affect the final result. The iron oxide layer, unlike rust, also protects the steel from corrosion through passivation.
Read more about this topic: Tempering, Tempering in Steel, Blacksmithing
Famous quotes containing the words tempering and/or colors:
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—Claude Monet (1840–1926)