page_title: Tempering (metallurgy)
section_title: Tempering colors
hierarchical_section_title: Tempering (metallurgy) / Carbon steel / Blacksmithing / Tempering colors
caption_attribution_description: English: These "tempering standards" were cut from the same stock of carbon-steel flatbar. They measure 1/2" wide by 2" long. The one on the left has been normalized and sandblasted, to provide a diffuse surface, (for better photographing). The piece second-from-the-left is untempered quenched-steel, also sandblasted to remove any oxidation. The rest have also been quenched and sandblasted, but then tempered in an electric oven to their corresponding temperature, for around an hour each to thoroughly "soak" the metal in the heat. Tempering standards are often used by blacksmiths, to compare to work being tempered, ensuring that the work is tempered to the right temperature. (Tempering colors may appear different in different forms of lighting, or at different angles. For instance, to get good color-rendering, these needed to be photographed in the diffuse, continuous-spectrum light of a cloudy day.)
context_page_description: Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.
context_section_description: 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.
caption: Pieces of through-tempered steel flatbar. The first one, on the left, is normalized steel. The second is quenched, untempered martensite. The remaining pieces have been tempered in an oven to their corresponding temperature, for an hour each. "Tempering standards" like these are sometimes used by blacksmiths for comparison, ensuring that the work is tempered to the proper color.
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