page_title: Heat treating
section_title: Tempering colors
hierarchical_section_title: Heat treating / Techniques / Tempering / 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: Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.
context_section_description: 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. These colors, called tempering colors, have been used for centuries to gauge the temperature of the metal.
  350˚F (176˚C), light yellowish
  400˚F (204˚C), light-straw
  440˚F (226˚C), dark-straw
  500˚F (260˚C), brown
  540˚F (282˚C), purple
  590˚F (310˚C), deep blue
  640˚F (337˚C), light blue
  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 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. Higher-carbon tool steel will remain much harder after tempering than spring steel (of slightly less carbon) when tempered at the same temperature. 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.
caption: Tempering colors of steel
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