The steel temper colour chart below indicates the various colours that you can expect when a steel element has been tempered. The colours listed are estimates and do not show the impacts that bright steel surfaces can have on the final colour.
When steel is heated after being newly ground, sanded, or polished, an oxide coating forms. The thickness of the iron oxide will increase as the temperature of the ground flat stock or steel rises. Even though iron oxide is generally opaque, such thin layers allow light to flow through and reflect off the upper and bottom sides of the coating. This results in a phenomenon known as thin-film interference, which results in the appearance of colours on the surface. The hues vary from a very light yellow to brown, purple, and finally blue as the thickness of this layer grows with temperature. These hues develop at precise temperatures, giving the blacksmith a unique advantage to identify the steel temperature by using the steel temper colour chart.
- Ligh yellow – 210 °C (410 °F) – knives, razors,
- Straw – 220 °C (430 °F) – rock drills, reamers, edge tools
- Yellow– 230 °C (450 °F) – scribers, planer blades
- Red brown – 260 °C (500 °F) – taps, dies, drill bits, hammers, cold chisels, press tools
- Purple – 260 °C (520 °F) – taps, surgical tools, punches
- Dark blue – 290 °C (550 °F) – screwdrivers, wrenches
- Light blue – 310 °C (590 °F) – springs, gears, wood-cutting saws
- Grey blue – 320 °C (610 °F) and higher – structural steel
The iron oxide loses its transparency beyond the grey-blue/grey-green tint, and you can no longer determine the tempering temperatures for steel past this point. The coating will also thicken with time, which is another reason for using overheating and fast cooling.
Even if the temperature in a tempering oven is kept at 205 °C (401 °F) for an extended period, steel will begin to turn brown, purple, or blue, even if the temperature was not higher than that required to achieve a straw colour. Heat sources that oxidise or carburise may also have an impact on the final product.
What is tempering?
Supplied tool steels often need a heat treatment performed on them to relieve internal stresses. Tempering is one heat treatment process that allows for the chemical properties of steel to be altered.
Tempering is the process of increasing the temperature of your steel to below its critical temperature, which changes depending on the steel grade you have.
To raise the temperature of steel, you will need a heating device. Most use a standard heating furnace, but other methods such as a gas furnace or induction furnace will also work. Once your steel has reached its recommended temperature, it will usually be held for a while before using a quenching method such as air to allow it to cool.