Steel tempering is a type of Metallurgy, or heat treatment process, which involves heating steel to the desired temperature, which is usually below the hardening temperature.
Then, the steel goes through a slow cooling process. The aim is to change the physical properties of the steel, including hardness, to make it easier to use during the machining or welding processes.
To find out more about the tempering process and how this can help you remove undesirable mechanical properties and make it easier for further hardening processes to achieve the results you need, keep reading.
What Is Tempering Steel?
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 but an 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.
When Is Tempering Used?
Tempering is used to remove excess hardness from steel, increasing its ductility after quenching and making it easier for the steel to be used in further processes.
Heat treating steel to temper it is usually used after the initial hardening process, to create products that require flexibility as well as strength.
This includes railway axels, shock absorbers, drill bits, aerospace components and even structural materials for the building industry.
Without tempering, the hardened steel would be hard and brittle, meaning it would be strong enough for its purpose but not flexible enough to withstand extreme pressure. By using tempering to restore ductility to steel hardened by prior operations, you can create a material that has the ideal properties for a range of construction and transport uses.
The Tempering Process
In basic terms, the tempering process involves heating steel to a high temperature, but below the melting point, then cooling it, usually using air.
The cooling rate and the heating temperatures used can determine the properties of the resulting material.
Tempering is carried out after quenching. Quenching steel involves heating the steel, then rapidly cooling it, with either water or air, to produce a hardened steel that lacks ductility.
To restore ductility, the steel is heated again in the same manner, but to a lower temperature, and then cooled again.
This precise practice requires skill and accurate temperature monitoring to ensure that the steel is always at the right temperature range.
It is best to carry out tempering after quenching to reduce your chances of warping or other issues. Tempering is commonly used on high carbon steel, to ensure it achieves the desired mechanical properties needed for its use in various applications.
Steel Tempering Colour Chart
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.
| ºF | ºC | |
| Grey Green | 630 | 330 |
| Grey Blue | 610 | 320 |
| Light Blue | 590 | 310 |
| Wedgewood Blue | 570 | 300 |
| Dark Blue | 550 | 290 |
| Violet | 530 | 280 |
| Purple | 520 | 270 |
| Red Brown | 500 | 260 |
| Yellow Brown | 480 | 250 |
| Dark Yellow | 460 | 240 |
| Yellow | 450 | 230 |
| Straw | 430 | 220 |
| Light Yellow | 410 | 210 |
| ºF | ºC |
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? A Brief Conclusion
Sheffield Gauge Plate is a leading supplier of tool steel and tool room services, so we understand the tempering process and how it can help businesses to make the most of their raw materials.
Whatever Rockwell Hardness score your steel has, we can help you find the right process to give it the physical properties you need. As well as explaining the tempering process, we also explore annealing vs tempering, so you can understand the difference between annealed steel and tempered steel, and how this affects the material’s mechanical properties.
For more information from our tool steel experts, check out our blog. You can also contact us today, and we’ll be happy to discuss your next project.