After the quenching process, steel becomes hard and brittle. In high-stress applications, the material can easily crack and shatter into pieces - very similar to what happens to glass when it drops.
These are undesirable effects that can compromise steel in most applications. To toughen up steel and make it strong again, steel is taken through tempering, which further hardens and increases the toughness of the material.
What is Tempering?
Tempering is a type of heat treatment used to increase the toughness of certain metals, most commonly iron-based alloys like steel.
The metal being treated, using this process, is heated under its critical point temperature and then air-cooled.
The tempering temperature depends on the application of the metal and the desired outcome. For instance, drill bits are tempered at a lower temperature than mattress springs.
There are other heat treatment options used on metals like annealing and normalising, but each of the processes have different outcomes. Tempering is best used on metal products that will be put under immense stress. It can be used on bolts, washers, nuts, screws, and even knives.
What are the Benefits of Tempering?
Improving toughness and hardness is one of the common benefits of tempering but the process has even more advantages:
It changes the physical properties of the material that help in other manufacturing steps like cold forging, welding, and machining.
Tempering helps to relieve stress making the metal easier to weld or machine.
Increases strength while making the material more flexible and ductile.
Increases hardness and introduces wear-resistant properties to the surface or through the entire metal.
Some metals can become weak and brittle when exposed to specific environments. Tempering helps to overcome the brittleness and increase toughness.
Tempering can also increase the magnetic and electrical properties of metals. This improves the chances of the metal becoming compatible with other materials.
When is Tempering Necessary?
As mentioned earlier, there are other types of heat treatments. Understanding when to use each method is vital to guarantee desirable results for the metal, depending on its application and requirements.
Tempering is often used after a quenching operation. When a carbon steel is heated then rapidly quenched, the material becomes hard and brittle. Tempering helps to restore ductility.
Metals that have gone through welding can have localised zones that are hardened because of the heat generated during the welding process. As a result, it can create undesirable mechanical properties and stress leading to hydrogen cracking. Tempering can help to reduce the hardness and relieve the stress removing the unwanted mechanical properties.
Processes like bending, drilling, rolling, and punching, among others, can lead to work hardened materials. Such materials have high residual stress, which can be alleviated by tempering the metal.
The Process of Tempering
Although steel is commonly mentioned because of strength and resistance, other iron-based alloys can be tempered as well.
The process starts with choosing your grade and the type of iron-based alloy you want to work on. After selecting the material, it is subjected to extreme heat.
After the metal is sufficiently heated, it is rapidly cooled. This act of quickly cooling the metal is called quenching. Theoretically, it sounds simple, but in practice it is a highly precise process.
After cooling, the steel is very hard but lacks ductility, which is crucial in a wide range of applications. This is why tempering is required.
The cooled steel is slowly and accurately re-heated to achieve the balance of ductility and hardness required for the application in question.
If there are any irregularities during this process, the material becomes damaged, distorted, or warped. It's best to carry out the steel tampering process immediately after the steel has been quench-hardened for the best results. This helps to avoid brittleness caused by quenching.
Common Applications of Hardened Steel
Tempered steel is commonly used in various industries and applications. Here are some of the most common applications of tempered steel:
Constructing buildings and bridges.
Building durable storage tanks.
Cutting edges for saws and drill bits.
Liners for chutes, dump trucks and trailers.
Tailored Tempering process for hot forming (partial press hardening)
Did you know that thyssenkrupp Steel Europe AG have developed the tailored tempering process? The innovation makes it possible to manufacture hot forming components in the die with precisely defined zones of varying strength or ductility – in one working step and from a sheet metal plate of hot forming steel.