What Is the Metallurgical Term Known as Tempering?

Tempering is a heat treatment process that is often used to improve hardness, strength, toughness, as well as decrease brittleness in fully hardened steel. 

A martensitic crystal phase is formed in steel when excess carbon is trapped in the austenitic lath and quickly cooled (usually by water quenching) at a suitable rate. This untempered martensite must be heated to below the lower critical temperature of the steel grade in order to allow carbon to diffuse out of the body-centered tetragonal structure, creating a more ductile and stable body-centered structure.

The goal of tempering is to bring forth the best combination of mechanical properties in ferrous materials. It is a common step in contemporary steelmaking. However, mild steel and medium carbon steel lack enough carbon to alter their crystalline makeup, so they cannot be toughened and tempered. 

Tempering Outside of Metallurgy

In cooking, the term "tempering" describes stabilizing a substance. When chocolate isn't tempered, it tends to be soft and sticky at room temperature and difficult to work with as a result. If you're having difficulty grasping the concept of metal tempering, the term's use in the culinary arts may improve your understanding.

It's essentially the same process as is used in metallurgy. When chocolate is tempered, it is simply cooled and heated to enable it to be dipped and the cocoa butter inside to become crystallized throughout. 

Benefits of Tempering

In precipitation-hardening alloys, such as aluminum superalloys, tempering causes evenly distributed alloying elements from the solution annealed product to react internally, creating inter-metallic phases known as precipitates. These precipitates are what strengthen the alloy, and in certain material systems, multiple tempers can yield multiple different precipitates, lending high-temperature strength to the alloy.

Aging in the Tempering Process

When tempering of a metallic material is carried out over extended periods of time in order to coarsen and increase the number of precipitates, it is called aging. Aging can actually occur at room temperature in some metals.

Why Tempering is Important

Since strength and toughness come at the expense of each other in a given material, tempering is a critical heat treatment process that can determine the balance of the two properties with careful temperature and time control.

After steel has been tempered, it can be easily shaped, cut and filed, which is important in the manufacturing process. Outside of manufacturing, heat treatment of steel is carried out in metal workshops for students.

When metal is tempered, it turns different colors based on the amount of heat to which it is exposed. Metal workers may be instructed to temper steel until it becomes a certain color.

While steel used for axes is tempered until it becomes purple, the steel used for woodturning tools is tempered until it becomes brown, and steel used for lathe tools for brass is tempered until it becomes pale yellow. Typically, the deeper the color, the higher the temperature at which it was tempered.