What is Tempering of Steel?

The successful heat treatment of a material plays a crucial role in ensuring parts are strong, hard or durable enough for their intended use. Tempering is one of these processes and is widely used in industry. But what exactly is tempering, and how is it performed?

What is Tempering?

Tempering is a form of heat treatment, its purpose is to increase the toughness and reduce the brittleness of a material. It is most commonly used on iron-based alloys like steel that have a medium to high carbon content.

Tempering is used to further control the material’s mechanical properties, usually after a high-heat process like hardening, forging or even casting.

How is Steel Tempered?

The consistent factor in tempering any material is heat, the part must be heated to a temperature below its critical point – the temperature at which it changes state – held at that temperature, then cooled gradually.

Temperatures vary depending on the type of material, and the final properties required, as does the dwell time.

Stage 1: Heating

Most materials are tempered at between 200°C and 600°C, depending on the type of material and the level of temper required. Heating is done in a furnace or oven, this could be gas, solid fuel or electrically heated.

To avoid cracking, the material must be heated gradually. It is also often done in an inert atmosphere, this is achieved by purging out all air with Argon or similar gas. This stops the metals from oxidizing.

Not all tempering processes are done in this way, though. Some are open-air and some even use a bath of salt crystals.

Stage 2: Holding or Dwelling

Dwell time has as much of an impact on the resultant material properties as the heating temperature. In general, the longer the part is held at the temperature, the softer it will become.

The dwell step is done inside the same inert environment. Maintaining the correct temperature for the duration requires accurate measuring and control of the furnace,

It can also be costly.

Stage 3: Cooling

Finally, the parts are allowed to cool. This is often done in still air but can be under the same conditions as the heating stage. The rate of cooling can also have a significant effect on the results of the tempering process.

What Temperature is Steel Tempered at?

Temperature has a large impact on the amount of tempering the material will receive. It varies from alloy to alloy.

Before the days of accurate electronic temperature monitoring, the color of the steel – as it heated – would tell the operator when to stop heating. The below table can be used as a rough guide.

Why is Steel Tempered?

Tempering is a vital part of the process of making components from high-carbon steels. It is a particularly important step on any components that have been hardened.

Hardening will often bring the overall hardness level to a point that makes the part brittle, with very little toughness. Tempering brings back some toughness, whilst retaining enough hardness for the component to function as intended.

What are the Benefits of Tempering?

There are a number of benefits to tempering metals, especially those that have been hardened:

• Reduced Brittleness – Brittleness is the tendency for a material to fracture and crack, something that can occur if a quenched component is not tempered before being subject to impact
• Increased Toughness – Not only does tempering reduce the tendency to crack, it increases its resistance to impact or shock loads, something vital for a component that is subject to harsh conditions
• Increased Ductility – Linked to the previous two points, increasing ductility allows the part to resist strain without breaking, this massively increases usability and durability
• Stress Relief – During both manufacture and the hardening process, heat can induce stresses into the material. Tempering allows the stresses to normalize and prevents them from resulting in warpage.

The Best Way to Temper Steel

Most tempering processes follow roughly the same outline, to ensure a temper that is controllable and produces reliable results, the following is required:

• A hardened component of known composition and hardness
• A tempering oven or furnace with precision control
• Inert gas purge system using a gas such as Argon or Nitrogen
• A cooling environment that allows a controlled reduction in temperature

The Importance of Tempering Steel

If a part has been hardened using a heating and quenching process, it will be extremely hard, but also brittle. If the tempering process is not done, the part is unlikely to last long in normal service before cracking.

Without tempering, parts cannot be hardened and durable.

Tempered Steel From Metal Supermarkets

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