Tempering explained: a low-temperature heat treatment

Heat treatments can modify the properties of materials. One of these procedures, the one that follows heating, is tempering. In this post we will tell you everything you need to know about it.

What is tempering?

After quenching, steel becomes too brittle. This can be corrected by tempering.

Tempering is a treatment that follows quenching, and consists of heating the part to a temperature much lower than that of the previous process in order to minimize the brittleness of the materials and also to reduce the internal stresses that occur.

Types of tempering

Tempering processes are divided into three main groups:

  • Low temperature tempering (between 160 and 300 ºC). This temperature is suitable for case hardening components and cold work tool steels. The hardness requirement is around 60 HRC.
  • Tempering of spring steels (between 300 and 500 ºC). Used in spring steels or similar applications, the hardness requirement is around 45 HRC.
  • High-temperature tempering (500°C or more) is used for quenched and tempered steels, for steels to be used as hot working tools and for high speed steel. The hardness requirement ranges from 300HB to 65HRC depending on the material.

Below is a list from the book Tratamientos térmicos de los aceros, by José Apraiz Barreiro, which lists correct tempering temperatures for various items:

Springs and steel strapping 350-500ºC
Wood saws
Metal saws
Cold chisels
Metal shears
Softwood planing blades
Wood chisels
Cold shearing blades
Hot shearing blades
Agricultural tools
Stone dressing tools
Wood planing blades
Hardwood blades
PunchesRiveting tools
Chisels and picks for stonemasonry
Rock picks
Drill bits
Large bladesLarge cutting tools that require flexibility and shock resistance in general
Iron mortising tools
Paper-cutting blades
Large knives
Cutting tools that require certain flexibility and resistance to shock in general
Stamping dies
Tools for engraving or carving steel
Blades that require some additional flexibility and good edge retention
Hard blades with low flexibility and high edge retention in general 175-220ºC

Benefits of tempering

Tempering offers numerous advantages:
· Increase of material strength
· Reduction of hardness and brittleness
· Ability to modify characteristics of material for specific applications

Goal of tempering

The main objective of tempering is to reduce the brittleness of steels in exchange for sacrificing their hardness and mechanical strength. It results in much tougher and more durable pieces.

Heat treatment procedure

As mentioned above, tempering is the next step after quenching. It should be carried out as soon as possible because, depending on the shape, material and other factors, cracks may appear during or after quenching, thus impairing the quality and integrity of the steel.

For the procedure, the piece must be sufficiently clean and polished to allow us to see the tempering colors produced.

In the past, this process took place in the forge, but times have changed and now it can also be done in a special furnace, which lends more precision to the heat treatment. In the latter case, protective gas should be deployed inside so that it does not oxidize. However, tempering outside the forge proper means the loss of a traditional method.

What is the right temperature for tempering?

Temperature is a very important factor in the tempering process. The longer the process lasts, the higher the temperature should be. Generally, the proper temperature range is between 160 and 500 ºC or more.

Steps in the process

The tempering process has three steps:

  • Heating to a temperature below the critical temperature. In this first step, the part is brought to a temperature somewhat below the desired temperature. In the case of carbon steels for construction, for example, the temperature should be between 450 and 600 °C, while for tool steels, the temperature should be lower, between 200 and 350 °C.
  • Temperature maintenance. The next step is to maintain the ideal temperature for the material over a period of time so that it is homogeneous throughout the piece.
  • Variable-speed cooling. To solidify the material and make it more durable, it must be cooled. The speed at which this is done has no impact on the material as long as its embrittlement range is avoided. If the tempering process reaches this range, the part should be put through a hot oil bath at about 150 ºC and then cooled either in a water bath or in the open air.

After forging and quenching, the piece is quite oxidized, so the tempering colors are not completely visible. When it is tempered, the surface needs to be sanded, polished or otherwise abraded so that the color of the metal is plainly visible. The more polished the surface the better the colors will show.

Why does this color transformation occur? The hues change according to the thickness of the oxidation layer covering the polished material. The piece retains its color when cooled, only changing if it is heated up again.

At ULMA Forja, we have been doing what we do best for 60 years. We create forged products of excellent quality that are used in the aerospace, naval and gas industries. If you want to know more about our procedures, do not hesitate to contact us, we will be happy to help you!