Materials engineering: what is quenching and tempering of steel?

We quench and temper steels to make them strong and tough. High strength means high resistance to plastic deformation. High toughness means high resistance to crack growth and fracture, precisely because the steel can deform plastically. So the steel should deform, yet at the same time it should not? How does that work?

Steel deforms through the movement of dislocations in its crystal lattice. Quenching and tempering means deliberately placing obstacles in the path of dislocations so that their movement is severely restricted (strength), but not made completely impossible (toughness). But how do we achieve this finely tuned balance?

Understanding steel: ferrite and cementite

First, we need to understand how steel fundamentally “works”. Steel is iron containing a small but crucial proportion of carbon.

At room temperature, unalloyed steel consists of a combination of almost pure iron (ferrite) and iron carbide (cementite). Cementite precipitates are iron-carbon compounds. Since the crystal lattice of ferrite can accommodate only very few carbon atoms, cementite forms and binds all the excess carbon. The particular combination, morphology and distribution of ferrite and cementite largely determine the material’s properties.

Strong and tough: a contradiction?

Steels that have not been heat-treated often contain structures in which relatively large ferrite and cementite plates are stacked alternately, as in pearlite. The soft, readily deformable ferrite provides toughness, while the hard carbides block dislocations and increase strength. A large number of small carbides, as spherical as possible, are much better than a few large, plate-shaped carbides, because they provide both greater strength and greater toughness. This is precisely what we aim to achieve through quenching and tempering.

Step 1: austenitising

First, we need to dissolve the cementite precipitates and distribute the carbon they contain throughout the steel. To do this, we heat the steel to temperatures above 700 °C, or considerably higher depending on the alloy. In this temperature range, a different crystal structure of iron forms: austenite.

Austenite can accommodate considerably more carbon than ferrite. At a sufficiently high temperature, carbides can be dissolved completely. This process is called austenitising.

Step 2: hardening – martensite forms

The steel is then cooled rapidly, so rapidly that the carbon has no time to form carbides again. We effectively “freeze” the carbon in the crystal lattice, which becomes strained and strong, but also extremely brittle. This produces martensite: a tetragonally distorted, highly strained structure with very high strength, but also high brittleness. The cooling process that leads to the formation of martensite is called hardening. Strength is now at its maximum, but toughness is lacking.

Step 3: tempering – adjusting toughness

To adjust the toughness deliberately, we reheat the hardened steel to around 500 °C, or considerably below or above this temperature depending on the desired properties. This step is called tempering. The carbon becomes mobile again and precipitates as iron carbide. The crucial point is that we control the size, number and distribution of the carbides through the tempering temperature and duration.

After quenching and tempering, that is, after austenitising, hardening and tempering, we once again have a combination of almost pure iron and cementite. Through tempering, the martensite has “transformed back” almost entirely into ferrite.

The difference from the initial condition, however, is the fine, uniform distribution of many small carbides that are as rounded as possible. The result is a material condition that combines high strength with high toughness.

An analogy from construction

The quenching and tempering process can be compared to stripping out and renovating a building. If we are not happy with how a house functions, we first remove all its internal walls. We then use the bricks from the old walls to build new ones according to our requirements. Depending on how many walls we build and where we put them, the layout and characteristics of the house change fundamentally.

During quenching and tempering, we “tear down” the original microstructure and rebuild it in a controlled way, aiming for optimum mechanical properties.

W.S. Werkstoff Service – expertise in materials engineering

If you would like to deepen your knowledge of material testing or materials engineering (metals), take a look at the seminars and courses offered by our W.S. TrainingCenter.

In our “GHA – Annealing, Hardening, Tempering” course, we cover the fundamentals of these heat treatment processes and their importance for optimising material properties.