Overview of heat treatment processes

This article provides answers to the following questions, among others:

  • What is a rolling texture?
  • How should a metal sheet be bent with respect to its rolling direction?
  • What is isotropy or anisotropy?
  • For what general purpose are heat treatments carried out?
  • Which process steps are characteristic of all heat treatments?

The microstructure of a steel has a particular influence on its properties. However, the steel microstructure is not only influenced by the solidification conditions (see article on microstructure formation). It can also be negatively influenced by subsequent production processes such as rolling, deep drawing, welding, etc.

For example, when rolling a steel sheet, the roundish grains are stretched in the rolling direction. Such a stretched microstructure is then also called a rolling texture. The rolled steel sheet behaves differently in a subsequent bending process in the rolling direction than tranverse to it. While the steel tends to crack when bent parallel to the rolling direction, the risk of cracking is considerably lower when bent transversely to the rolling direction. Formability has become direction-dependent as a result of rolling. This usually has a negative effect on manufacturing processes.

Rolling texture is defined as stretching of the microstructure in the rolling direction, which leads to a change in the material properties!

The directional dependence of a property (e.g. formability, machinability, current conductivity, reflectivity, etc.) is generally referred to as anisotropy. However, if a material behaves the same in all directions with respect to a certain property, this is called isotropy (“iso” = same). Anisotropy in the properties of a material is generally not desired, as this can lead to unforeseeable effects.

Anisotropy is the directional dependence of a property. With isotropy, however, the property is given equally in all directions!

But it is not only during rolling that the microstructure of a steel can be negatively influenced. Undesirable structural changes can also occur in the area of the joint between two welded sheets. These are due to high temperatures and uncontrolled cooling after welding. This can cause the weld seam to become brittle and tear under high loads.

For these reasons, certain heat treatment processes have been developed in which the steel microstructure can be subsequently changed in the desired manner by exposing it to high temperatures and specifically controlled cooling.

Heat treatment defines processes that specifically change the properties of materials by means of controlled heating and cooling!

In this way, undesirable microstructural changes can not only be reversed (e.g. recrystallisation annealing during rolling or stress relief annealing after welding), but completely new properties can also be achieved (e.g. quenching and tempering). Therefore, the most important heat treatment processes are described in more detail below:

Aim Heat treatment
Better formabilitysoft annealing
Better machinabilitycoarse-grain annealing, soft annealing
Homogenization of alloying elementsdiffusion annealing, solution annealing
Adjustment of grain sizerecrystallization annealing, coarse-grain annealing, normalizing
Reduction of residual stressesstress relief annealing
Controlling of strengthquenching and tempering, soft annealing, normalizing
Increasing hardnesshardening
Iron-carbon phase diagram, temperature range, overview, normalizing, soft annealing, coarse grain annealing, stress relief annealing, diffusion annealing, recrystallisation annealing
Figure: Temperature ranges of the heat treatment processes