Heat treatment

Process for obtaining specific improved material properties through exposure to heat, cooling or, quenching (e.g. hardening, tempering and quenching, bainitizing, surface layer hardening, etc.).

Moreover, with heat treatment it is possible to induce changes in the composition of the individual alloy components of the casting (e.g. C or N content).

The concept of “heat treatment” generally applies for all metal materials. Consequently, hardening also referred to as age hardening, e.g. of an aluminum alloy, is also classified as heat treatment process, however, DIN EN 10052 particularly applies for steel materials.

For heat treatment of the iron-carbon alloys, the iron-carbon phase diagram specifies the temperatures that must be complied with during treatment.

Figure 1 illustrates the temperature-time curves (see also TTT diagram) for the most important heat treatment methods for cast iron;
Figure 2 gives an overview of heat treatment methods for steel.

With classic heat treatment of steel, the following objectives are achieved by means of the respective treatment processes:

Change in strength:
Hardening / tempering and quenching; normalize annealling; softening annealing; recrystallization annealing

Improvement of toughness:
Tempering and quenching; normalize annealing; recrystallization annealing

Elimination of strain hardening:
Recrystallization annealing; normalize annealing

Reduction of internal stresses:

Stress relief annealing

Reduction of crystallization:

Homogenization annealing

Improvement of machinability:

Coarse grain annealing (Csoftening annealing (C>0.4%)

Additional references:
Heat treatment of hardenable aluminum alloys
Solidification mechanisms

  • Fig. 1: Annealing curves for heat treatment of cast iron
  • Fig. 2: Overview of heat treatment processes for steel (according to Anja Pfennig, Werkstofftechnik, FHTW)
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