Heat treatment of cast iron: a guide to choice

Compared with other metallic materials, cast irons offer users a wide choice of features at an attractive price, given the raw materials used and the ease with which they can be produced. However, some types of cast iron incorporate heat treatment into the manufacturing cycle, while others see their field of application significantly extended by appropriate heat treatments.

Malleable cast irons with ferritic or pearlitic matrices belong to this first group, as do gray cast irons. Similarly, alloyed cast irons with a graphitic or carburigenic structure usually require specific heat treatment to obtain a given matrix.

On the subject of graphitic or carburigenic structure, it's worth remembering that the fundamental difference between steels and cast irons lies in the possibility, for cast irons, of presenting eutectic carbon either in the form of more or less complex carbides (metastable state), or in the form of graphite (stable state).

By means of a special annealing process, the carburigenic structure can be converted into the graphitic form: this is the purpose of the malleabilization or graphitization treatment.

On the other hand, in no case does heat treatment significantly alter the shape of pre-existing graphite, so there is no need to use heat treatment to modify or improve the shape of graphite obtained during solidification.

For unalloyed or low-alloy cast irons, heat treatment allows :

• reducing internal constraints ;

• increased machinability ;

• obtaining good toughness;

• increased mechanical properties;

• increased wear resistance.

The choice of heat treatments to meet each of these objectives is the subject of this article, after a few reminders about cast irons.