Transformations in steels

As mentioned in the article in Techniques de l'Ingénieur Aciers. General [62] , the main factor in the hardening of iron is, in practice, the precipitation of carbides (which implies the addition of carbon to iron and thus the manufacture of steel), carbides which can be :

• iron, which is the case for non-alloy steels;

• carburigenic alloying elements introduced into the iron, as in the case of certain alloy steels.

These notions lead to the need to "condition" carbide precipitation correctly, i.e. to induce their formation within iron grains by giving them a size as close as possible to the optimum size [63] . From a general point of view, for many metals, this result is obtained by heat treatment operations that bring about structural transformations that can be summarized as follows:

• bringing precipitates into solution by heating to a sufficiently high temperature (depending on alloy composition and equilibrium conditions); in fact, these precipitates are generally too coarse in the as-solidified state;

• rapid cooling to prevent precipitation and thus maintain the alloy in a supersaturated state at room temperature (this is to avoid the heterogeneities that would result from the fact that any cooling is not uniform throughout the cross-section of a piece of metal; the conditions for germination and growth of precipitates are then different from one point to another);

• controlled reheating for optimum precipitation.

It is therefore essential to know how a precipitation-hardenable alloy behaves under these conditions, and to study its transformations.