Heat treatments in the mass of steels. Part 1

In this article, we focus on the heat treatment of structural steels. Other heat treatments are the subject of a second section.

Various hardening mechanisms can be used to give steel its high strength properties [M 245] but it has to be said that the most effective is that which involves the formation of precipitates (most often iron carbides or carbides of carburigenic alloying elements, but also, in a few cases, intermetallic compounds) within the iron grains. This hardening process, common to many metals, is carried out in three successive operations.

• The precipitates need to be brought into solution because, after solidification, they are generally much too large for the optimum dimensions required for maximum hardening. Here, in order to bring the carbides into solution, the steel must be heated to a temperature such that the iron has acquired its face-centered cubic structure (γ-iron), which can dissolve (in interstitial solution) the carbon present (in contrast to – iron α-centered cubic iron – in which the carbon is practically insoluble). Steel is thus transformed into a solid solution of carbon in γ-iron, austenite. The operation is called "austenitization".

• Rapid cooling brings the steel back to room temperature, preventing carbon precipitation in any form. This step is imposed by two facts: on the one hand, it is very difficult to achieve optimum precipitation during cooling, and on the other, it is impossible to cool a volume of metal in the same way in the skin and core; the core always lags behind the skin and cools more slowly (it is therefore not possible to achieve precipitation under the same conditions and thus obtain the same hardening). This operation, known as quenching, has a particular effect on carbon-containing steels: quenching transforms the steel into martensite, which becomes harder the higher the carbon content (on the other hand, quenching non-ferrous alloys softens them by removing the precipitates). Thus, in its unhardened state, steel is hard, even very hard, and consequently brittle (unless C% < 0.1). In addition, its structure is not stable and, for a steel to be used properly, it is necessary to induce carbon precipitation.

• Moderate reheating causes the precipitation of fine intragranular carbides that may be coherent with the α-iron matrix: this precipitation is achieved during reheating (this is "tempering") to which the previously quenched steel is subjected. Depending on the composition of the steel, these carbides are cementite Fe ₃ C, substituted cementite...