Heat treatment

The heat treatment of a material (by the heat or cold) is designed to modify certain properties (resistance in general). It may involve materials such as glass, wood, metal, or food.

Heat treatments also play an important role in the field of tribology.

In metallurgy :

The heat treatment of a piece of metal is to apply any structural changes with predetermined cycles of heating and cooling in order to improve the mechanical characteristics: hardness , ductility , yield strength , ...

This process is often coupled with the use of a controlled atmosphere during the heating part, either to prevent its oxidation , or to make a contribution or change of molecular surface ( surface treatment ).

General :

In metals, the atoms are organized in the form of crystals  : they form an ordered structure. Foreign atoms - impurities, alloying elements - can enter the network, either by substitution of the atoms 'core', or insertion, is the notion of solid solution .

In addition, there may be several types of crystals, such as inclusions, for example. The crystals are called minority "  rushed  ".

With increasing temperature, the atoms of the crystal s' agitate around their position and diverge from each other, causing expansion . This has several consequences:

the space between the atoms increases, which can accommodate more atoms in solution integration, and larger atoms;

therefore, there may be a precipitate dissolution: the atoms of these crystals pass in solid solution;

atoms are shaking, they become mobile and can move through the crystal, a phenomenon called diffusion  ;

in some cases, the atoms of the crystal reorganize in another crystallographic phase, we speak of allotrope .

It is these mechanisms that come into play during heat treatment.

Case of ductile materials :

A ductile material is a material that can deform plastically  , this is used for shaping ( rolling , drawing , forging , ...). This deformation defects of organization of atoms in the crystal, which hardens the material: this is the work hardening .

By heating the metal in a moderate way, it gives mobility to the atoms, they reorganize and eliminate the lack of organization. It softens the material as well. This process is called annealing .

Case of steels :

At low temperatures, two-phase steel is at steady state: it is composed of crystals of iron with carbon in solid solution (α or ferritic structure), and crystals of iron carbide Fe 3 C.

Steel has an allotropic transformation: it is low-temperature bcc (ferrite α) and fcc at high temperature (austenitic structure or γ). The austenitic structure of this largest insertion sites. When heating in the austenitic area, carbides dissolve (solution treatment).

If the carbon content is sufficient, then rapid cooling allows the iron atoms to reorganize (γ → α transformation) - so-called displacive transformation - but not to the carbon atoms move to reform carbides - called diffusive transformation. Was formation of a carbon-supersaturated structure which hardens the steel. This development is supported by the presence of alloying elements in low (chromium, nickel, molybdenum). According to the cooling rate, is formed of martensite or bainite .

If the cooling rate is very fast - splat - it freezes the structure γ. Is obtained an austenitic steel at room temperature. This is true of many stainless steels.

Case of alloys of aluminum :

Some aluminum alloys have a cure by forming precipitates with alloying elements: Al 2 Cu for copper-containing alloys, Mg 2 Si for alloys containing magnesium and silicon, ... This is called hardening .

With the rise in temperature, to 500  ° C , the precipitates dissolve, that is dissolving. Tempering prevents the reformation of the precipitates. Unlike steel, tempering therefore causes a softening of the alloy.