The materials are hardened in an inert gas atmosphere composed of nitrogen, methanol and natural gas. This atmosphere is matched exactly to the carbon content of the respective material. At temperatures that are normally above 780°C the starting structure of the component is first converted into an austenitic structure. The workpiece is maintained at this temperature so that the alloying elements can be incorporated homogenously in this austenitic structure. Then the workpiece is quenched in such a way a so-called martensitic structure is originated. Quenching is a rapid cooling of the workpiece and is done in oil, gas or other quenching media. Then the tempering creates the desired properties of the component, above all the necessary hardness and toughness. In this process, tempering takes place using high temperatures. An optional sub-zero treatment can be carried out after the protective gas hardening. It serves to transform the retained austenite and stabilize the martensite. Because of the high temperatures involved, there will inevitably be some degree of distortion.
It should be noted that Hardening means to fully through harden a material. Hardening and tempering is therefore quite different from surface hardening or precipitation hardening.
Most processes are performed in highly sophisticated furnace equipment, specially designed to give the best results possible.
Atmospheric hardening is the hardening of components in an inert atmosphere. This protects the surface of the component against scaling and oxidation and against carburization and decarburization. By means of a regulated carbon potential in the inert gas atmosphere, decarburization and carburization processes can be reversed.
the theory of microstructural transformation
Steel in an unhardened state has a body-centered cubic (bcc) structure, in which it can only dissolve very little carbon. After heating it up above approx. 720 °C austenite is originated. Austenite features a face-cubic centered (fcc) crystal structure (and occupies a smaller volume). It can dissolve considerably more carbon, which occurs at hardening temperature. By cooling the material then rapidly enough, converting it from a face-centered cubic structure back into a body-centered cubic structure, oversaturated carbon remains and martensite is originated. Due to the presence of supersaturated carbon, the bcc lattice is stretched out to a tetragonal lattice. The martensite thus possesses high internal stresses and a larger volume than non-hardened steel at room temperature. As a consequence, the high internal stresses result in a high hardness of the material. Upon tempering, a little carbon will be diffused from the tetragonal cube. Consequently, stress and volume, but also hardness decrease and the toughness increases significantly.