The heating process is an operation, or a series of operations, by which a metal or a metal alloy, obtains some mechanical properties.
The metals undergo one or more thermal cycles in a given environmental status, and there is a constant monitoring of: temperature, duration and speed of the temperature changes, below the casting point or the casting period. During the hardening process the component is heated over the austenization temperature of the AC1 metal (this phase must be extremely slow in order to minimize the changes of the temperature between the superficial zones and the core of the component) and after having exposed it to an appropriate period at this temperature, it is cooled down to the room temperature faster than the critical hardening speed, thus obtaining a martensitic structure.
- the martensitic structure gives the component a high mechanical resistance.
The hardened metals have a more or less high internal tension state, which in most cases, is necessary to eliminate or, at least reduce, before their use because they are very strong and are less tough. In this case, we resort to the tempering heat treatment that more or less changes the martensitic structure of the hardening, cancelling out the tensions and the fragility.
The tempering must be carried out after the hardening in order to avoid the danger of the creation of cracks (caused by the state of high tension of the hardened pieces). The tempering temperature must be chosen in order to obtain the best compromise between the hardness and the toughness characteristics. In fact, as the temperature increases, we obtain a progressive increase of the toughness, of the elongation and of the contraction and, correspondingly, a decrease in the hardness, of the resistance to the traction and of the exhaustion.
The period of the tempering temperature is decided according to the dimensions of the pieces and to the type of oven. The whole of the two heating treatments in sequence is called austempering. It is the typical treatment of special steels, the so-called reclamation steels; once they have been reclaimed they still have a good workability with the tools by the removal of shavings.
- elimination of the tensions and fragilities caused by hardening;
- elimination of the cracks in the hardened components.
The quench annealing allows the softening of the steel in order to render it more suitable for mechanic workings, to eliminate residual tensions and the effects of a plastic deformation, a welding or a previous heat treatment.
The quench annealing is absolutely necessary for the raw materials of casting in which it is necessary to eliminate the strong structural heterogeneities: in this last case, the heating is obtained at a higher temperature and with long periods of permanence.
This treatment helps the workability of the steels by cold plastic deformation, while it doesn’t help the workability for the tools due to their excessive softening.
- elimination of the residual tensions in castings;
- high simplification during the cooling phase;
- economic process.
This treatment is aimed to sharpen and homogenize the structure, particularly in the weakly alloy steels, where the structure obtained is mostly made up of ferrites and flaky perlites.
The structure and the hardness obtained are strictly connected to the dimension of the casting and the material used.
- homogeneity of the components used in the process.