Thermal Treatments
1.Definition:
It is a succession of combined heating and cooling operations: it is the variation of temperature as a function of time.
2-Aims: The heat treatments aim to:
1) To improve the mechanical characteristics of materials and make them more favorable for a given use, based on modifications of Ductility (capacity, resilience, or toughness, resistance, hardness;
2) To regenerate a metal which has a coarse grain: refine the grains, homogenize the structure;
3) To remove internal tensions from materials that have undergone cold plastic deformation.
3-Main heat treatments of steel:
The main heat treatment methods which modify in various ways the structure and properties of an alloy are: Annealing, Quenching, Tempering, Normalizing.
1. Annealing
Annealing consists of:
- Heat the part to a specific temperature called the annealing temperature (the choice of temperature depends on the objectives, it can range from 450 to 1100°C)
- Keep this room at this temperature for a given time
- Cool at the appropriate rate to obtain, after returning to room temperature, a structural state of the metal close to the stable equilibrium state.
- Annealing makes it possible in particular to eliminate or reduce residual stresses in the metal linked to a previous action (deformation, welding, etc.) or a previous heat treatment, to obtain the formation of a structure favorable to a subsequent action (deformation, machining, etc.) or a subsequent heat treatment.
2. Quenching
It consists of:
- Heat the part to a suitable temperature (austenitizing or solution heating)
- Keep this room at a specific temperature;
- Cool the part at an adequate speed by immersing it in water (water quenching) or oil (oil quenching) or by blowing in a gas such as air. Water-polymer emulsions and certain salts (nitrates, nitrites) can also be mentioned as quenching media. The purpose of quenching is to harden the metal. It produces very hard steels but in most cases not very ductile. It is therefore generally followed by tempering.
- 3- Income:
Income consists of:
- In one or more heatings of the part to a determined temperature (lower than the temperature at which the phase transformation of the steel begins ~ 700°C);
- Maintaining the room at this temperature for a given time;
- Appropriate cooling. Tempering can mitigate the effects of quenching by making the part more ductile and tougher. In special cases, it can allow secondary hardening. The use of the term tempering implies that the treatment is carried out on a previously quenched state. [A treatment at 500°C on a quenched steel is tempering, on an untreated steel is annealing].
- 4- Standardization
- The normalizing treatment (equivalent to high-temperature annealing followed by air cooling) is a structural refining treatment allowing softening and the elimination of stresses that appeared with the previous operations. In addition to obtaining homogeneous structures sought in certain mechanical applications, it leads to a reproducible state that is ideal for quenching. The so-called “normalized” state is a delivery state of steel products.
4-Conclusion:
Heat treating metals is a great way to improve their mechanical properties. Besides the physical, it can also improve the electrical and magnetic properties of a metal. This further improves the part’s compatibility with other materials.
Thermal Treatments
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