Heat treatment
The heat treatment of a part consists in making him undergo transformations of structure thanks to predetermined cycles of heating and cooling in order to improve the mechanical characteristics of them: hardness, Ductility, elastic Limit,…
This process is often coupled with the use of an atmosphere controlled at the time of the temperature setting of the part, either to avoid its oxidation, or to carry out a molecular contribution on its surface.
The heat treatments also play a big role in the field of the tribology , to see the specialized chapter of Wikilivre devoted to this subject.
Hardening
Hardening is carried out after a setting in solution of some compounds: It is a question of maintaining materials to be soaked at a sufficient temperature and sufficiently a long time. One plunges then the part in a liquid (bath of oil, water, Plomb melted, etc) or one cools it with a gas (nitrogenizes, air, etc).
In the case of steels, the goal is to avoid the precipitation of some compounds and thus to increase the hardness material. For other metals, hardening can have the opposite effect, for example with the Alliage S of Aluminum or stainless steels.
Case of the Steel S
The hardening of the Acier S comprises a heating until - beyond temperature of austenitization, a maintenance at this temperature during a given time and a cooling at a given speed. In this process the setting in solution of Précipité S intervenes which has, in a general way, of too important dimensions to obtain a optimal hardening finally. It is here that the process of austenitization plays its main role. This temperature must be selected so as to ensure a good distribution of the alloy elements, which ensures a homogeneous hardening. Therefore it is important, when designing part intended to be soaked, to take care to have the homogeneous shape of the part, in order to avoid matter concentrations in certain parts, which can pose problem during the heat treatment. In the austenitic field, iron has a structure Cubique with centered faces (iron γ) which has sites interstitial larger than in the cubic structure centered (iron α), which makes it possible carbon to much better dissolve in iron γ than in iron α. If one now subjects it to a slow cooling and in balance, there will be precipitation of carbon and one will turn over at the initial state before the austenitization, thing which does not interest us because desired hardening will not have been obtained. On the other hand, if cooling produces with speed enough fast (controlled by various variables which one will approach later), precipitation is prevented and consequently the iron matrix is forced by the carbon atoms. In this way, one obtains hardening. This treatment of hardening transforms the Austénite into Martensite, which presents a hardness proportional to the percentage of carbon. In the same way, according to the percentage of carbon of material, other elements can appear, like the Bainite. At the same time, this hardening causes also undesirable effects such as for example an increase of brittleness of the material (impact strength). For this reason after martensitic hardening, one always carries out an income (at least an income of relaxation around 200°C). After a bainitic hardening (or soaks isothermal), the income is useless.
Many variables influence quality and the mechanical properties of the tempered steel and they all are important to control:
- the temperature of hardening
- the time of hardening
- the rate of cooling (“cooling spleen”)
- the chemical composition of the material
Moreover, during hardening it appears certain problems that one must avoid or control according to the quality of the finished product to obtain. During the heating the temperature is not homogeneous in the part (hotter on the skin and colder in the middle). This variation in temperature causes internal stresses which can involve elastic strain even plastic.
During cooling there exists also a gradient of temperature, but of contrary direction. The allotropic transformation about which we spoke above (iron γ => iron α) implies also a deformation. At this time one witnesses an important voluminal contraction. One must pay much attention to this point because as the deformation is important, it can cause cracks on the surface of the part. The compressive strength is not the same one as with traction and the risk of cracking is thus different. For this reason the risk is present particularly during the heating (it generates tractive efforts on the surface), but one must also control cooling (it generates contractions on the surface). It is the liquid of hardening (inter alia) which determines the speed of cooling.
Another type of possible problems at the time of the realization of a hardening are the reactions with the atmosphere. If one puts steel in contact with the air, there can be Décarburation and formation of Calamine. Steel can be exposed in these conditions not only during the heating but also during cooling (the free air is also a medium of hardening). By knowing the advantages and disadvantages of air hardening, one can decide if it is to better choose a liquid of hardening which does not present these effects and to assume the costs of them. Here some arguments which justify the importance that liquids of hardening have in the processes of cooling.
Case of the alloys of aluminum
The aluminum alloys, like steels, see their characteristics increased by a thermique.
treatment
With regard to the parts of Foundry, the two big families of alloys are the ACES (Aluminum - Silicium) and them WITH (Aluminum - Cuivre) for which the cycle of heat treatment is différent.
- Cas of the ACES: Hardening (temperature of setting in solution about 540°C, lasted 5 with 12:00, according to alloy and the mass of the part), quenching water, is followed of an income (170°C approximately) during 3 to 10:00.
- Case of WITH: The hardening, launched in solution at a temperature slightly lower and always is followed of a maturation of several days than room temperature.
The speed of cooling can cause the formation of internal stresses. One often observes a deformation of the parts during hardening. What obliges to intercalate an operation called the rectification between hardening and the income (or right after hardening in the case of some WITH). This operation, which is carried out on a marble in cast iron, using mallets, hammers, masses or even of the presses for the large parts. To obtain the good geometry, the operator often uses a gauge of rectification or rules, squares, vé S, etc This operation must be carried out as soon as possible because at the end of a few hours, there is risk of breakage of the part because of the increase in hardness caused by the maturation of alloy.
As example, on an alloy AS7G06 (approximately Silicon 7%, Magnesium 0,6%),
- an untreated casting will have as mechanical characteristics (approximately)
Rm ~210MPa, RP0.2 ~160MPa, has ~1% for a hardness about 80HB (Alloy IN AC-42200 SF). - whereas the same quenched-tempered alloy gives results about
Rm ~280MPa, RP0.2 ~250MPa, has ~2% for a hardness higher than 95HB (alloy IN AC-42200 ST6).
For aluminum alloys, hardening causes to decrease hardness instead of increasing it. After hardening, by phenomenon of maturation, the mechanical characteristics increase naturally with the room temperature. Certain alloys pouvent to reach their mechanical characteristics of uses. This phenomenon is used during the installation of the rivets.
See the complete article on the Hardening.
Income
The income is practiced after a hardening, by heating at a temperature lower than that of hardening. It makes it possible to improve the mechanical resistance of the parts treated by decreasing hardness (by supporting the dissolution of some fragile compounds such as carbides) and the internal thermal stresses obtained during hardening. One heats at a temperature lower than that of austenitization, then one cools more or less quickly. In certain cases (alloys with age hardening) the income makes it possible after hardening to increase the mechanical characteristics.
Annealing
Annealing is done after a mechanical treatment, an operation of welding, etc in order to make more homogeneous material and to return part of its former properties to him. One heats until total austenitization of the part, then one lets cool slowly, which makes him find its old properties.
Curves of cooling
To help the metallurgists and originators with the realization of parts intended for hardening, there exist abacuses of cooling which make it possible to know according to the percentage of carbon, of the time and the type of cooling (oil, water…), to know the metallographic components present at the time of the transformation, and once the cold part, which makes it possible to make sure that the characteristics obtained correspond to those required. There exist two types of curves of cooling. First of all curves obtained in continuous cooling. Then curves obtained with isothermal stages.
Technologies of temperature setting and treatment
The temperature setting are carried out in furnaces generally with controlled atmosphere or salt bath.
The controlled atmospheres make it possible either to protect material, against oxidation for example, or to bring an additional layer to material (for example, carbon or nitrogen for steel to improve the external mechanical characteristics).
The systems of manufacture of atmosphere are called Générateur S which can be exothermic or endothermic. The exothermic generators consist of burners which impoverish out of oxygen the atmosphere of the furnace and thus avoid the oxidation of material. The endothermic generators are systems which produce active atmospheres (for example carbon monoxide fixing of carbon on steel). The risks related to the use of these techniques are considerably reduced by the use of Fours to low pressure (known as " furnaces with vide") who require few quantities of gas of atmosphere.
The Fours with salt bath allow another technique of contribution of active molecules on the surface of material, by contact with a liquid. For example, the baths of Cyanure make it possible to bring nitrogen atoms to carry out a Nitruration. The Fours with low pressure advantageously replace the salt baths of this type by allowing a nitrogen contribution in gas form and in small quantity, which is much less dangerous than the use of the Cyanure or other.
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