A heat treatment operation takes place in three stages, each of which involves different parameters, notably the duration of each stage, the temperature applied and the ambient atmosphere. When in contact with certain materials, the presence of a gas can provoke a reaction that modifies the chemical and physical characteristics of the parts placed in the treatment furnace. This phenomenon can be beneficial or, conversely, detrimental to the operation. Thus, some treatments require a neutral but protective atmosphere, with no impact on metals, while others require a reactive atmosphere to promote the appearance of new chemical elements.
Two forms of controlled atmosphere
Controlled atmosphere operation involves diffusing a gas or gas mixture into the environment surrounding the parts being processed. In other words, it involves inserting a special atmosphere into the treatment furnace to transform the physical and chemical properties of the parts inside. Ambient air can have an impact on materials during the process, and is one of the essential parameters to be defined before any heat treatment operation, along with the heating and cooling temperatures and the duration of each stage.
Depending on the purpose of the operation, it is important to determine in advance whether or not a special gas is to be inserted into the furnace. There are two types of controlled atmosphere: neutral or reactive. In the first case, the atmosphere plays no role in the process and must not affect the materials. In the second, on the other hand, it reacts with metals, and is subject to precise study and dosage.
Neutral atmosphere treatment
As the name suggests, neutral atmosphere does not react with the materials present in the treatment device. In most cases, the ambient air is simply sucked in to create a vacuum, but it can also be replaced by argon, a neutral gas. This technique is widely used for annealing, stabilizing and quenching operations , as it greatly contributes to eliminating the risk of oxidation, over-carburization or decarburization in the case of iron alloys (steels).
All metals can be treated in a neutral atmosphere. In fact, pure materials such as iron, aluminum, nickel or titanium alloys can withstand this type of operation very well. This method is particularly suitable for more sensitive materials, such as nickel alloys, titanium or stainless steels, which are often treated under argon (or vacuum), i.e. in the complete absence of atmosphere. For example, for certain alloys used in aerospace parts, argon treatment is still indispensable.
Controlled neutral atmosphere and vacuum have the considerable advantage of reducing the risk of oxidation, thanks to the absence of oxygen during the process. What's more, these are the most environmentally-friendly solutions, since they emit no gases harmful to the environment.
Reactive atmosphere treatment
A reactive atmosphere, on the other hand, acts on materials during treatment. During the operation, a specific gas is introduced into the furnace, at a predefined dosage and for a predefined time.
The best-known methods include carburizing and nitriding. In the first case, the treatment furnace chamber is filled with a certain quantity of carbon-releasing gas, and in the second case, with nitrogen. This process modifies the properties of the treated parts to a greater or lesser extent. Case-hardening, based on carbon, is primarily intended to improve the resistance of parts to wear and fatigue. Nitriding, on the other hand, is designed to enhance the surface hardness of metals and extend their service life, thanks to the formation of surface nitrides.
However, since these gases react with materials, not all metals can be treated in this way with a reactive atmosphere. For example, stainless steels can be case-hardened, but titanium can be nitrided but not case-hardened on an industrial scale. Nor is it possible to form carbon- or nitrogen-based compounds with copper, for example.
The atmosphere present during a heat treatment operation can be decisive. In the case of a neutral atmosphere, it has no impact on the metals, and protects them, but in the case of a reactive atmosphere, it modifies their physical and chemical properties. In the first situation, the air naturally present in the treatment furnace is pumped out to create a vacuum, or replaced by argon, a neutral gas. A reactive atmosphere, on the other hand, involves diffusing a specific gas, such as carbon or nitrogen, to induce the appearance of new chemical elements on the parts. For this reason, the nature of the materials to be treated must be carefully analyzed before choosing a controlled atmosphere heat treatment.
This article deals with the different types of atmosphere and is coupled with the one on the different types of alloys.
