Gas nitriding is a nitrogen-based heat treatment process. This technique was developed at the beginning of the last century in the armaments industry, and is now used in a wide range of industrial sectors. Thanks to its very low operating temperature and the many properties it confers on parts, this treatment is particularly interesting and durable, and is available in a variety of technologies.
Origins of nitriding
The term "nitriding" actually comes from the English name for the element nitrogen. Noted as N in the periodic table of elements, nitrogen is therefore at the heart of nitriding. Its use emerged in the 1910s in the United States and Germany, developed by the arms industry.
Prior to this invention, these manufacturers were faced with a major problem: the treatments used up until then were carried out at high temperatures and modified the structure of the gun tubes, which were automatically deformed. Gas nitriding made it possible to treat the inside of these tubes while preserving their shape.
This process represents one of the first low-temperature treatments used in industry for hardening purposes. This method makes it easy to treat the bores of mechanical parts, while causing very little deformation. Following promising trials of this technology on gun tubes, gas nitriding has gradually spread to become a standard metal heat treatment technique.
How does gas nitriding work?
Gas nitriding is based on a simple chemical reaction involving the decomposition of a gas, ammonia. Ammonia is composed of one nitrogen atom and three hydrogen atoms, so its decomposition results in the diffusion of nitrogen into the atmosphere.
Advantages of this technology
The first advantage of gas nitriding is its low processing temperature. Instead of the 1,000 degrees Celsius required for carburizing or hardening, this chemical reaction can take place at just 500 degrees Celsius, guaranteeing better dimensional control of the treated part.
Gas nitriding of steel also generates iron, chromium, vanadium or aluminum nitrides, depending on the chemical composition of the steel. These contribute considerably to the wear resistance of the part. The formation of this nitrided layer can be broken down into two: the combination layer, which is found at the extreme surface of the part, and the diffusion layer, which is created a few tenths of a millimetre deep. The former is exceptionally hard, exceeding 1000 Vickers, and extends to a depth of just a few microns. It gives the part excellent wear resistance and a very good coefficient of friction. The diffusion layer corresponds, as the name suggests, to the zone where the nitrogen diffuses, and can extend up to a millimeter below the surface of the part. Gas nitriding is one of the multi-purpose heat treatments, as it is in fact a particularly versatile method. In some cases, it is possible to add an oxygen diffusion step. This is known as post-oxidation, a process which helps to reinforce the corrosion resistance of parts. Finally, this treatment also has the advantage of being applied to finished parts, i.e. once treated, they do not require reworking. Gas nitriding can therefore save real time and money. It is also possible to diffuse carbon at the same time as the nitrogen. This is known as nitrocarburizing, an advantageous treatment as it can be applied to less alloyed steels, and therefore less costly.
Limits of gas nitriding
The main limitation of this solution is undoubtedly its processing kinetics. Indeed, the time required to complete the process can be very long. Depending on the steel used, a gas nitriding treatment up to 1 mm deep can take around a hundred hours, compared with just 10 hours for case-hardening. What's more, final nitriding performance also depends on the grade of steel being treated and the quantity of additive elements in the steel enabling nitride formation.
Various gas nitriding techniques
In line with its commitment to offering solutions to suit all needs, Thermi-Lyon offers three different gas nitriding techniques: atmospheric pressure, low pressure and high pressure. The first is particularly useful in the aerospace industry, for example, when treating aircraft engine bearings. These need to be treated to a depth of more than a millimeter, which is what atmospheric pressure gas nitriding naturally offers. The high-pressure treatment we offer is called Thermi SNT and is intended for stainless steels only. Finally, the low-pressure solution is performed in a vacuum.
Today, gas nitriding is used in a wide range of industries, including the treatment of plastic injection molds and forming tools, as well as parts for turbochargers, injection systems for automotive engines and car valves. Thanks to its ease of application, this treatment is used in almost all industrial sectors, from automotive to food processing machinery.
Developed over a century ago by the armaments industry, gas nitriding is now used in a wide range of sectors, because it is so easy to use. Performed at low temperatures, it is based on the formation of nitrides on the surface of steel parts through the diffusion of nitrogen, giving treated parts excellent wear resistance.
