Gas Nitriding is an excellent alternative to liquid nitriding with the proven ability to meet and exceed the capabilities of the liquid treatments.

Liquid and gas nitriding have historically been used to fill different niches, with gas nitriding typically being used for deeper cases which are impossible to produce with liquid nitriding. In actuality, the two processes are nearly identical, as they rely on the same fundamental principles to achieve hardening. Gas nitriding was the first process to be developed, with the first patent being in 1908. Liquid nitriding became popular in the 1930s because it offered better uniformity and required a lower capital investment than the gas nitriding furnaces of the time. These advantages do not hold true in the present day, as modern controlled gas nitriding has advanced to the point where it is displacing liquid nitriding as the method of choice for low temperature case hardening.

The “Post-Oxidation” or “Oxynitriding” process is the gas equivalent of the brand-name QPQ process typically offered with liquid nitriding. With both processes, a black Fe3O4 oxide layer is formed to improve the corrosion-resistance. AIHT’s Post-Oxidation Reaction (PROTM) process replicates the QPQ process of liquid nitriding.

Metallurgical Comparison

Surface hardness is dependent on material chemistry, and thus is equivalent between nitriding processes. Core hardness is dependent on initial tempering condition provided that the nitriding temperature used is significantly below the tempering temperature. The radial growth of the component is related to the amount nitrogen that is diffused into the surface (which can be approximated by case depth), and will be equivalent for comparable case depths

Gas Nitriding can be tailored to generate cases of virtually any depth including depths shallower than Liquid Nitriding. Since gas nitriding is computer controlled, it is much easier to meet thin case specifications as are commonly found in sour service applications

The thickness of the white layer can be controlled using gas nitriding. This is important, as this brittle surface phase can be prone to cracking, particularly at sharp edges where coarse nitride networks can form and serve as crack propagation sites. Some white layer is desirable due to its corrosion-resistant properties, but an excessive white layer can lead to premature failure, particularly in sour service. As Liquid Nitriding is a manual process, it isn’t possible to control the white layer thickness.

Liquid-Equivalent Stainless Steel Nitriding

Activation of some stainless grades can be quite difficult, which has resulted in the public perception that gas nitriding of stainless steels is impossible. This is not actually the case. Gas nitriding can also be used to nitride stainless steels provided the surface is properly treated prior to nitriding. Gas nitriding allows for nitriding to much deeper case depths on stainless steel than liquid nitriding, and white layer control results in far fewer nitride networks. Gas nitriding also allows for specialized procedures to nitride stainless steel without impairing the corrosion-resistance, but this comes at the expense of case depth.

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Sample Case Depth Reports

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