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There are various thermochemical processes used on steel to form surface conditions needed to reduce wear [1-3]. Increasing tribological and corrosion properties of numerous steel components and additionally enhancing their visual appearance at the same time is also possible when a final post-oxidizing step is used. There are a few processes available; gas nitriding and gas nitrocarburizing with a post-oxidizing step and traditional liquid nitriding of steel [1-3]. Advanced Heat Treat Corp.'s gas nitriding process called UltraOx® offers an extremely good combination of all the required properties [4].
Liquid Nitriding / Salt Bath Nitriding
Liquid nitriding is the case hardening method carried out in a molten salt fused-bath containing either cyanides or cyanates [2]. The method is old and it has several proprietary modifications such as: liquid pressure nitriding, aerated bath nitriding, liquid nitrocarburizing and others. A typical commercial bath for liquid nitriding is composed of mixture of various sodium and potassium salts/cyanates and the operating temperature is 565°C (1050°F) [3].
SAFETY NOTE:
“The compounds are highly toxic…Contact between the compounds and mineral acids also generates another hazard: the formation of hydrogen cyanide (HCN) gas, an extremely toxic product. Exposure to hydrogen cyanide can be fatal” [3].
Thus, the environmental effects of such chemicals can be very negative and therefore the process has to be under strict control. Also, liquid nitriding has to be carried out at comparatively high temperature, limiting its applications.
Gas Nitriding
Gas nitriding covers processes whereby, respectively, nitrogen (nitriding), nitrogen and carbon (nitrocarburizing), or nitrogen and sulfur (sulfonitriding) are introduced into the surface of a solid ferrous alloy, typically but not exclusively in the ferritic state of the material [1]. The source of nitrogen is ammonia. The process can be carried out in a broad range of temperature, 390-620°C (734-1148°F) to produce a typical compound zone + diffusion layer structure or the structure of the expanded austenite at low temperatures. Very precise control of the processing allows for strict control of nitrogen and carbon percentage in the compound zone, which is very important for a predictable formation of the structure and properties of the layer.
Post-Oxidizing
An additional post-oxidizing step is used for further enhancing corrosion properties as well as to form the desired blackness of the treated component. Those properties can be well-controlled when the UltraOx® is used [4].
Considering making the switch from Liquid Nitriding (Salt Bath Nitriding) to UltraOx® / Gas Nitriding?
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References
1. K.-M. Winter and J. Kalucki, “Gas Nitriding and Gas Nitrocarburizing of Steels”, ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes J. Dossett and G.E. Totten, editors Copyright # 2013 ASM International, pp. 647-679.
2. G. Pantazopoulos, “Liquid Nitriding of Steels”, ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes J. Dossett and G.E. Totten, editors Copyright # 2013 ASM International, pp. 680-689.
3. H.-J. Spies, “Corrosion behavior of nitrided, nitrocarburized and carburized steels”, Thermochemical Surface Engineering of Steels, Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014, pp. 267-309.
4. Advanced Heat Treat Corp. https://www.ahtcorp.com/services/nitriding-and-nitrocarburizing/ultraox/
Who is Doctor Glow?
Dr. Edward Rolinski, aka Doctor Glow, has been studying the plasma/ion nitriding phenomenon since the 1970s and is arguably one of the most knowledgeable people in North America when it comes to nitriding.
The doctor has written countless articles and whitepapers in industry publications and manuals. Some of his most noteworthy contributions include the chapter on "Controlling Plasma Nitriding" in ASTM International (2017) as well as "Nitriding of Titanium Alloys" in the ASM Handbook (2016).
Dr. Edward Rolinski is the Senior Scientist at Advanced Heat Treat Corp. (AHT) and has been employed at AHT since 1994.
LEARN MORE ABOUT DOCTOR GLOW VIEW HIS RECENT ARTICLES
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