Encyclopedia of Iron, Steel, and Their Alloys
The outcome of a nitriding or nitrocarburizing process depends on many different factors, like parts’ Preparation, cleanliness, loading, temperature, as well as atmosphere uniformity, but one of the most influencing parameters for a thermochemical process is, besides temperature, the chemical composition of the atmosphere, as the parts are exposed to it while being treated. This entry gives an overview on how the composition of the process atmosphere interacts with the parts’ surface, by explaining the relation between the partial pressures of ammonia, hydrogen, oxygen, carbon monoxide, and carbon dioxide and the resulting nitriding and nitrocarburizing potentials. Further, these specimens and their ratios, given the said potentials, will act as driving forces to induce or reduce the amount of nitrogen, carbon, and oxygen atoms either in solution or bound in different phases. The entry also presents a variety of control policies for nitriding and nitrocarburizing processes, what gases to use, and what properties might be aimed for.
For many years, nitriding and especially nitrocarburizing was taken as an easy to accomplish heat treatment. All that was needed was to expose parts at a given temperature to an ammonia-containing atmosphere, sometimes with an addition of carbon bearing gases, such as carbon dioxide or endogas in order to reach the typically not very detailed specifications. From time to time hardness, case depth, and compound layer thickness did not meet expectations; nevertheless, the process was considered to be nonproblematic.
With the SAE AMS 2759/6 (Aerospace[1]), issued in 1987, the requirements for performing a nitriding process were specified more clearly, defining one- or two-stage processes where temperature and the so-called dissociation rate have been matched to steels and tolerated thickness of compound layer. The dissociation rate was easily verified using a water burette.
In order to enable the use of more complex atmospheres, like nitrogen-diluted ammonia, and still come to predictable results, instead of defining the dissociation rate, the SAE AMS 2759/10 (Aerospace[2]), first issued in 1999, defined the so-called nitriding potential. This way, the process atmosphere could be controlled to
match the relation between the nitriding potential and ironnitrogen phase boundaries, as given in the Lehrer diagram (Fig. 1). Of course, nitriding potentials have to be adjusted to steel grades in order to allow for the shifts in phase boundaries caused by the presence of alloying elements.In 2006, the SAE AMS 2759/12 (Aerospace[4]) similarly specified how to perform a controlled nitrocarburizing process by defining stages where the nitriding and carburizing potentials have to be controlled concurrently in order to achieve a given porosity in the compound layer. Like in the SAE AMS 2759/10, these potentials vary depending on the steel to be treated. CHEMICAL POTENTIALS, ACTIVITIES, AND PARTIAL PRESSURES (E.G., FROMM AND GEBHARDT[5])
Nitriding and nitrocarburizing are thermochemical processes; therefore, the outcome of the treatment depends on temperature and chemical reactions. Such chemical reactions take place in the process atmosphere where gas molecules react with each other, but also between the atmosphere and the metal surface and within the metal structure (e.g., Fromm and Gebhardt[5]).
The reactions are driven by the chemical potentials of the reacting species that can be seen as some kind of potential energy that will either be released or has to be spent in order to make these reactions happen.
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