The Influence of Nitrogen versus Hydrogen Diluting Gas on the Compound Layer Phase Formation During Ammonia Gas Nitriding of Iron and Low Alloy Steels

In many large industrial furnaces, atmospheric ammonia gas nitriding processes use nitrogen as the diluting gas for safety, economic, and environmental reasons. The widely published Lehrer phase diagram for ammonia/hydrogen gas nitriding of pure iron has been used as a reference guide for selecting ammonia/nitrogen mixtures for nitriding of alloy steels, though the diagram is not precise or accurate when the diluting gas is not hydrogen.

Recent R&D gas nitriding studies on pure iron powders indicate that the use of nitrogen as the diluting gas produces considerably different compound layer (white layer) phase ratios compared to the standard Lehrer ammonia/ hydrogen mixtures, according to XRD analysis. Current data reveals that nitrogen shifts the Lehrer diagram phase boundaries to the left. This is pertinent to the topic as only the gamma prime phase (Fe₄N) is desired, not epsilon (Fe_2-3N). Literature indicates that Fe₄N has uniquely sought-after magnetic properties in some specialized industries.

Testing currently is being performed to compare the pure iron results to that obtained when nitriding common low alloy, medium carbon steel.