When nitriding a steel, it is advisable to have a core with a phase structure of tempered martensite. The tempered martensite will assist with the support of the formed case. It is the strength of the core that will enable the nitrided surface to support load-bearing applications. If the core is softer than the load-bearing application, the case will fail and collapse. The inference is that it is the case that has failed. This is an incorrect assumption because it is the core that has failed.
Low hardness on a plasma-nitrided case is generally a result of incorrect gas ratios of nitrogen/hydrogen. The formula for ammonia decomposition at operating temperature is expressed as follows:
2NH₃⇄2N + 3H₂
For this reaction, the ratio of decomposed nitrogen to hydrogen is one part nitrogen to three parts hydrogen.
Plasma nitriding enables the user to vary the processed gas ratios of nitrogen to hydrogen. Therefore, if the formed nitrided case exhibits a low hardness value, it is usually indicative that the nitrogen ratio has been low in relation to hydrogen. It could perhaps be a ratio of one part nitrogen to (for instance) six parts hydrogen. For AISI 4140, this could mean that a nitrided case will form, but because of the low nitrogen availability there will be few stable nitrides formed, resulting in low surface hardness.
The advantage of plasma nitriding is that one can vary the nitrogen-to-hydrogen ratios to one that best suits the steel being treated for its particular application.
