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Ferritic Nitrocarburizing of machined threads

I have a part that failed due to multiple origin fatigue at external threads that have been FNC treated to a depth of approximately 0.001"(compound zone). The threads are on a tension nut that is highly loaded. Some of the literature suggests that FNC will increase fatigue life, but I am wondering if such a thin, brittle surface treatment would not actually reduce fatigue life. If the threads undergo any plastic deformation, I can imagine cracking of the compound zone, resulting in stress concentrations at these shallow crack tips. What is the general feeling regarding FNC of highly loaded cut threads?

David Christie
Metallurgist - Lansing, NY, USA
June 1, 2009

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