I would appreciate if anyone could indicate where to find
the S-N curves of Inconel 600 strip or plate plotted for several stress ratios.
Nickel Institute and Special Metals have been contacted, but the information received was not satisfactory.
You might want to try in the "Atlas of Fatigue Curves". I would try a local engineering university library to make sure the Atlas contains the curves you need. I don't have a copy of this book at my present location.
Wow, if neither of these entities was able to answer the question, then you will likely find it difficult to obtain a satisfactory answer in the public domain. Are you a customer of Special Metals? Surely they have this data available.