The optimum hardness for any gear submited to normal stresses is between 28 - 32 HRC, because in this domain, you will have troostite and bainite as the main structural components. However, the best durability for a gear can be obtained by gazeous or plasma nitriding (but it is not cheap!)...
Is somebody informed about the existence of industrial applications of this technology in Europe or USA? I could not find any new literature about this technology from 1994.
Vlad Marius Dorin, UNIMET-CUG Co., Cluj-Napoca, Romania
