Bending fatigue strength of case carburized and nitrided gear steels for aeronautical applications

A long-term research program was performed in order to experimentally characterize the bending fatigue resistance of high strength, high cleanliness gear steels typically used in the main gearbox of helicopter transmissions. The investigation was focused on the two main classes of gear steels adopted in this type of high power density transmissions, i.e. case carburizing and nitriding steels. A total of eight test variants were tested considering three different case carburizing and two nitriding steel grades along with shot peened and unpeened conditions. STBF (Single Tooth Bending Fatigue) tests were conducted on gear specimens having the same geometry and produced with the same manufacturing cycle in order to obtain results directly comparable between each other, as well as with the limit values given by gear standards. The results showed that fatigue limits significantly higher than those provided by gear standards can be achieved for both case hardened and nitrided steels for aeronautical applications. Moreover, the significant increase of the bending fatigue strength obtainable by means of shot peening, even for this class of high strength steels, was quantified for both case carburized and nitrided gear steels.