Alternative microstructures and their influence on mechanical properties of case-hardened gears

Case hardening is one of the most common heat treatment processes for highly loaded shafts and gears. Due to numerous investigations, a microstructure consisting of martensite with retained austenite (less than 30%) represents a high sustainable microstructure.

Besides this, there are some alternative microstructures compared to the commonly specified martensitic layer. The central question is how they influence the load carrying capacity of gears. A former research project was focused on the mechanical properties of carbonitrided parts by varying the amount of retained austenite. The investigations revealed that a microstructure containing a significantly increased amount of retained austenite can increase the flank load carrying capacity in comparison to standard case carburized gears. The tooth root bending strength was not influenced in a negative way.

A microstructure containing an increased amount of retained austenite can also be reached by gas carburizing. Furthermore, a variant containing bainite in the case hardened layer was adjusted. In an actual research project, the influence on the load carrying capacity of gears made out of the material 20MnCr5 was investigated in experimental test-runs, using spur gears. The results of the variants with alternative microstructure show some differences, e. g. in the hardness profiles, compared to results of standard case hardened gears. In comparison to the case hardened reference, the flank load capacity of the gears with the alternative microstructures showed increased flank load capacity numbers, whereas the tooth root bending fatigue was not influenced in a negative way.