AGMA 19FTM16 PDF

Case hardening is one of the most common heat treatment processes for highly loaded components such as shafts and gears. Due to numerous investigations and according to the material requirements for quality grade MQ and ME in part 5 of ISO 6336, a microstructure consisting of martensite with less than 30% retained austenite is favorable for a high load carrying capacity.

A former research project focused on the load carrying capacity of carbonitrided gears with alternative microstructures. In this research project, the carbonitrided gears with an increased amount of retained austenite of up to 65% showed a higher tooth flank load capacity than standard case carburized gears. At the same time, the tooth root bending strength was not influenced in a negative way. The question arises, how different alternative microstructures influence material properties and thus affect the tooth root bending strength of gears.

This report states the results of current investigations on material properties such as hardness depth profile, residual stress condition and amount of retained austenite as well as the tooth root bending strength of gear variants with different alternative microstructures. All gear variants are shot blasted after the heat treatment and made out of the materials 20MnCr5 and 18CrNiMo7-6.

The tooth root bending strength in the high cycle fatigue regime of these gear variants is not inferior compared to standard case carburized gears. In the cycle regime of limited life, the tooth root bending strength can be increased as well as decreased by the alternative microstructures.

As consequence, when regarding the tooth root bending strength, certain alternative microstructures, which are different to the recommendations of part 5 of the standard ISO 6336, can be tolerated. This means consequently, that if the tooth root bending strength is tested and acceptable, alternative microstructures can increase the tooth flank load capacity for shot blasted, case carburized gears.