ABSTRACT

Today, case hardening represents the standard heat treatment process for highly stressed gears. The microstructure in the surface layer of the gears generally consists primarily of martensite with a certain amount of retained austenite. In national and international standards, retained austenite contents of up to 25 to 30 vol. % are allowed with regard to optimum tooth root and tooth flank load carrying capacity. In addition, precipitations are tolerated only up to certain limits.

In a research project between IWT Bremen and FZG, Technical University of Munich, the application and recommended ranges of the standards were deliberately exceeded in order to determine results on the gear load carrying capacity of material structures characterized by comparatively high contents of retained austenite on the one hand and with different carbide respectively carbonitride precipitations on the other. Different heat treatment processes were used. A selection of the experimental results is presented in this paper, with the focus on the results for the material 20MnCr5. In addition to varying the heat treatment, different blasting conditions were investigated.

The variants with alternative surface layer structures show in some cases significantly increased load carrying capacity numbers compared to the gas carburized reference variant, although the surface hardness is significantly below typical values for case hardened surface layers. The paper evaluates the experimentally determined strength values in the context of the current state of knowledge and shows possible explanations for the increased lifetime of the alternative surface layer structures.

Author(s): Niklas Blech, Thomas Tobie, Karsten Stahl, Holger Surm, Matthias Steinbacher

ISBN: 978-1-64353-150-2