Investigations on the Tooth Root Bending Strength of Larger-Sized Induction Hardened Gears

Surface hardening is an economical and technological alternative to case hardening. This is especially true for larger sized gears. Due to the necessary high case hardening depths required for larger case hardened gears and due to technological boundaries (e.g., heat treatment furnace size and heat treatment duration) typical surface hardening processes such as flame or induction hardening can exhibit their benefits for these parts. While flame hardening usually results in mostly through hardened gear teeth, contour-hardened teeth can be achieved by induction hardening. As a result, the properties of surface hardened gears significantly differ in the surface and in the core region. However, the achievable tooth root bending strength strongly depends on the gear properties, such as the surface hardening depth and the microstructure. In the framework of this paper, the influence of induction hardening on the tooth root bending strength of larger-sized gears is investigated. Therefore, different variants of larger gears which were induction hardened gap-by-gap are compared. In order to gain a deep understanding, a systematical variation of the surface hardening depth, gear size (m_n = 14 mm and 20 mm), and surface condition was carried out. For example, as for the surface condition, one variant is additionally shot blasted after the hardening process. In addition, the experimental results for the induction hardened variants are compared to a flame hardened variant. The experimental investigations were done using a pulsator test rig and all variants are characterized by metallographic analysis and the determination of hardness depth profiles. The results are compared to the state-of-the-art for induction hardened gears according to ISO 6336, part 5 and are additionally contrasted to experimental results for case hardened gears with an equivalent size found in the literature.