Residual stress in case hardened steel gears

Aerospace gear components are required to demonstrate excellent load carrying and endurance characteristics. Case hardened steels are often utilised for these parts. During the manufacturing process of case hardened steel gears residual stresses are developed. The nature and distribution of these stresses are known to have a significant effect on distortion during heat treatment and machining processes. The fatigue performance of a gear subjected to cyclic loading is also strongly dependent on the nature and magnitude of manufacturing induced residual stresses.
In this work the development of residual stress during the heat treatment and machining process has been assessed in case hardened steel alloy spur gears. Gears have been manufactured from two different initial conditions; as-received bar material, and hot forged billet. A spur gear geometry was rough machined prior to full heat treatment, and subsequent surface finishing. The evolution of bulk residual stress distribution within the gears throughout the manufacturing process was measured using the Contour Method. Surface residual stresses, complimentary to contour method, have been measured by X-ray diffraction.
Gears from both manufacturing routes (i.e. machined from forged billet and as received bar) were found to develop tensile residual stress in the core of the gear following full heat treatment. High compressive stressed regions develop at the carburised case region at the exterior of the gear. The final residual stress magnitude and distribution within the gear was found to be independent of the initial forming process.