Residual stress profiles and tooth flank breakage

 

Extended calculation of the flank fracture risk of case-hardened gears with special consideration of the residual stress state in larger material depths

Motivation

Tooth flank fracture is an industrially relevant damage pattern that can occur in gears. In the case of cyclic stresses, it occurs at high cycle numbers and initiates in the material depth. The fracture depends on residual stresses that are built up during the heat treatment of case hardening. Due to local differences in cooling conditions and phase transformations, residual stresses develop in the steel which influence the occurrence of the damage pattern flank fracture. There are hardly any reliable data and results available for residual stresses in deep areas, as only complex and partly unreliable measuring methods can be applied here.

Procedure

• Carrying out heat treatments for small- and large-modulus gears under varied process conditions (carburisation profiles, quenching medium) and characterising the results of case hardening (microstructure, hardness and residual stress depth profiles)
• Material characterisation for heat treatment simulation: Determination of material data of the case hardening steel 18CrNiMo7-6 as a function of temperature, carbon concentration and microstructure
• Development and validation of a finite element model for the simulation of microstructure and residual stress development during case hardening of gears
• Carrying out simulative parameter studies and deriving regression models for predicting the residual stress depth profiles as a function of the essential geometrical and process-technical data of case-hardened gears
• Development of precise and standard-compliant calculation methods for the assessment of the flank fracture risk as a function of the heat treatment-related residual stresses.
• Derivation of recommendations for action to minimise the risk of tooth flank fracture

Results

• Validated measurement data of residual stresses in the component depth of case-hardened gears as a function of geometric and process conditions
• Extensive material data set for the steel 18CrNiMo7-6 in various carburised states
• FE simulation model and practical regression models for calculating residual stress depth profiles at the tooth flank

Project partner

• Research Centre for Gears and Gear Systems (FZG) Munich
• Leibniz Institute for Materials Oriented Technologies (IWT) Bremen

  Copyright: © FVA-AiF

Funding: Research Association for Drive Technology FVA, Member of the German Federation of Industrial Research Associations AiF

  Copyright: © Witzig, J.

Schematic characteristics of a tooth flank breakage. Indicated are the primary and secondary cracks as well as their direction of propagation, adapted from [1]

[1] Witzig, J.: Flankenbruch – Eine Grenze der Zahnradtragfähigkeit in der Werkstofftiefe. Dissertation. Technische Universität München, (2012).