A novel investigation into edge effect reduction of 4340 steel spur gear during induction hardening process

Abstract

Induction hardening, a promising approach for selective hardening of metal parts, is widely used for surface hardening, where a hard surface is required alongside a tough core. Regarding the complexity of this process, parts’ geometry deeply affects the temperature distribution and hardness profile accordingly. In this study, two magnetic flux concentrators are introduced to our induction machine set in order to control the magnetic flux and consequently hardness profile (case depth) of spur gears. The performance of magnetic flux concentrators is examined by the effect of machine parameters on the case depth and the edge effect of AISI 4340 steel-made spur gear. Design of experiments based on Taguchi method is primarily used to optimize the number of experimental trials. Then, the hardness profiles of heat-treated gears at the tip and root of gears are measured by microindentation hardness tests. The results are analyzed using analysis of variance (ANOVA) and response surface methodology (RSM) to determine the main effect of process parameters, also the best combination of process parameters that maximizes the case depth and minimizes the undesirable feature of edge effect. Finally, the predicted case depth models versus process parameters are developed based on linear regression method. To this end, four predictive models of case depth at tip and root in the edge plane and middle plane of spur gears are generated. Results imply that maximum case depth with minimum edge effect at root and tip is achieved by setting up the highest machine power, longest heating time, and minimum axial gap between concentrators and the spur gear. This study provides a good exploration of case depth in presence of magnetic flux concentrators under various process parameters and gives a reliable guideline towards edge effect during induction hardening process.