Increase in fatigue life of gears hardened by contour induction using double shot peening

Countour Induction Hardening (CIH) method is in the limelight as a new possible surface refining method for gears as it reduces quenching distortion considerably and increases compressive residual stress. Also, Double Shot Peening (DSP) method, especially its advanced technique of using extensively small shots (less than 100 m) at the secondary peening, enables increased compressive residual stress under the surface of the gears.

Therefore, the new Compound Surface Refining Method of using CIH + DSP is expected to give much higher compressive residual stress than the conventional methods. On studying the application of this new method, the authors evaluated their surface-refining characteristics and the fatigue limits. The main results of the study are as follow: 1) Very small austenitic grain size and no abnormal surface structure in CIH and CIH + DSP gears. 2) Extremely high compressive residual stress under the surface of CIH + DSP gear. 3) Considerably increased fatigue limit of CIH + DSP gear, i.e., 154% and 36% higher fatigue limit than the Carburized and CIH gear, respectively.