Residual stress relaxation by bending fatigue in induction-hardened gear studied by neutron Bragg edge transmission imaging and X-ray diffraction

The compressive residual stress on the gear tooth's surface, a vital parameter to control mechanical properties, such as strength, has a beneficial effect on the component's fatigue life. A novel procedure, double induction quenching (DIQ), effective for improving the fatigue strength of gear products, has been used for producing gears with steep gradients of compressive residual stress generated in the tooth surface. We performed a Bragg edge imaging experiment at a pulsed neutron source to determine the spatial distribution of the {110} lattice spacing (d110) and the broadening of the {110} Bragg edge (w110) on the DIQ gear product after tooth-bending fatigue tests to which different loading cycles were applied. No significant difference occurred in the d110 and the w110 at Hofer's critical section (tensile side) of the teeth with different loading conditions within the accuracy of data analysis. However, we detected a decrease in the w110 and changes in the residual lattice strain distribution in the axial direction (through the thickness) along the tooth root directions at the opposite side of Hofer's critical section for both teeth after 3 x 105 and 8 x 105 cycles, relieving the compressive residual stresses during the fatigue process. The residual stress close to the gear tooth surface determined by X-ray diffraction using sequential polishing showed a slight relaxation and redistribution from the tensile side in the hoop direction, complementary to the neutron Bragg edge imaging information in the axial direction.