Study on Residual Stress and Optimum Hardening Condition of Induction Hardened Gears

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Temperatures during induction heating process of a shaft with uniform cross-section were calculated by FEM electromagnetic field analysis and heat conduction analysis method, considering changes of the magnetic permeability and the resistivity with the temperature, and measured. The validity of this FEM calculation program was confirmed by comparing the calculated temperatures with the measured ones. An electromagnetic field analysis, a heat conduction analysis, and an elastic-plastic stress analysis during induction heating and water cooling processes of shafts with uniform cross-section and with shoulder fillet, of the inner surface of thick-walled cylinder, of the worm gear, and of the spur external and the internal gear were carried out by the FEM,considering changes of the magnetic permeability, the resistivity, the thermal expansion coefficient, and the yield stress with the temperature. The effects of the electric power and the frequency on the temperature and the stress during … More the induction hardening process and the residual stress due to the hardening were examined.The main results obtained from this investigation are summarized as follows.1. The calculated temperatures during induction heating process of a shaft with uniform cross-section by the FEM program developed in this investigation agree well with the measured temperatures. 2. Amount of heat of surface of the part of major diameter of the shaft with soulder fillet due to induction heating is greater in the case of high frequency but smaller in the case of low frequency than that of the part of minor diameter. 3. Residual stresses sigma_Z^<**> and sigma_<rheta>^<**> in the axial and the circumferential direction of the surface of the part of major diameter of the shaft with shoulder fillet due to the induction hardening become large compressive stresses and sigma_z^<**> and sigma_<rheta>^<**> of the part of minor diameter small tensile stress in the case of high frequency, while sigma_z^<**> and sigma_<rheta>^<**> of both parts become compressive stresses in the case of low frequency. 4. The temperatures of the inner surface during induction heating process of the inner surface of a thick-walled cylinder are higher than those of the outer surface. The difference between temperatures of both surfaces increase with electric power and frequency. 5. Residual stresses sigma_z^<**> and sigma_<rheta>^<**> in the axial and the circumferential direction of the inner surface due to induction hardening of the inner surface of a thick-walled cylinder decrease with increasing electric power. 6. The difference between temperatures at the tooth tip and the tooth bottom during induction heating process of worm gear increases with electric power and frequency. 7. It should be taken into consideration in the strength design of gear tooth that the residual stress along the tooth surface due to the induction hardening of spur external and internal gear becomes tensile stress in some cases at the tooth bottom under the heating condition of high frequency and large electric power. 8. Induction hardening by dual frequency method is considered to be valid for a shaft with shoulder fillet and a spur gear. Less