Contact fatigue and bending fatigue are two main failure modes of steel gears. Surface pitting and
spalling are two common contact fatigue failures, which are due to the alternating subsurface shear
stresses from the contact load between two gear mates. When a gear is in service under cyclic load,
concentrated bending stresses exist at the root fillet, which is the main driver of bending fatigue failures.
Heat treatment is required to increase the hardness and strength of gears to meet the required contact
and bending fatigue performance. Induction hardening is becoming more popular due to its process
consistency, reduced energy consumption, clean environment, and improved product quality. It is well
known that an induction hardening process of steel gears can generate compressive residual stresses in
the hardened case. Compressive residual stresses in the hardened case of tooth flank benefit the contact
fatigue performance, and residual compression in the root fillet benefits the bending fatigue. Due to the
complex gear geometry, the residual stress distribution in the hardened case is not uniform, and different
induction hardening process can lead to different residual stress pattern and significant variation of
fatigue performance. In this paper, an innovative approach is proposed to flexibly control the magnitude of
residual stress in the regions of root fillet and tooth flank by using the concept of preheating prior to
induction hardening. Using an external spur gear made of AISI 4340 as an example, this concept of
innovative process is demonstrated with finite element modeling, using commercial software DANTE.

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