Steel gears are heat treated to obtain enhanced properties and improved service performance. Quench
hardening is one of the most important heat treatment processes used to increase the strength and hardness
of steel parts. Defects seen in quenched parts are often due to high thermal and phase transformation
stresses. Typical defects include excessive distortion, surface decarburization, quench cracks, large grain
growth, and unfavorable residual stresses. Gear geometries with large section differences may suffer high
stress concentrations and crack during quenching. Surface decarburization before quenching may lead to
high surface residual tension and possible post heat treatment cracking. In this paper, the commercial heat
treatment software DANTE is used to investigate three examples of heat treatment defects. Improved
processes are suggested with the help of modeling. The first example is an oil quench process for a large
gear. Peeling cracks were observed on the gear surface during grinding of the quench hardened gears.
Computer modeling showed that surface decarburization was the cause. The second example is a press
quench of a large face gear. Unexpected large axial bow distortion was observed in quenched gears, and
computer modeling indicated that an incorrect press load and die setup were the reasons. The third example
is an in-process quenching crack caused by high concentrated tensile stress from unbalanced temperature
and phase transformations in a spiral bevel pinion gear. The quenching process was modified to solve the
problem. This example also emphasizes the need for heat treatment modeling in gear design to reduce the
possibility of heat treatment defects. The three examples ilustrate how to effectively use heat treatment
modeling to improve the quality of the gear products.