Analysis of deformation in the carburizing-quenching heat treatment of helical gears made of SCM415H steel

Helical gears made of low alloy carbon steel are widely used in automotives as components of power transmission because their hardened surfaces enhanced their strength and wear resistance. The hardened surface of the gear is commonly obtained by the carburizing and quenching heat treatment. However, deformation of teeth of the gear caused by the heat treatment significantly affects the dimensional accuracy of the precision part, making noise, causing vibration, and hastening malfunction of the component in the power transmission. In this study, investigation of deformation caused by the heat treatment of a helical gear made of SCM415H steel was carried out by a numerical simulation with the finite element method (FEM). In order to examine the effect of the overall characteristics of the heat treatment on the deformation, temperature history, carbon diffusion, phase transformation, and axial stress were analyzed at the same time. In addition, diffusion time and quenchant temperature were adopted specifically as process parameters to study their effects on the deformation. The simulated results revealed that the volume of the gear and the shape and lead of the tooth became somewhat smaller by heat treatment than before the process. Results showed that when the coolant temperature was increased enough, deformation of the gear was reduced.