Distortion Mechanisms During Carburizing and Quenching in a Transmission Shaft

Distortion control during industrial carburizing and quenching operation of precision transmission components is of utmost importance due to their direct impact on performance, such as efficiency, noise and vibrations. The importance of controlling various heat treatment process parameters for mitigating distortion is well accepted, but their specific influence on mechanisms is less understood. In the present work, an integrated finite element-based model is used to simulate gas carburizing and quenching operation on a typical transmission shaft. Investigation is carried out on the effect of raw materials, carburizing and quenching process parameters to predict, analyze and minimize distortion. The effect of phase transformation and generation of thermal strain during heating and cooling stage of heat treatment is investigated, and the mechanisms of bending, diameter and length distortion of a shaft are analyzed. The displacive nature of transformation of bainite at higher temperature with its inherent large shear component of deformation was identified to be responsible for bending distortion in a shaft. Bainitic transformation, martensitic transformation and thermal strains developed during quenching cause volume expansion which leads to diameter expansion and lesser length shrinkage. Finally, bending distortion of a shaft and bore distortion of a gear are contrasted, both in terms of mechanisms and distortion control strategies.