Origin of non-uniform tooth flank hardening distribution in SCM440 mobile induction heat–treated steel spur gears—a parametrical study with experimental–numerical coupled investigation

The design of a hardening process that can achieve the desired level of hardening quality is paramount for spur gear teeth, as a poorly executed process may result in a variety of defect schemes. The mobile induction hardening technique has emerged as a promising and cost-effective method for large spur gears. However, achieving the desired output quality remains challenging. This study aims to comprehensively evaluate the results of gear tooth hardening using the tooth-to-tooth mobile induction hardening process. The evaluation process focuses on the tooth flank, which is the area most prone to failure. The study investigates the effects and interactions of crucial process parameters, such as flank length, scanning speed, and air gap, on the hardening results. Numerical and experimental measurements are used to characterize the hardening results. The study’s results demonstrate high accuracy in the modeled numerical simulation, with prediction errors ranging from 3.02 to 4.05% across different experiment-numerical validation scenarios. The induction heating and spray cooling design employed in the study generates sufficient heating energy to achieve an average austenite distribution of 97.13% in the heat-affected zones and an average martensite phase of 82.21% during the quenching process. A tempering process is then carried out as a standard procedure to enhance the material’s ductility, resulting in a decrease in material hardness from a maximum of 64.77 HRC initially to a maximum of 61.98 HRC. Multivariable non-linear regression analysis confirms the significant influence of the studied process parameters on flank hardening quality, with the scanning speed parameter having the most substantial impact. The quantitative results indicate that reducing the scanning speed, air gap, and flank length leads to better hardening quality in terms of longer hardened flank, deeper hardening depth, and smaller edge effects. Insights provided in this study are very beneficial to build intuitions in obtaining desired hardening quality of tooth flank using mobile induction hardening.