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Prematurely fractured teeth of SCM440 large‐sized spur gear are commonly found among gears of a mechanical press machine. The macroscopic inspection and microstructure examination of a fractured tooth are established to show that the previous induction hardening process produced two serious issues; insufficient hardening in the root areas and nonuniform hardened depth along the tooth circumference. The numerical simulation is executed to replicate the previous induction hardening process, which consists of simulations for tooth‐to‐tooth mobile induction heating, water spray quenching, and the tempering process. This study highlights the influence of most adjustable parameters, such as the scanning speed of the inductor coil and air gap, to collect beneficial insights for further improvements. Experimental and numerical results are compared to validate the numerical model used. The adjustment of scanning speed and air gap highly affects the magnitude and uniformity of hardened depth along the tooth profile. Promising improvements can be achieved by slower, smaller scanning speeds and thinner air gaps, where the distortion is still in the allowed allowable range. This research provides a method and insights for future explorations to deal with relevant issues in the tooth‐to‐tooth mobile induction hardening process.

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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 3.02–4.05% across different experiment-numerical validation scenarios. The induction heating and spray cooling design employed in the study generate 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 nonlinear 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 is very beneficial to build intuitions in obtaining desired hardening quality of tooth flank using mobile induction hardening.