<p>The deployment of heavy machinery in the mining industry is a typical practice, and machine components such as drive shafts and gears frequently fail when operating together. However, such failures limit the plant’s productivity, resulting in economic losses for the companies. In the present study, we investigated the failure of an IGB pinion shaft made of case-hardened steel that was utilized to drive the bull gear of a massive ball mill. According to firsthand information, it was suspected to be an abrupt failure caused by a shock load, and circumstantial evidence was gathered prior to the failure analysis. Both metallurgical and numerical methods were used to determine the type of failure that happened with the power transmission shaft. Samples from diverse areas were tested for metallography (inclusion, microstructure, hardness) and mechanical properties (tensile, fatigue, impact) in accordance with ASTM standards. Additional studies, such as FEA analysis, were conducted on the CAD models to better understand the stress conditions based on input parameters such as input power, gear ratio, and shaft rotations. During the fractography examination, the huge, failed shaft and cyclically loaded test samples underwent both visual inspections and SEM studies, respectively. The material tests and chemical analysis reports indicated that all of the parameters met the standards for a case-hardened steel. The current failure study was concluded as a normal fatigue failure by rejecting the earlier hypothesis of sudden shaft fracture because of low fracture toughness of the material. Because it was believed that the material possesses a low fracture toughness but later after impact test, the value found to be high (147 J), which is reasonable for a case-hardened 18CrNiMo7-6 grade steel.</p>