During the annual maintenance of tracked vehicles operated by the army, large amounts of unidentified metallic foreign substances are observed in the transmission oil filter when exchanging the transmission oil and filter. After the transportation of the transmission from the field military base to the army consolidated maintenance depot, a systematic disassembly and inspection revealed that the hub assembly in the planetary range pack of the transmission was destroyed. The primary and secondary damaged parts were analyzed to investigate the root cause of the failure, and large amounts of metallic foreign substances collected in the oil filter were also analyzed. Owing to the intergranular fracture mode observed at the fractured surfaces of the hub assembly, it was speculated that a catastrophic fracture occurred owing to the action of an unknown instantaneous external force larger than the allowable load. To prevent the recurrence of similar cases in the future, a welding procedure specification (WPS) was designed, and a preheating procedure was performed before the welding process.