Failure analysis of an idler gear of diesel engine gearbox

A failure investigation has been conducted on an idler gear of diesel-engine gearbox used in a truck, which is made from 20CrMnTi steel. The gear was plasma-carburized. The inner circle is splined. The crack origin zone is situated at the root fillet location of the weld region of the splined inner circle. The macro-fracture surfaces show brittle cracking characteristics and the micro-fracture exhibits intergranular cracking features. The delayed brittle fracture is the dominant failure mechanism of the idle gear. The detailed metallurgical examinations on the carburized layer and the core zone were conducted. The failure causes were assessed.