Effect of Steel Inclusion on Heat Treatment Quench Crack - Case Study

Abstract

Hypoid pinion is a major member in the heavy truck axle as it transmits the power from propeller shaft to hypoid ring gear. This is the first member in axle assembly to receive the power from transmission. Metallurgical quality is inevitable parameter in this part. In this case, a fracture analysis of hypoid pinion carried out, root cause is found & corrections are addressed. The quench crack is generated during the case carburizing treatment. The fractography shows that this crack is initiated from the gross non-metallic inclusion present at the sub surface of the drive pinion. SEM EDS analysis conducted & identified inclusion as Aluminium Oxide predominantly with a dimension 7 mm in length & 1mm in width caused the longitudinal crack on the pinion. The ishikawa diagram plotted for steel mill process to understand the cause. Since the type of inclusion is Al2O3, focused more on ladle refining station, VD process & continuous casting parameters in steel mill process where entrapment could occur. The appropriate corrective action taken in steel mill on VD process, post VD soft rinse, multiple sampling for inclusion from first & last billet, NDT method UT phased array system is introduced by developing the master sample having size same as inclusion. The Inclusion went undetected whole the process starting from steel mill to pinion case carburizing heat treatment & is opened up after quenching due to volumetric difference in crystal, happens during heating & cooling. The mechanism of crack generation is discussed which involves crystallographic changes takes place during carburizing cycles.