Investigation of the fracture of a car’s drive shaft

A car’s drive shaft, machined from case-hardened steel that was carbo-nitrided, quenched and tempered, ruptured at an early stage of its use. Both sides of the fractured shaft were delivered for an investigation in order to discover the cause of the rupture. The fractured area was partially damaged due to the mutual rubbing of the shaft’s sides. The initial part of the fracture surface was darker and corroded, suggesting it originated either during the quenching or as a result of a shock impact early on during the shaft’s useful life. Scanning electron microscopy revealed an inter-crystalline fracture in the carbo-nitrided region of the shaft. Auger electron spectroscopy (AES) was used to analyse specimens that were fractured in-situ in order to avoid any contamination. The obtained fracture was probed for grain-boundary segregation. A possible explanation for the origin of the grain-boundary weakening in the shaft’s production process is put forward.