A heavy vehicle turbine shaft fractured in service. The manufactured called in Metallurgical Associates to determine both the cause of this failure and the potential for failures in other turbines in service that could lead to a costly re-call.
MAI engineers performed a systematic visual examination of the multiple fracture orientations, reconstructing the order in which they occurred to identify the first fracture and origin location. This area, the longitudinal crack adjacent to a bearing diameter, was sectioned for further analysis by SEM and optical microscopy. Other locations were selected for hardness testing, chemical analysis and additional optical microscopy to determine if the shaft was in conformance with design and heat treatment specifications.
SEM examination of the origin revealed fatigue fractures initiating at the surface from multiple locations due to torsional loads in service. Chemical analysis identified the shaft material as the specified SAE 5150 alloy steel. However, the sulfur content was at the maximum allowable level. This resulted in numerous manganese sulfide inclusions which acted as fatigue initiation sites.
MAI engineers provided a range of cost-effective options to prevent future shaft failures. A non-destructive testing program was developed using magnetic particle inspection to identify and replace shafts in service that contained cracks but had not yet failed. Shot peening at critical, highly stressed areas on shafts in inventory was recommended, significantly increasing fatigue resistance and allowing manufacturing to continue uninterrupted. Finally, the material specification was revised to a lower maximum allowable sulfur content, preventing the formation of gross manganese sulfide inclusions which acted as initiations sites for the multiple fatigue crack origins.