Failure of a helical gear in a power plant

• The results showed that the helical gear failed by fatigue fracture.
• The presence of oxide–carbide inclusions in the structure is thought to be the root cause of the failure.
• Fatigue crack was initiated inside the material at the inclusion–matrix interface.
• The carbide was formed during the solidification of the ingot due to segregation of alloying elements to the liquid surrounding existing oxide prior to the completion of ingot solidification.
• It is recommended that improved steel making practice be employed.

Failure of high reliability components such as gears in power generation plants is a rare occurrence. This paper reports the results of failure analysis of a helical gear used in a power plant in Thailand. The gear failed after approximately nine years of service. Visual inspection as well as macro- and microscopic examinations of fracture surface were performed. Microstructure of the gear material was examined and microanalysis were carried out. Microhardness at the case of the gear at various distances from the surface were measured. The results showed that the fracture surface exhibited fatigue fracture characteristics. There were inclusions in the microstructure of gear material. Microanalysis results indicated that the inclusions are iron oxide surrounded by (FeMo)3C carbide. It is concluded that the gear failed by fatigue fracture. Fatigue cracks were initiated at the inclusions and propagated through the material until finally the gear tooth was overloaded and fractured. The inclusions were thought to be formed during the casting of the ingot that was used to make the gear blank, possibly due to inadequate degassing or deoxidation of liquid steel prior to ingot casting. It is recommended that cleaner steels be used to make such high reliability gears which could be achieved via improved steel making practice.