Assessment of overheated generator end bells

Abstract

18%Mn18%Cr steel end bells from a 350 MW generator were briefly overheated during an operational fault. Many heat affected zones from electrical arcing were found on the inner surfaces of the shrink fit areas. In situ hardness testing revealed that the heat-affected areas were significantly softer than the base material. Replication also showed that localized areas had been heated to temperatures in excess of 400 °C, which is known to significantly reduce tensile strength, fracture toughness and resistance to aqueous stress corrosion cracking. Two of the most severely damaged zones were found to be less than 8 mm deep after progressive excavation, hardness testing and replication. The excavations were located in a section thickness that varied from 38 to 50 mm. The option of temporarily re-installing the end bells was explored using a fracture mechanics approach, assuming that the heat affected zones were equivalent to multiple cracks. The risk of fracture and fatigue crack growth during operation was found acceptable. With operational restrictions, the end bells were re-installed and ran successfully for 8 months until replacement end bells were available.