Failure Analysis of Large Stroke Welded Disk Bellows in Storage

Five out of ten welded disk bellows manufactured of AM350 stainless steel for the control rod drive mechanism of the fast breeder test reactor showed an increase in leak rate using helium leak testing from 10 −9 to 10 −4 Pa-m 3 /s during storage. Though the leak rate is so small that no leak can be detected by soap bubble test, it is more than the acceptable value for deploying in reactor (10 −8 Pa-m 3 /s). This paper presents the results of a systematic failure analysis carried out and recommendations to prevent similar failures in the future. In service, the bellows are deployed in subcooled and tempered (SCT) condition conforming to SCT1000 condition in ASTM A693. The effectiveness of the heat treatment was verified by mechanical properties and x-ray diffraction investigations and compared with the values reported in the standard. Confocal and scanning electron microscopy of the failed bellows at the location of the failure was carried out and few pits were observed on the failed bellows both on the base metal and the weld. Confocal microscopy of the bellows from the other lot did not show significant pits. The potentiodynamic anodic polarization studies of AM350 indicate that AM350 in the solution annealed condition possesses less pitting resistance compared to that in the SCT1000 condition. This also explains the reason for the failure of bellows manufactured in a specific lot with a significant delay between the manufacturing to the heat treatment. This paper also presents recommendations to prevent similar failures in the future.