Failure analysis of isolation valve stems in steam generator blow down system

Two isolation valves failed prematurely in the steam generator blow down system of a nuclear power plant. Metallurgical analysis of the damaged stems in the isolation valves was made. Heat treatment tests were carried out to verify the reliability of stems in comparison with the required mechanical specifications. The results of the analysis showed the abnormal precipitation of nonmetallic inclusions, MnS, in the matrix. These nonmetallic inclusions which could acts as notches were distributed randomly to 0.2 vol.%, two times higher than those of typical materials. In addition, the hardness of the damaged stems was twofold higher and the elongation was reduced by half compared to those values attainable from a proper full annealing. It is concluded that improper tempering applied to the damaged stems resulted in second hardening and decrease in toughness, consequently shortening the life of the stems.