ABSTRACT Several sockolets of Alloy 625 operating at 540°C in contact with ammonia gas in a heavy water production plant showed cracking after 115,000 h of service‐life. A sockolet is a branch connection used to connect a smaller branch pipe to a larger main pipe. A failure investigation was done to identify the root cause of failure; it involved microstructural examination, fractography, hardness measurement, and X‐ray diffraction analysis. During service, the sockolet material developed a hard and brittle surface nitrided layer (40–65 µm), composed mainly of chromium nitride. The hardness of the nitrided layer was 1300 HV 0.2 as opposed to the hardness of 430 HV 0.2 for the base alloy. The failure of the sockolets was caused by intergranular cracking and fracture. The thick and highly brittle nitrided layer formed intergranular surface cracks, which propagated along grain boundaries into the base alloy substrate due to the presence of brittle Cr‐rich carbides.
Chandra et al. (2026) studied this question.