Failure of a hydrogenerator reactor inlet piping by high- temperature hydrogen attack

A case of cracking in carbon steel inlet piping of a hydrogenerator reactor by high-temperature hydrogen attack that resulted in a leak after 47 years of service is examined. A transverse crack formed in the flange neck close to a non-post-weld-heat-treated butt weld, just outside the heat-affected zone (i.e. in the sub-critical region). Although the main factor in the failure was a recent increase in hydrogen partial pressure that placed the operating conditions above the pre-2016 API 941 Nelson curve for carbon steel, there were some pre-existing/dormant contributing factors, namely lack of post-weld heat treatment, bending stress on the piping, the presence of H2S in the process fluid, coarse ferrite-pearlite microstructure (i.e. the grain size effect) and some impurities in the vintage steel. An interesting observation was that the grain boundaries were decorated with a film of light-etching phase that was identified to be cementite by electron backscatter diffraction as opposed to being ferrite that could form upon internal decarburization (i.e. pearlite decomposition). An explanation is provided for the formation of cementite along the grain boundaries. Overall, the findings of this investigation attest to the usefulness and relevance of the (simplistic and imperial) Nelson curve for carbon steel, which has been questioned over the last few decades.