Metallurgical examinations were conducted on both a channel head of a combined heat exchanger and a piece of piping that had been in long-term hydrogen service under conditions well below those of the Nelson curve for 0.5 Mo steel. The examinations revealed that the channel head material had sustained intergranular cracking, localized internal decarburization, and bubble cavities along grain boundaries. In the case of the piping, cracks were found in the welds and base metal adjacent to welds. However, base metal located away from the welds of the piping was completely sound.

In order to determine the major contributing factor to hydrogen attack, carbide reactions and microstructure changes from heat treatment were investigated and, subsequently, resistivity to hydrogen attack was evaluated by performing hydrogen exposure tests at temperatures from 300 to 440 C under a pressure of 9.81 MPa (1422 psi). The results of these tests confirmed that fissuring or cracking as a result of hydrogen attack can be prevented if stabilized carbides such as M23C6 or Mo2C are present in the steel.

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