Effects of Heat Treatment and Chemical Composition on the High Temperature Hydrogen Attack (HTHA) Resistance of C-½ Mo Steels

C-½ Mo steels have been used in the construction of petrochemical vessels and piping, many of which are still in active service. High temperature hydrogen attack (HTHA) is one of the major forms of degradation encountered in these steels, in which atomic hydrogen from the service environment diffuses into steel and reacts with carbon/carbides. This results in formation of methane bubbles and coalescence leading to fissuring/cracking and potentially to unexpected component failure. Metallurgical evaluation and heat treatment studies were conducted on two C-½ Mo heats to investigate the effect of heat treatment, microstructure, and chemical composition on HTHA resistance. Unique box-like assemblies were fabricated for autoclave hydrogen exposure experiment to simulate the single-sided exposure conditions for typical pressure vessel components. Four box-like assemblies were fabricated, two from each of the two C-½ Mo heats. One sample from each heat was given a normalizing and tempering heat treatment while the other underwent an annealing and tempering heat treatment. A higher molybdenum to carbon ratio provided improve resistance to HTHA, a result of the formation of a greater fraction of stable Mo₂C carbides. A more HTHA resistant, stronger microstructure in the normalized and tempered box-like assemblies led to greater HTHA resistance in comparison to the annealed and tempered box-like assemblies.