The roles of globular M23C6 carbides in hydrogen attack resistance in C-0.5Mo steels

Metallographic observations on the C-0.5Mo steel components in petroleum re fineries and experimental studies revealed that unusual ferrite-pearlite st ructures with quasi-M23Co carbides were more susceptible to hydrogen attack than standard ferrite-bainite structures with M3C carbides. Post weld heat treatments (PWHT) were found to promote methane bubble formation. In order to explain these microstructural effects on hydrogen attack susceptibility , thermal stability of carbides and the strength of interface between carbi des and the matrix were examined. C-0.5Mo steel specimens with varied microstructures were exposed to high pr essure hydrogen, and the temperature dependence of the emitted methane conc entration was measured with a gas chromatograph. The temperature at which m ethane evolution took place and the changes in methane concentration with t emperature were both unaffected by the microstructures, In low strain rate tensile tests, void formation around carbide particles started at lower str ain in a ferrite-pearlite microstructure with quasi-M23Co carbides than in a ferrite-bainite microstructure with M3C carbides. Voids increased in numb er more rapidly with strain in the former than the latter. After PWHT. void s started to form at lon-er strain compared with before PWHT probably due t o coarsening of carbides. Therefore, it can be concluded that hydrogen attach in C-0.5 Mo steels is n ot governed but stability of carbides but by the difference in total area o f methane formation site between the two.