1999 Volume 85 Issue 4 Pages 332-339
Metallographic observations on the C-0.5Mo steel components in petroleum refineries and experimental studies revealed that unusual ferritepearlite structures with quasi-M23C6 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 carbides and the matrix were examined.
C-0.5Mo steel specimens with varied microstructures were exposed to high pressure hydrogen, and the temperature dependence of the emitted methane concentration was measured with a gas chromatograph. The temperature at which methane evolution took place and the changes in methane concentration with temperature were both unaffected by the microstructures. In low strain rate tensile tests, void formation around carbide particles started at lower strain in a ferrite-pearlite microstructure with quasi-M23C6 carbides than in a ferrite-bainite microstructure with M3C carbides. Voids increased in number more rapidly with strain in the former than the latter. After PWHT, voids started to form at lower strain compared with before PWHT probably due to coarsening of carbides.
Therefore, it can be concluded that hydrogen attack in C-0.5Mo steels is not governed by stability of carbides but by the difference in total area of methane formation site between the two.
