H-induced C–Cr cluster redistribution in MANET steel

Internal friction was applied to trace the formation and propagation of the hydrogen-induced degradation of commercial mild and 5% Cr steels. Hydrogen charged and then degassed specimens were subjected to internal friction measurements at the grain boundary relaxation temperature range and the irreversible effects of hydrogen precharging on the grain and phase boundary relaxation processes were studied. From the comparison of the internal friction and the hydrogen permeation data, the critical limits of hydrogen pretreatment corresponding to the different state of the grain boundary degradation were evaluated. At lower critical limit (associated with the minimum of hydrogen apparent diffusivity), the formation of near grain boundary dislocations and the grain boundary decohesion occurred. At critical hydrogen precharging causing the formation of microvoids, annihilation of dislocations and vacancies within the grain boundary voids led to the recovery of internal friction and hydrogen diffusivity. The obtained results agreed with the earlier observed effects of hydrogen precharging on pure iron and low alloy steel, despite the apparent difference in the grain boundary structure and chemistry of those materials.