Grain boundary relaxation of hydrogen precharged mild and Cr steels

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 me asurements at the grain boundary relaxation temperature range and the irrev ersible effects of hydrogen precharging on the grain and phase boundary rel axation processes were studied. From the comparison of the internal frictio n and the hydrogen permeation data, the critical limits of hydrogen pretrea tment corresponding to the different state of the grain boundary degradatio n were evaluated. At lower critical limit (associated with the minimum of h ydrogen apparent diffusivity), the formation of near grain boundary disloca tions and the grain boundary decohesion occurred. At critical hydrogen prec harging causing the formation of microvoids, annihilation of dislocations a nd vacancies within the pain boundary voids led to the recovery of internal friction and hydrogen diffusivity. The obtained results agreed with the ea rlier observed effects of hydrogen precharging on pure iron and low alloy s teel, despite the apparent difference in the grain boundary structure and c hemistry of those materials. (C) 2000 Elsevier Science S.A. All rights rese rved.