Abstract
The resistance to HE of Cr–Mo low-alloy SA 372 steels fabricated by adding different amounts of Nb was investigated by slow-strain-rate tensile and H-permeation tests. NbC carbides played an important role in hindering grain boundary migration, leading to a significant grain refinement from 28.3 to 13.1 μm. The Nb-free steel exhibited a significant loss of elongation (Elloss), and its fracture surface contained several secondary cracks and quasi-cleavage facets. In the 0.08 wt%-Nb-added steel, the Elloss decreased significantly due to the facilitated precipitation of additional NbC carbides, resulting in a ductile-dimpled fracture mode. The effective H-diffusivity of Nb-added steels was lower than that of the Nb-free steel, indicating that H-diffusion was considerably hindered in the Nb-added steels by the presence of NbC precipitation as well as grain refinement. This hindered H-diffusion contributed to an improvement of the resistance to HE in SA 372 steels.
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