An increase in macroscopic flow stress, i.e., hardening, due to hydrogen charging has been observed by many researchers, while there are fewer reports of decreased macroscopic flow stress, i.e., softening, due to hydrogen. Birnbaum and Sofronis attempted to resolve the contradictory hydrogen effects, especially the decrease in “microscopic” flow stress (softening) vs the increase in “macroscopic” flow stress (hardening) they used a combination of TEM experiments and theoretical analysis of hydrogen-dislocation interaction.
Gavriljuk and co-workers report hydrogen-induced softening of austenitic steels, as revealed by internal friction measurement, whereas tensile tests showed hardening in hydrogen-charged specimens.
The precise studies by Kirchheim and co-workers showed that hydrogen has two contradictory effects, i.e., both resisting and enhancing dislocation motion. There are more articles that report increased macroscopic yield strength of austenitic stainless steels due to hydrogen than those that report decreased yield strength. However, hydrogen effects are not necessarily limited to the enhancement of dislocation mobility, namely, to assisting crystallographic glide. Birnbaum and co-workers found that hydrogen decreased the microscopic yield stress this finding was based on in-situ transmission electron microscopy (TEM) observation of increased dislocation movement caused by hydrogen. It is pertinent to regard the interaction between hydrogen and dislocations as being strongly correlated with microscopic plastic deformation behavior in almost all metallic materials, and also with strain-induced martensitic transformation in austenitic steels. With respect to the relationship between hydrogen and microstructure, localized plasticity due to hydrogen-enhanced dislocation mobility and crystallographic slip localization by hydrogen have also attracted considerable attention in the field of materials science.
#CASE HARDENING HAS ZEBRA PATTERN CRACK#
Hydrogen-induced degradation in fracture toughness, fatigue strength, and fatigue crack growth properties has also caused concern in various industrial sectors. Among deleterious effects of hydrogen, ductility loss is a well-known phenomenon. From a macroscopic viewpoint, most research showed undesirable effects in the deterioration in strength properties caused by hydrogen. This finding will, not only be the crucial key factor to elucidate the mechanism of HE, but also be a trigger to review all existing theories on HE in which hydrogen is regarded as a dangerous culprit.ĭ uring the past 40 to 50 years, many articles on hydrogen embrittlement (HE) have been published, and several hypotheses have been proposed. Competition between these two roles determines whether the resulting phenomenon is damaging or, unexpectedly, desirable. Hydrogen can play two roles in terms of dislocation mobility: pinning (or dragging) and enhancement of mobility. Although this mysterious phenomenon has not previously been observed in the history of HE research, its mechanism can be understood as an interaction between hydrogen and dislocations. A dramatic phenomenon was found in which charging a supersaturated level of hydrogen into specimens of austenitic stainless steels of types 304 and 316L drastically improved the fatigue crack growth resistance, rather than accelerating fatigue crack growth rates. However, this article shows, surprisingly, that hydrogen can have an effect against HE. The well-known term “hydrogen embrittlement” (HE) expresses undesirable effects due to hydrogen such as loss of ductility, decreased fracture toughness, and degradation of fatigue properties of metals.
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