Hydrogen embrittlement mechanisms in steel and their synergy: Unified HELP+HEDE model

According to Scopus, our paper on hydrogen embrittlement in steel from 2021 reached 120+ citations.

"Influence of hydrogen-enhanced plasticity and decohesion mechanisms of hydrogen embrittlement on the fracture resistance of steel", Engineering Failure Analysis, (2021) 123:105312

by Wasim Muhammad, Milos Djukic, and Tuan Duc Ngo,

Free download - ResearchGate:

https://www.researchgate.net/publication/349158455_Influence_of_hydrogen-enhanced_plasticity_and_decohesion_mechanisms_of_hydrogen_embrittlement_on_the_fracture_resistance_of_steel

Link - Elsevier: https://lnkd.in/d2Zx-P3

Highlights:

• Low carbon steel was long-term exposed to corrosive hydrogen-containing solutions.
• The coupled effect of corrosion and hydrogen embrittlement (HE) was investigated.
• Models for the degradation of fracture toughness of steel were developed.
• Activity of both localized plasticity (HELP) and decohesion (HEDE) mechanisms were confirmed according to the HELP+HEDE model for the synergistic action of HE mechanisms proposed by Djukic et al.

A model for the coexistence of hydrogen embrittlement (HE) mechanisms (unified HELP + HEDE model) is proposed, including a novel "local HEDE micro-incidents" concept.

The unified HELP?+?HEDE model for HE in low carbon steel, shown in Fig. above, is based on the specific obtained fracture modes at various global hydrogen concentrations depending on the dominant HE mechanism during the synergistic action mode of both HELP and HEDE mechanisms.

The coexistence of the HE mechanism according to the HELP?+?HEDE model, proposed by Djukic et al., defined that the previous HELP mechanism is not always necessary for the activation and the dominance of the HEDE mechanism (“non-HELP mediated decohesion” process activation) at high local/global hydrogen concentrations in steels.

The "local HEDE micro-incidents" (grain boundary decohesion, fissures, and initial IG micro-cracks), at a moderate global hydrogen concentration in steel, probably initiated locally at ferrite-pearlite interfaces and their triple junctions, are in their incipient stage as discrete micro-scale incidents in the macro-volume of the specimen. Hence, the HELP mechanism is still predominant.

It seems that for the full macro HEDE mechanism manifestation (sharp drop in macro-mechanical properties) and its dominance (HELP?+?HEDE, HEDE?>?HELP), the necessary prerequisite is “the macro-volume effects” of HEDE. This means the appearance and accumulation of a large enough number of the micro HEDE incidents locally in a small volume (only local and initial IG microcrack incidents within the brittle TG fractured areas) which finally provokes the macro-volume HEDE manifestation and dominance (HELP + HEDE, HEDE> HELP) at the global hydrogen concentration higher than the critical.

An empirical relation for the degradation of the fracture toughness KQ as the function of the hydrogen content and pH with time is limited in the literature. To address, this limitation an expression for the change in fracture toughness of low carbon steel incorporating hydrogen content is also developed.

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