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Effect of Hydrogen Flux on the Plasticity and Damage Mechanisms of Martensitic Steels

Picture for Effect of Hydrogen Flux on the Plasticity and Damage Mechanisms of Martensitic Steels
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The depletion of easily exploitable oil and gas resources increases the demand for high strength steels able to withstand the high pressures of deeper offshore exploitation. These steels need also to resist a sour environment that enables hydrogen stress cracking. Hydrogen embrittlement is the consequence of a complex mechanism involving adsorption/absorption diffusion and trapping phenomena. Although hydrogen dramatically affects the fracture the mechanisms responsible for generating ductile andbrittle hydrogen-induced fracture surfaces are not well understood. Therefore it is critical to study the impact of mobile and trapped hydrogen on both ductile and brittle fracture of martensitic steels. The present work combines a new design of permeation testing under tensile loading and FEM modelling to investigate the interactions between hydrogen and damage mechanisms focused on plasticity. More precisely the impact of mobile and trapped hydrogen on ductile and brittle fracture of martensitic steels is inspected using a local approach of fracture and a specific analysis of defects evolution under hydrogen flux (vacancies and dislocations). Damage investigation was performed for specimens submitted to several mechanical states (different notch shapes) and hydrogen concentrations (pre-charged and under hydrogen flux). The results showed that the trapped hydrogen favors a ductile fracture while the mobile hydrogen promotes a quasi-cleavage damage at a scale of the martensitic laths. Further analysis revealed that whereas the trapped hydrogen promotes the germination and growth of voids around inclusions and precipitates the mobile hydrogen enhances decohesion along interfaces. The role of plasticity in both fracture mechanisms wasinvestigated. It has been found that plasticity is crucial not only for the ductile fracture but also to the quasi-cleavage process. EBSD images at the quasi-cleavage regions reveal that the majority of the fracture paths occurred on slip planes emphasizing the plasticity contribution to this brittle fracture.

Product Number: 51319-12739-SG
Author: Livia Cupertino Malheiros
Publication Date: 2019
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