Hydrogen-assisted fatigue crack growth (HAFCG) in steels is an obstruction for the reliable design of pressure vessels or pipelines used for storage and transportation of gaseous hydrogen. Plenty of researches have been carried out to elucidate the rationales of HAFCG on ferritic and martensitic steels, although none of the proposed models can work to make sense overall crack growth acceleration characteristics and its dependences on mechanistic/environmental variables including stress intensity or testing temperature. In this study, pure iron was selected as a model system of ferritic steels, for circumventing any microstructural complexity and simplifying the interpretation of microscale fracture mechanisms occurring inside the crack tip fracture process zone. Fatigue crack growth (FCG) tests were performed in 0.2~90 MPa hydrogen gas, followed by crack-wake deformation substructures analyses via electron backscattered diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM).&nbsp;&nbsp; The FCG rate in hydrogen gas exhibited a distinct two stage behavior depending on the level of stress intensity factor range, ΔK. That is, the lower ΔK regime where FCG rate was equivalent with that in air (Stage I), and the higher ΔK with substantial acceleration of FCG reaching up to 30 times relative to the reference (Stage II). The predominant fracture modes were intergranular (IG) fracture for the former, and transgranular quasi-cleavage (QC) for the latter. The electron microscopy techniques revealed well-evolved dislocation cell or sub-grain structures immediately beneath the IG surface in Stage I, generating small voids even along the peripheral un-cracked grain boundaries (GBs) in which the IG fracture might commence in virtue of the linkage of these GB micro-voids. Meanwhile, the QC was characterized by weakly-evolved dislocations with the fracture path lying parallel to {001} crystallographic planes. Implications are thus made that the important process for IG is the nucleation of GB ductile damages owing to hydrogen-dislocation-GBs interactions, while the dislocations pinning and resultant suppression of plastic relaxation by concentrated hydrogen at the crack tip zone triggers microscopic cleavage, thereby resulted in a greater FCG acceleration in Stage II.

Fatigue Crack Growth in Ferritic Microstructure Under the Exposure to Gaseous Hydrogen: An Overview

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

ASME 2021 Pressure Vessels & Piping Conference (PVP2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.