Session: MF-02-03 Materials for hydrogen service III (Joint with C&S)

Paper Number: 61717

Start Time: Tuesday, July 13, 2021, 08:00 PM

61717 - Fatigue Crack Growth in Ferritic Microstructure Under the Exposure to Gaseous Hydrogen: An Overview 

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).

   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.

Presenting Author: Yuhei Ogawa Kyushu University

Authors:

Yuhei Ogawa Kyushu University
Osamu Takakuwa Kyushu University
Hisao Matsunaga Kyushu University