Kim Verbeken

In the study of the liquid metal embrittlement (LME) of the T91/lead-bismuth eutectic (LBE) system, it is observed that LME occurs in a temperature interval which is similar to the temperature range where dynamic strain aging (DSA) is observed. However, the potential correlation between DSA and LME has not yet been satisfactorily investigated. This investigation for the T91/LBE system is exactly the topic of this work. For the evaluation of DSA and LME, slow strain rate tensile tests are conducted in the temperature range between 200 °C and 450 °C with strain rates of 5×10−5 s−1 and 5×10−6 s−1 in reference and a molten oxygen-depleted LBE environment. The resulting tensile properties, as well as the fracture surfaces and lateral surfaces of the failed samples, suggest a correlation between DSA and LME in the T91/LBE system. The maximum mechanical degradation of T91 is observed in the case where the effects of both DSA and LME on material properties are found to be at maximum. However, the observation of DSA was not identified as a prerequisite for LME to take place. Therefore, these results may indicate that DSA partly contributes to the ductility minimum observed in the T91/LBE system. In addition, the results of this work show that changes in the fracture surface and lateral surface are more sensitive features to claim for the potential occurrence of LME than the changes in total elongation.

Hydrogen can degrade the mechanical properties of steel components, which is commonly referred to as “hydrogen embrittlement” (HE). Quantifying the effect of HE on the structural integrity of components and structures remains challenging. The authors investigated an X70 pipeline steel through uncharged and hydrogen-charged (notched) tensile tests. This paper presents a combination of experimental results and numerical simulations using a micro-mechanics-inspired damage model. Four specimen geometries and three hydrogen concentrations (including uncharged) were targeted, which allowed for the construction of a fracture locus that depended on the stress triaxiality and hydrogen concentration. The multi-physical finite element model includes hydrogen diffusion and damage on the basis of the complete Gurson model. Hydrogen-Assisted degradation was implemented through an acceleration of the void nucleation process, as supported by experimental observations. The damage parameters were determined through inverse analysis, and the numerical results were in good agreement with the experimental data. The presented model couples micro-mechanical with macro-mechanical results and makes it possible to evaluate the damage evolution during hydrogen-charged mechanical tests. In particular, the well-known ductility loss due to hydrogen was captured well in the form of embrittlement indices for the different geometries and hydrogen concentrations. The limitations of the damage model regarding the stress state are discussed in this paper.

Herein, a numerical solution of Fick's second law in one dimension with experimentally determined diffusion coefficients at different constant loads is combined with slow strain rate tensile tests and subsequent fractography on dual‐phase steel. From the latter, the depth of the hydrogen embrittled region is determined and correlated to concentration profiles determined by the numerical model. The concentration profiles indicate that incorporating stress dependency of the diffusion coefficient results in different concentration profiles compared to using a constant diffusivity. Additionally, these allow to more accurately determine a critical local hydrogen concentration based on the total diffusible hydrogen concentration at saturation.