The role of titanium and vanadium based precipitates on hydrogen induced degradation of ferritic materials

Abstract

The hydrogen induced damage of generic Fe-C-Ti and Fe-C-V ferritic alloys was investigated to assess the influence of precipitates on the hydrogen sensitivity of a material. The precipitates, formed during heat treatment, were evaluated by scanning transmission electron microscopy (STEM). The hydrogen/material interaction was evaluated by: 1) melt and hot extraction to determine the total and diffusible hydrogen content, respectively, 2) permeation experiments to calculate the diffusion coefficient, 3) thermal desorption spectroscopy to determine the hydrogen trapping characteristics of the materials. Furthermore, two different types of hydrogen induced damage were evaluated, i.e. hydrogen assisted cracking and blistering, resulting from electrochemical hydrogen charging with and without the application of an external load, respectively. Evaluation of the hydrogen induced damage and the role of the precipitates was performed by combining optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). An important though divertive role of diffusible hydrogen is observed in both damage mechanisms for the investigated microstructures. On the one hand, a large amount of diffusible hydrogen compared to strongly trapped hydrogen promotes hydrogen assisted cracking of materials, while on the other hand, the blistering phenomenon is delayed under such conditions.

Initiation of hydrogen induced cracks at secondary phase particles

“…A clear interaction of cracks with large Ti-based precipitates was demonstrated as well with EBSD ( Fig. 14) [39]. Most likely the hydrogen induced cracking initiated at the large carbides/carbonitrides, due to hydrogen build-up at the interface or in the precipitates.…”

“…Large incoherent carbonitrides (2-5 µm), medium-sized carbonitrides (Ti(C,N)) (100-700 nm) and small carbides (2-24 nm) were found in the material ( Fig. 3 and 4) [39]. with permission from Ref.…”

“…13). Most often clusters of precipitates were involved [39]. Terasaki et al [24] also observed a greater HIC sensitivity at clusters of inclusions.…”

“…In summary, crack nucleation during hydrogen charging has been related to a localized concentration and subsequent recombination of hydrogen at suitable heterogeneities such as grain boundaries, second phase particles, microvoids and tangled dislocations [38]. This investigation gives an overview of previous work [6,9,39,40] in order to bring together results obtained on different materials, compare the initiation mechanism for hydrogen induced cracking and as such give new insights on an important topic. The study compares the blistering behavior of four different types of materials, i.e.…”

“…The study compares the blistering behavior of four different types of materials, i.e. ultra low carbon (ULC) steel [6], TRIP (transformation induced plasticity)-assisted steel [40], generic Fe-C-Ti alloy [39], and pressure vessel steels [9]. These materials exhibit strongly varying microstructural features, such as different type of inclusions, precipitates and phases.…”