The impact of hydrogen on the ductility loss of bainitic Fe–C alloys

The influence of hydrogen on the mechanical properties of generic lab-cast Fe-C bainitic alloys is studied by tensile tests on notched samples. The bainitic microstructure is induced in a 0.2% C and 0.4% C Fe-C alloy by an appropriate heat treatment. The hydrogen embrittlement susceptibility is evaluated by mechanical tests on both in situ hydrogen pre-charged and uncharged specimens. The observed ductility loss of the materials is correlated with the present amount of hydrogen and the hydrogen diffusion coefficient. In addition to the correlation between the amount of hydrogen and the hydrogen-induced ductility loss, the hydrogen diffusion during the tensile test, quantified by the hydrogen diffusion distance during the test, appears to be of major importance as well

FeS corrosion products formation and hydrogen uptake in a sour environment for quenched & tempered steel

Surface corrosion product formation is one of the important factors affecting the corrosion rate and hydrogen uptake in a H2S environment. However, it is still unclear how the base material composition will affect the corrosion products that are generated, and consequently their impact on the corrosion rate. In this paper, corrosion product formation and the impact of the Mo content of the base material on the composition of the corrosion products and hydrogen absorption in a sour environment was investigated. The corrosion layer was composed of a double layered mackinawite (FeS1−x) structure, which was enriched with molybdenum and chromium. The layers were formed via two different mechanisms, i.e., the inner layer was created via a general oxide film formation corrosion mechanism, whereas the upper layer was formed by a precipitation mechanism. The presence of this double corrosion layer had a large influence on the amount of diffusible hydrogen in the materials. This amount decreased as a function of contact time with the H2S saturated solution, while the corrosion rate of the materials shows no significant reduction. Therefore, the corrosion products are assumed to act as a physical barrier against hydrogen uptake. Mo addition caused a decrease in the maximal amount of diffusible hydrogen

Electrochemical hydrogen charging of duplex stainless steel

This study evaluates the electrochemical hydrogen charging behavior and interaction between hydrogen and the microstructure of a duplex stainless steel. A saturation level of approximately 650 wppm is reached after 10 d of charging. The data are compared with a model resulting in a diffusion coefficient of 2.1 x 10(-14) m(2)/s. A two-step increase of the concentration is observed and ascribed to saturation of ferrite followed by charging of austenite grains. Microstructural changes are observed during charging, i.e., formation and interaction of dislocations, as a result of the high residual stresses inherent to the production process of duplex stainless steels

Initiation of hydrogen induced cracks at secondary phase particles

The goal of this work is to propose a general mechanism for hydrogen induced crack initiation in steels based on a microstructural study of multiple steel grades. Four types of steels with strongly varying microstructures are studied for this purpose, i.e. ultra low carbon (ULC) steel, TRIP (transformation induced plasticity) steel, Fe-C-Ti generic alloy, and pressure vessel steel. A strong dependency of the initiation of hydrogen induced cracks on the microstructural features in the materials is observed. By use of SEM-EBSD characterization, initiation is found to always occur at the hard secondary phase particles in the materials

The effect of microstructural characteristics on the hydrogen permeation transient in quenched and tempered martensitic alloys

This work evaluates the permeation curve characteristics for four quenched and tempered generic, ternary alloys, each containing one specific carbide. The different carbides (W2C, Cr23C6, TiC, and V4C3, respectively) are induced by a quench and tempering treatment. The correlation is made between the different microstructural characteristics, including the carbides and the martensitic matrix, and the observed hydrogen diffusivity and thus the permeation transient. The permeation curves, obtained via the Devanathan and Stachurski method, are therefore compared with thermal desorption spectroscopy and hot extraction results. The delay of the permeation transient can be associated with the overall trap density, while the slope is related to the amount of reversible trapping sites. Generally, the obtained hydrogen permeation transient of the different ternary or Fe-C-X materials correlates with the hydrogen trapping ability. The following order of hydrogen diffusion is determined, i.e., Fe-C-V < Fe-C-Ti << Fe-C-Cr < Fe-C-W. The hydrogen trapping ability of the tempered induced carbides plays a decisive role in the value of the hydrogen diffusion coefficient