Hydrogen Desorption in Steels

Abstract

It is generally accepted that hydrogen embrittlement involves interaction with hydrogen and defects sites and hence requires the hydrogen to be in a diffusible form. One of the experimental techniques to investigate the trapping is thermal desorption spectroscopy (TDS) which monitors the hydrogen desorption rate during continuous heating. However, it is indirect so the resulting data of the curve require interpretation.

A numerical method that incorporates both diffusion and detrapping processes has been implemented to account for the practical experiments. The detrapping process was simulated using two different models; the first model assumes local equilibrium between trapped and free hydrogen and the other model considers only kinetics of detrapping without the local equilibrium assumption. However, the results of the local equilibrium model and kinetic models were found to be consistent; i.e., an equilibrium distribution of hydrogen is maintained at all temperatures. It was also found that the desorption peak is separated in the samples with multiple traps, if the difference in binding energies is large enough.

This model was tested against new experimental data and the influence of plastic strain on hydrogen desorption in ferrite was assessed. In addition, the effects of carbon segregation on hydrogen desorption in ferrite was investigated using bake hardening steel.

This model was applied to unsaturated austenite samples and the effects of aluminum addition were assessed. First-principles calculation yielded the binding energy of aluminum in austenite, and the trap density was calculated by considering the concentration of aluminum. From the simulation with those parameters, hydrogen thermal desorption was predicted and compared with the experimental results. In addition, the effects of grain size were analyzed with reported data, and the grain boundaries in austenite were confirmed to be much less potent than those in ferrite.

Finally, the effects of austenite in hydrogen desorption in TRIP-assisted steel were studied. The austenite contents were controlled using a different intercritical annealing temperature. The results were also analyzed with the numerical model.