Hydrogen induced mechanical degradation in tungsten alloyed steels

The present work considers the effect of hydrogen on the mechanical properties of generic Fe-C-W alloys. Thermal treatment aimed at a martensitic microstructure and two conditions are compared; i.e. an as-quenched state and a quenched and tempered state with lower dislocation density and with W based carbides formed during tempering. The hydrogen induced mechanical degradation is evaluated by in-situ tensile tests, while thermal desorption spectroscopy, hot/melt extraction and permeation experiments are performed to understand the observed hydrogen embrittlement degree. The hydrogen induced ductility loss increases with increasing carbon content due to the higher amount of hydrogen trapped by the denser martensitic lath boundaries. Furthermore, the quenched and tempered condition shows a lower susceptibility to hydrogen. This is correlated to the reduced hydrogen content when tempered due to a decreased dislocation density and the fact that the tempered induced W2C particles did not trap hydrogen. Moreover, corresponding observations to this interpretation are perceived by permeation experiments since, on the one hand, the diffusion coefficient decreases with carbon content due to the increasing trapping ability of alloy A ➔ B ➔ C, and, on the other hand, the diffusivity increases when tempering is applied, which is also linked to the decrease in dislocation density and the disability of the carbides to hinder hydrogen diffusion by efficient trapping.