5th International High Manganese Steel Conference - HMNS 2022

Hydrogen embrittlement (HE) is a persistent mode of failure in high-strength steels
which hinders their applicability in industry. The role of retained austenite (RA) in the
HE susceptibility of these steels is still unclear. There are two possible scenarios, firstly
since the solubility of H in austenite is remarkably larger than that of martensite, the
RA phase can act as a trap for H atoms and hinder them from diffusion to the critical
regions such as grain boundaries and junctions and damage the material. On the other
hand, if during the lifetime of the steel RA undergoes martensitic transformation it could
release excess trapped hydrogen atoms which might damage the material in the vicinity
of the former RA.
To shed more light on the role of RA, we probed the role of H atoms on the relative
stability of the fcc/bcc/hcp phases in Iron using the ab initio thermodynamics approach.
The results indicate that at low hydrogen chemical potentials regime the stability of the
fcc phase is slightly enhanced while at high hydrogen chemical potentials the bcc phase
dominates. The different excess volumes of the hydrogen-rich phases in steels can lead
to phase transformation under the cyclic load. Furthermore, for the scrutiny of the
decisive role of the interface, the interplay of the hydrogen atoms and carbon atoms at
the bcc-fcc phase boundary is investigated using density functional theory. Results
indicate that the phase boundary is indeed a trap for both H and C atoms. Interestingly,
the interaction of the C and H atoms at the phase boundary is repulsive i.e. presence of
C at the phase boundary prohibits binding of H atoms to it