  Hydrogen embrittlement in 304L (18wt.% Cr, 8-10wt.% Ni) austenitic stainless
steel (ASS) fabricated by laser powder-bed-fusion (LPBF) was investigated by
tensile testing after electrochemical hydrogen pre-charging and compared to
conventionally available 304L ASSs with two different processing histories, (i)
casting plus annealing (CA) and (ii) CA plus thermomechanical treatment (TMT).
It was revealed that hydrogen-charging led to a significant reduction in
ductility for the CA sample, but only a small effect of hydrogen was observed
for the LPBF and CA-TMT samples. Hydrogen-assisted cracking behavior was found
to be strongly linked to strain-induced martensitic transformation. In
addition, the amount of alpha' martensite was much higher in the CA sample than
in other samples, suggesting that severe hydrogen embrittlement can be
correlated with the low mechanical stability of austenite. Detailed
microstructural characterization showed that low austenite stability of the CA
sample was mainly attributed to the retained content of delta ferrite and the
chemical inhomogeneity inside the gamma matrix (gamma close to delta has ~2
wt.% higher Cr but ~2 wt.% lower Ni), but TMT enhanced the chemical homogeneity
and thus austenite stability. By contrast, the LPBF process led directly, i.e.
without any thermomechanical treatment, to a fully austenitic structure with
homogeneous elemental distribution in the ASS. These results confirmed that the
presence of delta and the chemical inhomogeneity inside gamma matrix, which
promoted the deformation-induced martensitic transformation and the associated
H enrichment at the gamma-alpha' interface, was the primary reason for the
severe H-assisted failure.
