The short crack propagation behavior in the low and high cycle fatigue regime was examined
and the influence of hydrogen on the deformation-induced phase transformation from austenite
to martensite was investigated on the metastable austenitic steel AISI304L.The aim of the present
investigation is to characterize the predominant mechanisms of hydrogen embrittlement and their
influence on the short fatigue crack propagation morphology and rate.
To reach this purpose, cyclic in-situ experiments in both a scanning electron microscope and a
confocal laser microscope were employed in combination with related techniques such as EBSD
(electron backscatter diffraction) and roughness monitoring. Reference samples
(without increased hydrogen content) as well as hydrogen-precharged samples were tested in
air and vacuum. In order to obtain a hydrogen concentration of about 30 wppm, the samples
were precharged for 2 weeks at 185°C in an atmosphere of pure hydrogen of 19 MPa.
Cyclic loading above the classical fatigue limit was applied and led to a local transformation
of the face-centered cubic (fcc) y-austenite into the body-centered cubic (bcc) a'-martensite.
This transformation takes place in local areas near the crack tip and in areas with high plastic deformations.
The results show that the hydrogen concentration established by hydrogen gas charging has a
strong effect on the phase stability and the crack growth rate of the investigated material.
The martensitic phase transformation is more restricted to the vicinity of the crack and the
crack growth rate is significantly enhanced by interstitial hydrogen. These observations indicate
a strong localization of the plastic deformation according to the HELP mechanism.