Advanced characterization of cryogenic 9Ni steel using synchrotron radiation, neutron scattering and 57Fe Mössbauer spectroscopy

The bcc ferritic Fe-Ni steels and in particular the commercial 9Ni alloys were developed and widely used at cryogenic temperatures for liquefied natural gas storage. The excellent thermomechanical properties and cryogenic toughness of these materials are closely related to the presence of metastable fcc austenitic phase whose stability increases in the presence of austenitic components (Ni, Cr, Mn, etc.) and with decreasing fcc grain size. Numerous reports are related to quenching and tempering technological procedures improving the cryogenic performance, however, very little attention has been paid to the structural characterization of austenitic minority species except for a few papers revealing rather contradictory results. Using high-energy X-ray diffraction, small-angle neutron scattering and 57Fe Mössbauer spectroscopy we show that the minority austenitic particles in the commercial X8Ni9 steel are enriched in nickel up to 27 ± 6 at.% Ni and have a characteristic size of 620 ± 25 Å. The austenitic phase is highly sensitive to mechanical deformation and stress. Disappearance of the fcc grains during the treatment implies a mechanical failure while mechanically-resistant regions are characterized by a higher intensity of the austenitic reflections probably related to the generated texture. The last finding was never reported previously.