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Phase transformations in austenitic stainless steels during low temperature tribological stressing
(2001)
The tribological behaviour at low temperatures is unknown for many materials even for those widely used in cryogenic engineering. Because of the extension of applications of low temperature technologies there is a need for investigations in this field. It is the aim to study the stability of the austenitic structure of FeCrNi alloys under these harsh conditions. This paper deals with tribological experiments with alloys of different stability of the austenitic structure in the temperature range between room and liquid helium temperature. It was found that alloys stable at room temperature show martensitic transformation under tribological stressing and decreased service temperature. The obtained transformation behaviour does not exhibit a linear tendency down to liquid helium temperature. A maximum could be stated.
Hydrogen represents an important alternative to fossil fuels. Hydrogen storage is possible as a gas, at room temperature (RT) at about 20 MPa pressure, and in a liquefied form, at cryogenic temperatures of about 20 K. The latter form is particularly attractive due to the possibility of stocking a large quantity of hydrogen within a small volume. In moving parts (e.g. of transport vehicles) cryogenic temperature and the presence of hydrogen strongly enhance wear processes and subsequently component failure.
The present work deals with the deformation behaviour and the microstructural deterioration of austenitic CrNi- and CrMn high nitrogen-steels during friction in liquid hydrogen at 20 K. The modified microstructure within the wear scar is studied by scanning electron microscopy and X-ray diffraction methods. Diffraction studies of wear scars reveal the importance of twinning during deformation at 20 K. This increase of twinning can be attributed to a hydrogen-induced reduction of stacking fault energy (SFE) in the austenitic steels. Interactions between twin boundaries and planar dislocation structures along with locally increased stresses led to the formation of extensive crack networks. The amount of hydrogen-induced surface cracks depends on the alloy composition and is not necessarily correlated to the wear resistance of the austenitic steels.