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Title: The effect of hydrogen on the structure and properties of austenitic stainless steels
Author: Maulik, P.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1978
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A study has been made of the effects of hydrogen introduced cathodically on the structure, stability and tensile properties of three austenitic stainless steels. Two of the alloys studied were basically Fe-Ni-Cr alloys (types 304L and 310) and the third a manganese substitute steel - type 21-6-9. Electron microscope and X-ray observations show that hydrogen induces martensitic transformation, the details of which depend upon the composition (i.e. austenitic stability) temperature and hydrogen content. It is suggested that the martensite phases form as a result of overlapping of stacking faults formed by the hydrogenation process. A detailed study was made of the effect of hydrogen on the Ms temperature of a range of austenites in which it was found that any effect was negligible. It is suggested that the martensite which is formed under appropriate experimental conditions is essentially strain induced as a result of the extensive internal strain generated by the severe hydrogen concentration gradient. In addition to the standard tensile properties measurements were also made of the activation volume and work hardening index as a function of hydrogen content during charging red outgassing. Activation volume, work hardening index, U.T.S. and ductility decreases on hydrogenation and recovers on outgassing. The proof stress increases on hydrogenation and does not completely recover on outgassing. The fracture process was studied principally by means of the scanning electron microscope. Three components were identified (a) brittle fracture of grain boundaries normal to the tensile axis initiated from the surface, (b) a brittle-ductile transitional zone thought to be associated with the plastic blunting of propagating cracks and (c) a ductile failure in the core of the sample occurring by the coalescence of voids, the rate of nucleation and/or growth of which is accelerated by hydrogen. Concurrent elastic stress during exposure to hydrogen does not appear to change the magnitude of the hydrogen induced property changes. The effect of prior plastic deformation is complex depending upon the type of deformation process. These observations are discussed in terms of a model in which hydrogen interacts with and is transported by moving dislocations and so cause high local concentrations at dislocation obstacles.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available