PHASE-TRANSFORMATIONS AND RELAXATION PHENOMENA CAUSED BY HYDROGEN IN STABLE AUSTENITIC STAINLESS-STEELS

Stable austenitic stainless Cr18Ni16Mn10 steels alloyed with different nitrogen content were studied by means of X-ray diffraction, internal friction and Mossbauer spectroscopy. Cathodic charging induces the ep silon(H) phase. No evidence is obtained for occurrence of the hydrogen -induced gamma(H) phase which could be interpreted in terms of spinoda l decomposition of the hydrogen solid solution. Nitrogen retards the f ormation of the epsilon(H) phase and increases the fraction of the hyd rogen-rich gamma solid solution. The beneficial effect of nitrogen on hydrogen-caused losses of the mechanical properties can be attributed to the suppression of the epsilon(H) phase formation. The internal fri ction spectrum of hydrogenated stable austenitic stainless steels cons ists of five peaks three of which are of the relaxation nature and oth er two correspond to hysteretic processes. The hydrogen-induced relaxa tion in austenite can be described by the Snoek-like mechanism concern ed with the reorientation of complexes of hydrogen atoms with substitu tional solutes. The ideas of H-H reorientation and hydrogen-induced Sn oek-Koster relaxation in austenite are not confirmed. Short range atom ic order changed by means of cold work or electron irradiation affects the relaxation spectrum. Addition of nitrogen prevents the relaxation concerned with a short range migration of the hydrogen atoms, which c an be attributed to the nitrogen suppression of the epsilon(H) phase. Hysteretic damping is caused by outgassing of hydrogen from the sample s.