International Hydrogen Conference (IHC 2012): Hydrogen-Materials Interactions
70 Strain-Induced Phase Transformation and Change of Structure in Hydrogen-Charged Austenitic Stainless Steel
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The effect of hydrogen on the strain-induced phase transformation from γ-austenite to α´-martensite in type 304 steel was studied using X-ray diffraction. Tensile strain of more than 5% induced the α’-martensite, and the fraction of α´-martensite formed depended on deformation degree. Subsequent hydrogenation at 543 K under hydrogen pressure of 100 MPa during 200 hours reduced the martensite fraction, which was accompanied by grain refinement. Ab initio atomic calculations of free energies for the α- and γ-iron performed in the harmonic approximation demonstrate that hydrogen decreases the temperature of equilibrium for these phases. It is suggested that the formation of ultrafine-grained structures through recrystallization is related to the hydrogen-decreased inverse α´-γ transformation during the heating of the deformed steel.