HYDRIDE FORMATION AND DECOMPOSITION IN ELECTROLYTICALLY CHARGED METASTABLE AUSTENITIC STAINLESS-STEELS

An investigation of phase transformations in hydrogen-charged metastab le austenitic stainless steels was carried out. Solution-annealed, hig h-purity, ultralow-carbon Fe18Cr12Ni (305) and laboratory heat Fe18Cr9 Ni (304) stainless steels were examined. The steels were cathodically charged with hydrogen at 1, 10, and 100 mA/cm(2), at room temperature for 5 minutes to 32 hours, in an 1N H2SO4 solution with 0.25 g/L of Na AsO2 added as a hydrogen recombination poison. Changes in microstructu re and hydrogen damage that resulted from charging and subsequent room -temperature aging were studied by X-ray diffraction (XRD) and transmi ssion electron microscopy (TEM). Hydrides from hydrogen charging (hcp epsilon in 305 SS and fee gamma* and hcp epsilon* in 304 SS) were obs erved. The evidence suggests the following mechanisms for hydride form ation during charging: (1) gamma --> epsilon --> epsilon hydride and (2) gamma --> gamma hydride. These hydrides were found to be unstable and decomposed during room-temperature aging in air by the following suggested mechanisms: (1) epsilon hydride (hcp) --> expanded epsilon (hcp) phase --> alpha' (bcc) phase and (2) gamma hydride --> gamma ph ase. The transformation from epsilon to alpha', however, was incomple te, and a substantial fraction of epsilon was retained. A kinetics mod el for hydride decomposition and the accompanying phase transformation during aging is proposed.