Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels

An investigation of phase transformations in hydrogen-charged metastable austenitic stainless steels was carried out. Solution-annealed, high-purity, ultralow-carbon Fel8Crl2Ni (305) and laboratory-heat Fel8Cr9Ni (304) stainless steels were examined. The steels were cathodically charged with hydrogen at 1, 10, and 100 mA/cm², at room temperature for 5 minutes to 32 hours, in an lN H₂SO₄ solution with 0.25 g/L of NaAsO₂ added as a hydrogen recombination poison. Changes in microstructure and hydrogen damage that resulted from charging and subsequent room-temperature aging were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Hydrides from hydrogen charging (hep ε* in 305 SS and fcc γ* and hcp ε* in 304 SS) were observed. The evidence suggests the following mechanisms for hydride formation during charging: (1)γ → ε → ε* hydride and (2) γ → γ* hydride. These hydrides were found to be unstable and decomposed during room-temperature aging in air by the following suggested mechanisms: (1)ε* hydride (hcp) → expanded ε (hcp) phase →α′ (bcc) phase and (2) γ* hydride →γ phase. The transformation from ε* toα′, however, was incomplete, and a substantial fraction of ε was retained. A kinetics model for hydride decomposition and the accompanying phase transformation during aging is proposed.