Abstract: X-ray diffraction (XRD) techniques were applied to investigate hydrogen induced martensite and transformation in weld metals of stainless steels during cathodic hydrogen charging and subsequent aging.The results indicate that hydrogen charging can induce austenite lattice expansion in weld metal of stainless steel corresponding to about2.7% (308L) and 2.9% (347L) respectively,greater than about 1.2% (304L) in lattice parameter.The transformation from the fcc γ phase to the hcp ε phase occurred during cathodic charging.During aging,transformation to bcc α' phase from the hcp ε phase, however,was incompletd and some ε was restained.Martensite transformation followed the sequence γ→ε→α' During cathodic hydrogen charging and subsequent aging,the volume fraction ε+α'remained essentially constant.Even if after long time aging, e and a' martensite did not disappear.