Transmission electron microscopy (TEM), light microscopy and X-ray diffraction investigations of the effect of cathodic hydrogen charging on microstructure of 316L stainless steel were conducted. The objective of this study was to explain in more detail the mechanism of hydrogen induced microstructure changes which affect the ductility loss of this material.

X-ray diffraction examination confirmed that cathodic H-charging of 316L steel, like in other grades of stainless steels induces partial phase transformation of austenite to several hydride phases. The solution annealed (initial state) and 50% cold worked specimens were examined by TEM before and after hydrogen charging. It was found that hydrogen charging introduces numerous dislocations and causes microtwins formation. Examination of the solution-annealed hydrogen charged specimens revealed that their microstructure exhibits similar features to that of the uncharged but 50 % cold-worked specimens. In both cases, a high dislocation density increase and microtwins formation took place. It means that cathodic hydrogen charging produces heavy plastic deformation of the matrix.

The similarity of the microstructure of the annealed and hydrogen charged material to the microstructure resulting from 50 % cold-working indicates that the amount of plastic deformation induced by hydrogen charging is of the same order