Phase changes related to hydrogen-induced cracking in austenitic stainless steel

Hydrogen-induced phase transformation of the γ-f.c.c. phase to the ε{lunate}-h.p.c. and α′ b.c.c. martensitic phases in austenitic stainless steel were investigated. By means of X-ray diffraction and transmission electron microscope (TEM) techniques, the austenitic stability of AISI type 316 stainless steel was studied over various aging times following 24 h of cathodic charging at room temperature. Splitting of the γ:γ* and ε{lunate}:ε{lunate}* diffraction peaks was observed. The α′-phase was formed in the early stage of aging. There was no evidence of an α′-phase shift during aging. The weight fraction of the α′-martensite increased from 7% in a coarse-grained (ASTM 2) specimen to 28% in a fine-grained (ASTM 11) type 316 steel. After cathodic charging with hydrogen, induced cracks propagated mainly along the {111}_γ planes. TEM observations of the surface cracking revealed that the cracking propagated mainly along the regions having a mixed structure of α′- and ε{lunate}-martensitic phases.