Hydrogen embrittlement susceptibility of tempered 9%Cr–1%Mo steel

The influence of the subsurface hydrogen activity on the hydrogen embrittlement (HE) susceptibility of a tempered 9%Cr–1%Mo ferritic-martensitic steel (T91) has been studied by constant extension rate tests (CERT) performed under cathodic charging during straining at 20 °C. Changes in the hydrogen activity on the surface were obtained by varying the cathodic current density imposed during a tensile test completed to fracture. The broken samples were observed by scanning electron microscopy (SEM). The results show a strong dependence of the extent of the brittle zone with the subsurface hydrogen activity. This effect could be attributed to an enhancement of the apparent diffusion coefficient of hydrogen when increasing H activity. Increasing H activity could enhance both the trapping rate on pre-existing and strain-induced traps and H dragging by moving traps such as dislocations.

► Role of H activity on H absorption and H embrittlement investigated by CER tests and SEM. ► Increasing H activity favors an increase of H penetration depth and the corresponding brittle zone. ► Evidences of an enhancement of H trapping and H dragging by dislocations with increasing H activity.