Hydrogen-induced intergranular embrittlementt of ni-base alloys used for steam generator tubes

The hydrogen-induced intergranular rupture of alloys 600, 690 and 82 has been investigated in order to improve the understanding of the possible role of hydrogen on the Stress Corrosion Cracking (SCC) sensitivity of these alloys when exposed to primary water in Pressurized Water Reactors (PWR). Small tensile specimens were hydrogenated at 150°C by cathodic charging in molten salts in order to introduce a controlled amount of hydrogen homogeneously distributed in the samples. The hydrogen concentration in the samples and its dependence on the alloys chemistry and microstructure and on additional desorption annealing was measured by using the thermal fusion method. The role of H on the strain hardening and on the tensile properties of the alloys was investigated at room temperature. SEM was used to characterize the extent of H-induced intergranular cracking. Hydrogenated samples exhibit a strong H-induced ductility lass associated with intergranular fracture. The extent of both the ductility lass and the intergranular rupture strongly depends on the chemical composition and on the microstructure of the alloys. Alloy 82, representative of the weld alloy 600, exhibits the largest HE susceptibility whereas alloy 690 is the less affected by H absorption. The tensile properties recorded after desorption annealing illustrate the reversible character of H-induced intergranular rupture and demonstrate the predominant role, in the mechanism responsible for the ductility lass, of diffusible hydrogen and its interaction with defects involved in the plastic deformation such as dislocations.