Intergranular strains in Inconel-600 and the impact on interpreting stress fields in bent steam-generator tubing

Neutron diffraction measurements are reported of the intergranular (type 2) strains developed in situ under an applied stress in Inconel-600. The magnitudes and signs of the intergranular strains are interpreted in terms of a simple Taylor model with qualitative success. The results are applied to the interpretation of the residual stress fields in severely bent and slightly bent Inconel-600 tubing for steam generators. The anomalous nature of the strains in strongly bent tubing obtained with the (200) reflection is explained naturally in terms of the presence of a combination of type 1, macrostrains and type 2, intergranular strains in the measurement. The impact of this finding on any diffraction measurement of strain and its interpretation in terms of stress is considered.