The study of stress application and corrosion cracking on Ni-16 Cr-9 Fe (Alloy 600) C-ring samples by polychromatic X-ray microdiffraction

Microscopic strains associated with stress corrosion cracks have been investigated in stressed C-rings of Ni-16 Cr-9 Fe (Alloy 600) boiler tubing. Polychromatic X-ray microdiffraction was used to measure deviatoric strain tensors and the distribution of dislocations near cracks that had been propagated in electrochemically accelerated corrosion tests. An associated investigation of the C-ring-induced strains prior to corrosion showed significant tensile strain in the stress axis direction by the torsional closure of the alloy tube section in the C-ring test. Significant grain lattice rotation and pronounced plastic strain at some grain boundaries were noted. Stress-corrosion-cracking-generated intergranular cracks were produced in two Alloy 600 specimens after 6 h and 18 h tests. The diffraction patterns and resultant strain tensors were mapped around the cracked area to a 1 μm spatial resolution. The strain tensor transverse to the crack growth direction showed tensile strain at the intergranular region just ahead of the crack tip for both specimens. Both cracks were found to follow grain boundary pathways that had the lowest angle of misorientation. Dislocation distributions within each grain were qualitatively obtained from the shapes of the diffraction spots and the effect of “hard” and “soft” grains on the crack pathway was explored for both 6 h and 18 h specimens. The Schmid factor of one of the grains adjacent to the crack at the 6 h and 18 h initiation sites was found to be the lowest, compared to Schmid factors calculated for surface grains away from the initiation site, and also along the crack path into the bulk.