Effect of short-range ordering on stress corrosion cracking susceptibility of Alloy 600 studied by electron and neutron diffraction

Slow strain rate tests (SSRTs) were conducted on low-temperature mill-annealed Alloy 600 tubes at 250 and 360 °C in water with either 8 ppm or below 10 ppb of dissolved oxygen (DO). A special tensile specimen design with a hump was employed for these tests. During SSRT in 360 °C water, stress corrosion cracking (SCC) or intergranular cracking of Alloy 600 is enhanced at DO contents below 10 ppb but suppressed at 8 ppm DO. The SCC susceptibility of Alloy 600 is observed to be related to the degree of lattice contraction by short-range ordering, which is enhanced in the presence of hydrogen. By analyzing electron and neutron diffraction patterns before and after SSRT in 360 °C water, for the first time, definitive evidence is presented for the short-range ordered phase with a d-spacing of 2.1 Å being formed in Alloy 600 during SSRT in 360 °C water, which is manifested by the forbidden reflections at the 1/3{4 2 2} positions in 〈1 1 1〉 selected area diffraction patterns.