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 8ppm or below 10ppb 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 10ppb but suppressed at 8ppm 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 2.1Å being formed in Alloy 600 during SSRT in 360°C water, which is manifested by the forbidden reflections at the 1/3{422} positions in 〈111〉 selected area diffraction patterns.