Relationship Among Dislocation Density, Local Strain Distribution, and PWSCC Susceptibility of Alloy 690

The effects of local strain distribution on primary water stress corrosion cracking of cold-rolled Alloy 690 with an inhomogeneous microstructure were investigated by measuring dislocation densities using transmission electron microscopy. Many intragranular carbides with a Cr-rich M₂₃C₆ structure were dispersed in the fine grains. The results showed that the dislocation densities near the intragranular carbides were high, regardless of the degree of cold-rolling. Below 20% cold-rolling, the dislocation densities near grain boundaries were higher than that in the grain interior. Meanwhile, the dislocation densities in the grain interior increased to similar value of the grain boundary with increasing degree of cold-rolling up to 40%. The results indicate that the intragranular carbides dispersed in the fine grains play an important role in the local strain distribution of a cold-rolled Alloy 690 with an inhomogeneous microstructure. It is suggested that the high local strain in the grain interior in a severely cold-rolled Alloy 690 induced by interaction between dislocation and intragranular carbides could be responsible for the mixed cracking mode and the high crack growth rate.