Microstructural characterization on initial stage of IGSCC and TGSCC of Type316L stainless steel in PWR primary water environment

Abstract

The initiation of IGSCC and TGSCC on Type 316LN stainless steel (SS) in PWR primary water environment were examined by SSRT. 20% and 40% cold rolled Type316LN (SS) flat tensile specimens, the surface of which were finished with colloidal silica suspension, were elongated for 10% in a simulated primary PWR environment at 340 and 360 ℃. Many grains for each specimen after SSRT were observed by SEM and characterized using EBSD to reveal the crystallographical features of initiation for TGSCC and IGSCC.

Slip planes appearing at the specimen surface with slip direction vector which is perpendicular to surface tend to be crack initiation site of TGSCC. The stresses operating at the grain boundary (GB) between two adjacent grains were estimated assuming that the primary slip system was active for each grain. It is revealed that the stress at GB not necessarily correlates with the initiation of IGSCC. On the other hand, IGSCC preferentially occurs when the slip band appearing at surface is nearly parallel to GB.

The results of susceptibility of IGSCC on Type 316LN SS was compared with that obtained for Alloy 600 in the same environment. IGSCC of Alloy 600 initiates depending on misorientation angle of GB, and also on tensile and shear stresses generating at GB.