Avoidance Method Study for SAGBO Cracking During Heat Treatment of a ITER CS Model Coil Conductor Using Incoloy 908 Jacket

Abstract

Incoloy 908, which is chosen as the ITER CS model coil conductor jacket material, exhibits a characteristic problem that is oxygen embrittlement along grain boundaries as a result of heating in an oxygen atmosphere in the presence of tensile surface stress, causing intergranular cracking during heat treatment from oxidation along grain boundaries (SAGBO:, Stress Accelerated Grain Boundary Oxidation). To consider real manufacturing process of the CS model coil, it was found that avoidance of SAGBO during an actual CS model coil conductor heat treatment is only to attain a SAGBO-free oxygen atmosphere such as 0.1 ppm for both of the conductor jacket outer surface and the inner surface. A baking process and an impurity gases burn-out process are developed for an actual conductor heat treatment, which will accomplish such oxygen atmosphere. These processes are just applied to the Japanese first heat treatment of the conductor.