Abstract
The Incoloy 908 jacket material exhibited intergranular cracking during superconductor reaction heat treatment at 650°C. The mechanism of this intergranular cracking was attributed to stress-accelerated grain boundary oxidation (SAGBO). Incoloy 908 is the jacket material for the cable-in-conduit superconductor for the International Thermonuclear Experimental Reactor (ITER). Fracture surfaces mapped by wave dispersive X-ray (WDX) analyses indicated an accumulation of oxygen at the crack initiation region, but did not show any presence of oxygen around the crack tip. A computational simulation of the conductor manufacturing process indicated the presence of residual surface stresses in the jacket material. Residual surface tensile stresses are created on the jacket outer surface, as a result of plastic deformation during conductor fabrication (known as the jacket compaction process). It has been concluded that the successful heat treatment of the jacket material can be accomplished by:
(1) Careful shotpeening on the outer surface of the jacket material to create compressive surface stresses and
(2) Evacuation of oxygen by removing oxygen sources such as the oil contaminants and water vapor from the conductor at temperatures below 550°C.
