Microstructure and Microdeformation Effects on IGSCC of Alloy 600 Steam Generator Tubing

Microdeformation characteristics in alloy 600 tubing have been examined after various tensile deformations. Microstructure developed during processing was found to control subsequent microdeformation behavior. Grain boundary carbides were the most effective source of dislocations, activating at lower macrostrains and continuing to operate at higher macrostrains than other sources. Ledges within grain boundaries, twin boundaries, and matrix carbides also acted as dislocation sources. Most dislocation activity at low strains was confined to planar arrays.A conceptual model is presented to explain the effects of interfacial and matrix microstructure on microdeformation and primary side stress corrosion cracking (SCC) of alloy 600 tubing. Microstructure is linked to intergranular stress corrosion cracking (IGSCC) resistance through its influence on microdeformation behavior and the resultant crack tip stress state. Dislocation source activity at grain interfaces is proposed to be critical in controlling IGSCC susceptibility. Effective sources such as grain boundary carbides promote crack blunting, decrease the crack tip stress state, and increase resistance to cracking.