Effects of grain boundary composition and structure on IGSCC of Alloy 600

The metallurgical factors responsible for intergranular stress corrosion cracking (IGSCC) of alloy 600 on the secondary side of steam generators are identified and evaluated for their known and potential impact. These include grain boundary composition: chromium and segregated impurities such as phosphorus and boron, chromium carbides, solid solution carbon, grain boundary structure and grain size. Each of these factors is defined and described in the context of commercial alloy 600 steam generator tubing. Each is then evaluated in terms of its known or potential effect on IGSCC in the secondary side environment. A higher bulk chromium concentration is known to impart resistance to IGSCC through increased passivity and solid solution strengthening. Grain boundary chromium depletion is detrimental in acidic solutions, but not in neutral solutions. In caustics, chromium depletion combined with carbon segregation may act to lower resistance to IGSCC. Carbon is a potent solid solution strengthener and increases the strength and resistance to creep deformation in inert and high temperature water environments. However, segregation at the grain boundary may be detrimental in caustic. While phosphorus and boron are the most important grain boundary impurities, P is benign in all environments and B may have an effect if not incorporated into the carbide. Grain boundary misorientation has a profound effect on the creep behavior, strength and IG cracking resistance of Ni-16Cr-9Fe. It has the potential for development as a tool for control of IGSCC. A smaller grain size correlates with increased IGSCC resistance, most likely through an increase in flow stress. 123 refs., 35 figs., 2 tabs