Grain-boundary microchemistry and intergranular cracking of irradiated austenitic stainless steel.

摘要

Constant-extension-rate tensile tests and grain-boundary analysis by Auger electron spectroscopy were conducted on high- and commercial-purity (HP and CP) Type 304 stainless steel (SS) specimens from irradiated BWR components to identify mechanisms of irradiation-assisted stress corrosion cracking (IASCC). Susceptibility to intergranular fracture could not be correlated with radiation-induced segregation of impurities such as Si, P, C, or S, but a correlation was obtained with grain-boundary Cr concentration, indicating a role for Cr depletion. Grain-boundary chemistry was conducted on BWR neutron absorber tubes fabricated from two similar heats of HP Type 304 SS of virtually identical bulk chemical composition but exhibiting different susceptibility to IASCC after irradiation to (approx)2 (times) 10(sup 21) n/cm(sup 2) (E > 1 MeV). Grain-boundary concentrations of Cr, Ni, Si, P, S, and C of the cracking-resistant and -susceptible HP heats were virtually identical. However, grain boundaries of the cracking-resistant material contained less N and more B and Li. This observation indicates that, besides the deleterious effect of grain-boundary Cr depletion, synergism between grain-boundary segregation of N and B and transmutation to H and Li plays an important role in IASCC.