Kinetics and Mechanism of Thermal Aging Embrittlement of Duplex Stainless Steels

摘要

Microstructural characteristics of long-term-aged cast duplex stainless steel specimens from eight laboratory heats and an actual component from a commercial boiling water reactor have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), small angle neutron scattering (SANS), and atom probe field ion microscopy (APFIM) techniques. Three precipitate phases, i.e., Cr-rich alpha ' and the Ni- and Si-rich G phase, and gamma sub 2 austenite, have been identified in the ferrite matrix of the aged specimens. For CF-8 grade materials, M sub 23 C sub 6 carbides were identified on the austenite-ferrite boundaries as well as in the ferrite matrix for aging at greater than or equal to 450 sup 0 C. It has been shown that Si, C, and Mo contents are important factors that influence the kinetics of the G-phase precipitation. However, TEM and APFIM analyses indicate that the embrittlement for less than or equal to400 sup 0 C aging is primarily associated with Fe and Cr segregation in ferrite by spinodal decomposition. For extended aging, e.g., 6 to 8 years at 350 to 400 sup 0 C, large platelike alpha ' formed by nucleation and growth from the structure produced by the spinodal decomposition. The Cr content appears to play an important role either to promote the platelike alpha ' (high Cr content) or to suppress the alpha ' in favor of gamma sub 2 precipitation (low Cr). Approximate TTT diagrams for the spinodal, alpha ', G, gamma sub 2 , and the in-ferrite M sub 23 C sub 6 have been constructed for 250 to 450 sup 0 C aging. Microstructural modifications associated with a 550 sup 0 C reannealing and a subsequent toughness restoration are also discussed. It is shown that the toughness restoration is associated primarily with the dissolution of the Cr-rich region in ferrite. (ERA citation 13:008831)