Effect of Residual δ-Ferrite on SCC Behavior of 321 Stainless Steel

摘要

The effect of residual δ-ferrite (～2%) on the stress corrosion cracking (SCC) behavior of 321 stainless steel (SS) base metal from a dissimilar weld joint was investigated by direct current potential drop (DCPD) method with an "on the fly" change of water chemistry in simulated light water reactor (LWR) water environment. The results show that 321SS is a highly SCC resistant material. The improved SCC crack growth rate (CGR) performance, from 3.3 × 10~(-8) mm/s to 1.6 × 10~(-9) mm/s, and the absence of a Cl~- acceleration effect on CGR were caused by the dispersed island-shape δ-ferrite. The SCC resistant δ-ferrite inhibited the growth of cracks and forced the crack to propagate along the phase boundary, where neither elevated residual strain nor significant Cr depletion could be found. The dispersed island-shape δ-ferrite works as a Cr source to facilitate the less soluble Cr-rich oxides formation at phase boundaries. Furthermore, the dispersed Ti(CN)x particles at the γ/δ phase boundaries in Ti-modified austenitic 321SS tends to blunt the crack tip, inhibits phase boundary oxidation, and facilitates the less soluble Cr/Ti rich oxides formation, thus impeding the SCC growth when the crack was forced to propagate along the γ/δ phase boundary.