Long Term Thermal Aging of Type Cf 8 Stainless Steel

Abstract

Atom Probe Field-Ion Microscopy and Transmission Electron Microscopy have been used to characterize the microstructure and to determine the changes that occur during long-term low-temperature aging of primary coolant pipes used in pressurized water nuclear reactors. The cast CF 8 type stainless steel forms a mixture of austenite and ferrite. On aging 10 nm diameter precipitates, tentatively identified as G-phase, formed in the ferrite. The ferrite matrix also decomposed into a fine scale mixture of a chromium-enriched phase and an iron-rich phase similar to the transformation products in iron-chromium-X systems that spinodally decompose

Microstructural Characterization of Primary Coolant Pipe Steel

“…This larger volume of G-phase is related to the differences in initial composition between the two alloys. The G'-phase silicide is rich in nickel and molybdenum [1,5,6] which had higher levels in the CF 8M steel than the CF 8 steel, Table 1 .…”

“…The nominal compositions of this alloy [2] and the previously examined CF 8M alloy [1] are given in Table 1. The CF 8 steel was examined after laboratory aging for 70,000 h at "300, 350 or 400°C.…”

“…These precipitates were identified as G-phase from their cubeon-cube orientation relationship with the ferrite, the face centered cubic structure (Fm3m space group), the lattice parameter of 1.11 nm, and the absence of the 400 reflection [1,5].…”

“…A previous combined atom probe field-ion microscopy (APFIM) and analytical electron microscopy (AEM) investigation of a duplex CF 8M alloy indicated that two phase transformations had occured during aging which could contribute to the changes in mechanical properties [1], The ferrite, after aging for 7500 h at 400°C, had spinodally decomposed to form an interconnected network of iron-rich a phase and chromium-enriched a 1 phase. The precipitation of a G-phase nickel silicide was also observed in the ferrite.…”

“…The precipitation of a G-phase nickel silicide was also observed in the ferrite. It was suggested that small changes in the composition of the alloy could affect the quantities of the phases present by altering the position of the miscibilty gap or by suppressing the formation of G-phase [1].…”