Phase separation study of in-service thermally aged cast stainless steel – atom probe tomography

QC 20210527
Embrittlement of Duplex Stainless Steels by thermal aging shortens the service life of structural components in LWRs. This is an important issue when life extension programs are aiming at 60-80 years in service. Cast and welded austenitic stainless steels, which contain some ferrite, are known to be affected by thermal aging. Historically, many LWR components of complex geometry have been cast in the Mo-containing quality CF8M. Aging is attributed to two types ofphase transformations; Demixing of the ferrite by spinodal decomposition into Cr-rich ´ and Fe-rich  regions; and precipitation of G-phase, carbides and other secondary phases.A study was conducted on two in-service aged large casting CF8M elbows exposed for 72 kh at 291ºC and 325ºC, respectively, followed by 22 kh at a reduced service temperature. Atom Probe Tomography was used to characterize the decomposition of the ferrite for both aging states. Spinodal decomposition and nucleation of precipitates, i.e. G-phase, have been identified. The extent of phase transformation increases with exposure temperature, and the mechanical properties follow the same trend.

Ingår i:International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors: Canadian Nuclear Society (CNS)9781510813953

Av författaren/redakt...: Bjurman, Martin; Thuvander, Matti ...; Lui, Fang; Efsing, Pål, 196 ...

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