Kinetics and mechanism of thermal aging embrittlement of duplex stainless steels

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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 ..cap 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 ... continued below

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Pages: 32

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Chung, H.M. & Chopra, O.K. June 1, 1987.

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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 ..cap 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 ..cap 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 ..cap alpha..' (high Cr content) or to suppress the ..cap alpha..' in favor of ..gamma../sub 2/ precipitation (low Cr). Approximate TTT diagrams for the spinodal, ..cap 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.

Physical Description

Pages: 32

Notes

NTIS, PC A03/MF A01; 1.

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  • 3. international symposium on environmental degradation of materials in nuclear power systems: water reactors, Traverse City, MI, USA, 30 Aug 1987

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  • Other: DE88002987
  • Report No.: CONF-870839-6
  • Grant Number: W-31109-ENG-38
  • Office of Scientific & Technical Information Report Number: 5856937
  • Archival Resource Key: ark:/67531/metadc1104269

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  • June 1, 1987

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  • Feb. 18, 2018, 3:59 p.m.

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  • April 18, 2018, 11:28 a.m.

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Chung, H.M. & Chopra, O.K. Kinetics and mechanism of thermal aging embrittlement of duplex stainless steels, article, June 1, 1987; Illinois. (digital.library.unt.edu/ark:/67531/metadc1104269/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.