Composition effects on the degradation and microstructure evolution of aged duplex stainless steels

Research output: Chapters, Conference Papers, Creative and Literary Works (RGC: 12, 32, 41, 45)32_Refereed conference paper (with ISBN/ISSN)

1 Scopus Citations
View graph of relations

Author(s)

Detail(s)

Original languageEnglish
Title of host publicationProceedings of the 6th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors
PublisherPubl by Minerals, Metals & Materials Soc (TMS)
Pages391-397
ISBN (Print)873392582
Publication statusPublished - 1993
Externally publishedYes

Conference

TitleProceedings of the 6th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors
CitySan Diego, CA, USA
Period1 - 5 August 1993

Abstract

The possibility of thermal aging embrittlement in ferritic stainless steels led to an investigation on duplex stainless steels. A particularly intractable problem, phase instability, which has been discovered in duplex stainless steels characterized by the phase transformation to austenite (denoted γ2) in ferrite (denoted α), a complex nickel silicide known as G phase, carbides, and spinodally decomposed into a fine scale network of an iron rich phase and a chromium enriched phase. The phase instability is also found in CF8 duplex stainless steels when long term thermally aged between 300 °C and 500 °C. The room temperature impact strength can decrease by 80% after aging for 8 years at a temperature as low as 300 °C. These phase transformations are considered to be responsible for the low temperature degradation in mechanical properties. The extent of the degradation was found to be strongly dependent on the ferrite composition, and that the exact evolution of microstructures drew large attention. In this study, altering the ferrite composition while increasing the solution annealing temperature, degradation and microstructures of CF8 alloys aged at 300, 400, and 500 °C up to 10000 h were investigated. For CF8 grade materials at 500 °C aging, M23C6 carbides were identified on the austenite-ferrite boundaries as well as in the ferrite. Many phases precipitate and transformations in ferrite also were investigated. CF8 steels age embrittlement at 500 °C due to ferrite degradation and carbides formed on α/γ interface. However, TEM analyses show that the embrittlement during aging at 400 °C is associated with spinodal decomposition and G phase precipitation, only spinodal decomposition at 300 °C. The End-of-Life toughness of components survived below 300 °C should not be extrapolated by aging data obtained at ≥400 °C for the CF8 grade steels. It has been shown that ferrite aging embrittlement below 400 °C could be characterized by microhardness. The Cr content appears to play an important role to microhardness, and Ni content in ferrite influences the kinetics of embrittlement. Ni appears to accelerate the spinodal decomposition and G phase precipitation.

Citation Format(s)

Composition effects on the degradation and microstructure evolution of aged duplex stainless steels. / Shiao, J. J.; Tsai, C. H.; Kai, J. J.; Huang, J. H.

Proceedings of the 6th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors. Publ by Minerals, Metals & Materials Soc (TMS), 1993. p. 391-397.

Research output: Chapters, Conference Papers, Creative and Literary Works (RGC: 12, 32, 41, 45)32_Refereed conference paper (with ISBN/ISSN)