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Microstructure Characterization Of Alloy 600TT Steam Generator Tubing

This paper describes the evolution of production standards for Alloy 600 tubing, the historical performance of steam generator tubing, and the results of microstructural analyses of archive and pulled tubing samples from commercial PWRs to address these issues. Alloy 600 is a corrosion-resistant nickel-base alloy that is used in a variety of applications that require good resistance to general corrosion, high strength, and good formability. It has been used extensively for steam generator tubing in commercial nuclear power plants, and this experience led to the use of several different types of Alloy 600 material.

Product Number: ED22-17239-SG
Author: Amanda Jenks, Chuck Marks, Brent Capell
Publication Date: 2022
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Alloy 600 has been used for steam generator tubing in both the mill annealed (MA) and thermally treated (TT) conditions, the latter of which results in generally improved stress corrosion cracking (SCC) resistance. Still, SCC has been observed in all types of Alloy 600 tubing in numerous plants. While there is no consensus on the particular mechanism of this degradation mode or what specifically induces SCC resistance, the microstructure of the material could be a significant factor. One concern is the uniformity (or lack thereof) of the microstructure of in-service Alloy 600TT steam generator tubes, which could explain why greater numbers of flaw indications have been observed at some plants than at others. Additionally, microstructure information could be used to assist predictions of future performance. 

Alloy 600 has been used for steam generator tubing in both the mill annealed (MA) and thermally treated (TT) conditions, the latter of which results in generally improved stress corrosion cracking (SCC) resistance. Still, SCC has been observed in all types of Alloy 600 tubing in numerous plants. While there is no consensus on the particular mechanism of this degradation mode or what specifically induces SCC resistance, the microstructure of the material could be a significant factor. One concern is the uniformity (or lack thereof) of the microstructure of in-service Alloy 600TT steam generator tubes, which could explain why greater numbers of flaw indications have been observed at some plants than at others. Additionally, microstructure information could be used to assist predictions of future performance.