Nuclear Executive Update   
An EPRI Progress Report, July 2011
TECHNICAL HIGHLIGHTS
Freeze Sealing Guide to Address Concerns with Potential Brittle Behavior

New research indicates that martensitic transformation should not have an impact on the ductility of austenitic stainless steels when subjected to freeze sealing.

An updated guide on freeze sealing will reflect EPRI research on martensitic transformation in austenitic stainless steels after exposure to extremely low (freeze sealing) temperatures. Such research is needed to address questions regarding whether martensitic transformation in some types of stainless steels could lead to brittle behavior.

EPRI tested both non-sensitized and sensitized Type 304 SS samples exposed to liquid nitrogen (-320 F) for 1 hour, 8 hours, 24 hours, and 168 hours (1 week) to determine how low temperatures could impact material properties. Type 304 stainless steel (SS) was selected because it is affected more by martensitic transformation than 316 SS, thus providing a "bounding" material. Prior to exposure, one-half of the 304 SS samples were heat-treated to create a sensitized microstructure. Photo micrographs confirmed the sensitization of the material by comparison of "before" and "after" heat treatment.

To determine if martensitic transformation had occurred to a degree to materially affect the overall ductility, EPRI conducted Charpy V-notch testing. As shown in the figure, the data indicates no effect on toughness values for non-sensitized materials and only a slight drop in values for sensitized materials. Therefore, the test results indicate that martensitic transformation should not have an impact on the ductility of commonly used materials such as 304 and 316 SS in nuclear power plants. Based on this testing, the current guidance contained in EPRI report TR-016384 R1, Freeze Sealing (Ice Plugging) of Piping, will be updated and published in late 2011.

For more information, contact Marty Bridges at 704.595.2672 or mbridges@epri.com.