Session: MF-06-02 Materials and Technologies for Nuclear Power Plants II

Paper Number: 62049

Start Time: Tuesday, July 13, 2021, 08:00 PM

62049 - Residual Lifetime Assessment of Service-Aged Dissimilar Forge 91-Pipe 91 Steel Header Welds With High Temperature Digital Image Correlation 

Condition assessment of welded steam components is crucial activity for power plant operation and maintenance. In this work, creep performance of an ex-service large bore girth weld between an SA-182 F91 reducer to a SA-335 P91 header was tested with a specially designed high temperature digital image correlation (DIC) system. Feature cross-weld creep samples were assessed using full field creep strain monitoring with DIC which permitted for analysis of local strain accumulation through the duration of the each test. The results provide insight regarding failure in the heat affected zone in 9-12% Cr creep strength enhanced ferritic (CSEF) steel welds. Metallurgical characterization was executed in the as-received and post-test conditions to determine the contribution of pre-existing creep damage, the so-called ‘soft zone’ in the HAZ, and the microstructural regions in the HAZ to the final observed failure location. The in situ DIC strain measurements confirmed a highly nonuniform creep strain distribution across the F91-P91 weld. The cross-weld specimen eventually failed through the F91 HAZ exhibiting a so-called “Type IV failure”. Important observations in this work clearly show that while the nominal creep strain averaged over the gauge length is consistently below 3%, while the localized creep strain value in the HAZ where failure occurs is ~10%. Post-test characterization of the ruptured specimens shows that creep failure occurred in the most pre-damaged region and this location is adjacent to, but not inside the documented soft zone prior to the test. Roles of underlying microstructure features such as pre-existing cavities, coarsened precipitates, and large inclusions will also be discussed to clarify the creep failure mechanism.

Presenting Author: Yiyu Wang Oak Ridge National Laboratory

Authors:

Yiyu Wang Oak Ridge National Laboratory
Wei Zhang Oak Ridge National Laboratory
Yanli Wang Oak Ridge National Laboratory
John Siefert Electric Power Research Institute
Alex Bridges Electric Power Research Institute
Steven Kung Electric Power Research Institute
Zhili Feng Oak Ridge National Laboratory