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Microstructural Evolution in Dissimilar Metal Weld Failures Involving Grade 91 Steel

Dissimilar metal welds (DMWs) between ferritic and austenitic stainless steels are commonly used in power generation applications for steam pipes operating at temperatures up to 550°C

Grade 91 is a Creep Strength Enhanced Ferritic Steel which was designed for higher temperatures than its Grade 22 predecessor and is commonly welded to nickel based superalloys as filler metals

The presence of a carbon concentration gradient across the partially mixed zone creates a chemical potential gradient which causes carbon to diffuse from the base metal into the weld metal at elevated temperatures

DMW failures typically occur due to the difference in coefficient of thermal expansion and creep strength which change when aged in service due to carbon diffusion

Grade 91 DMWs have been reported to fail prior to their expected lifespan. This failure always occurred on the base metal side of a thin 1-3μm carbide free ferrite band along the fusion line

 

Industry Sponsor: EPRI, Special Metals, ThermoCalc

Faculty: John DuPont

Graduate Student: Sean Orzolek

Industry Contact: John Siefert, Jack deBarbadillo, Adam Hope