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An Analysis of Microstructure and Corrosion Resistance in Underwater Friction Stir Welded 304L Stainless Steel An Analysis of Microstructure and Corrosion Resistance in Underwater Friction Stir Welded 304L Stainless Steel

by Clark, Tad Dee

Abstract (Summary)
An effective procedure and parameter window was developed for underwater friction stir welding (UWFSW) 304L stainless steel with a PCBN tool. UWFSW produced statistically significant: increases in yield strengths, decreases in percent elongation. The ultimate tensile strength was found to be significantly higher at certain parameters. Although sigma was identified in the UWFSWs, a significant reduction of sigma was found in UWFSWs compared to ambient FSWs. The degree of sensitization in UWFSWs was evaluated using double loop EPR testing and oxalic acid electro-etched metallography. Results were compared to base metal, ambient FSW, and arc welds. Upper and lower sensitization localization bands were identified in the UWFSWs. Although higher sensitization levels were present in UWFSWs compared to the arc weld, ambient FSW, and heat treated base metals, the UWFSWs were found less susceptible to corrosion than arc welds due to the subsurface location of the sensitization bands. A SCC analysis of UWFSWs relative to base metal and arc weldments was performed. U-bends were exposed to two 3.5% NaCl cyclic immersion experiments at 21 C and 63 C for 1000 hours each. A tertiary test was conducted in a 25% NaCl boiling solution. The UWFSW u-bends were no more susceptible to SCC than base metal in the cyclic immersion tests. In the boiling NaCl test, the SCC of the UWFSWs showed significant improvement over the SCC of arc welds. Arc u-bends cracked entirely within the weld bead and HAZ, while SCC in the UWFSWs showed no cracking localization.
Bibliographical Information:

Advisor:

School:Brigham Young University

School Location:USA - Utah

Source Type:Master's Thesis

Keywords:friction stir weld fsw underwater uwfsw stainless steel zone 304l austenite austenitic procedure process window welding parameters corrosion intergranular stress cracking scc u bend sensitization pcbn double loop epr current ratio potentiostat electrochemical reactivation sampling area quantification tensile strength ductility elongation statistical significance microstructure optical microscopy oxidation surface finish base metal sigma murakami oxalic acid electro etch arc comparison cyclic immersion test boiling nacl subsurface location residual improvement superiority astm standard haz heat affected

ISBN:

Date of Publication:06/09/2005

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