Effect of Alloying on Microstructure and Precipitate Evolution in Ferritic Weld Metal
Author
Narayanan, Badri Kannan
Degree
Doctor of Philosophy, Ohio State University, Materials Science and Engineering, 2009.
Committee / Advisors
Michael Mills Dr (Advisor)
Sudarasanam Babu Dr (Committee Member)
Hamish Fraser Dr (Committee Member)
Marie Quintana P.E (Committee Member)
.
Abstract
   
The effect of alloying on the microstructure of ferritic weld metal produced with a self-shielded flux cored arc welding process (FCAW-S) has been studied. The welding electrode has a flux core that is intentionally alloyed with strong deoxidizers and denitriding elements such as aluminum, titanium and zirconium in addition to austenite formers such as manganese and nickel. This results in formation of microstructure consisting of carbide free bainite, retained austenite and twinned martensite. The work focuses on characterization of the microstructures and the precipitates formed during solidification and the allotropic phase transformation of the weld metal. Aluminum, manganese and nickel have significant solubility in iron while aluminum, titanium and zirconium have very strong affinity for nitrogen and oxygen. The effect of these alloying elements on the phase transformation and precipitation of oxides and nitrides have been studied with various characterization techniques. In-situ X-ray synchrotron diffraction has been used to characterize the solidification path and the effect of heating and cooling rates on microstructure evolution. Scanning Transmission Electron Microscopy (STEM) in conjunction with Energy Dispersive Spectroscopy (EDS) and Electron energy loss spectroscopy (EELS) was used to study the effect of micro-alloying additions on inclusion evolution. The formation of core-shell structure of oxide/nitride is identified as being key to improvement in toughness of the weld metal. Electron Back Scattered Diffraction (EBSD) in combination with Orientation Imaging Microscopy (OIM) and Transmission electron microscopy (TEM) has been employed to study the effect of alloying on austenite to ferrite transformation modes. The prevention of twinned martensite has been identified to be key to improving ductility for achieving high strength weld metal. Here's the link to the .pdf:

http://search.ohiolink.edu/etd/send-pdf.cgi/Narayanan%20Badri%20Kannan.pdf?osu1243965903

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Henry