Dawei Yu



Document Type


Degree Name



Dept. of Materials Science and Engineering


Oregon Graduate Center


Fundamental aspects of welding metallurgy and microstructural relationships controlling fracture toughness improvement were studied during a series of experimental electroslag (ES) welding practices for joining A36 and A588 base materials with 50mm and 76mm thickness. The weld metal solidification study revealed a correlation between the solidification structure and the as-deposited weld metal morphology. Based on this finding, a model describing the formation sequence of ES weld metal has been suggested. The cellular dendritic to columnar dendritic transition and the kinetics of austenitic growth were the key factors affecting the products of solid state transformation in the weld metal. Upon cooling, the cellular dendritic region first transformed to coarse columnar austenite grains, followed by the formation of a high percentage of acicular ferrite. The columnar dendritic region first transformed to thin columnar austenite grains, followed by the formation of a high percentage of grain boundary and side plate ferrite. Variation in the form factor of the weld pool influenced the morphology of solidification structure and consequent as-welded microstructure. The effect of alloying additions to the weld metal were intensively studied in order to develop an effective method for upgrading the toughness of as-deposited weldments. Small additions of Cr, Mo and Ni tended to promote equiaxed dendritic growth at the weld center of mild steel welds, which substantially improved resistance to hot cracking. Also, the presence of Cr, Ni and Mo in the weld metal effectively inhibited the formation of grain boundary ferrite and side-plate ferrite. However, Cr-Mo additions resulted in a bainitic ferrite weld metal with low CVN toughness. While the Ni addition, which was a strong promoter of acicular ferrite formation, improved the weld metal toughness. Ni-Mn-Mo alloying in A36 and A588 steel ES welds created a fine acicular ferrite-predominant weld metal microstructure with impressively improved toughness relative to conventional practices. The metal powder-cored tubular filler metal is recommended for ESW, because of its advantages over the solid filler metal such as, lower heat input, lower base metal dilution, higher deposition rate, higher form factor, and higher resistance to hot cracking. A specially designed Ni-Mn-Mo alloyed tubular filler metal exhibited not only dramatically upgraded CVN impact toughness in the ES deposits of both A36 and A588 steel joints, but increased the K1c toughness values of A588 steel weldments. Detailed investigations revealed that electroslag welds have special thermal Transfer characteristics. Its large molten slag pool delivers an enormous heat to the surrounding base metal. The HAZ size in ES weldments was controlled by not only the total heat input but also the geometry of the molten weld pool. It was also demonstrated that, for ES weldments, the measured CVN impact toughness of coarse HAZ is a sensitive function of the distance from the fusion line.





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