Author

Hee-Sung Ann

Date

August 1987

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Materials Science and Engineering

Institution

Oregon Graduate Center

Abstract

Principles of solidification mechanics were applied to study solute band formation, dendrite reorientation and renucleation mechanisms during weld metal solidification of electroslag (ES) welds. Dendrite reorientation and renucleation were significantly affected by solute banding. Development of secondary and tertiary dendrite arms was dependent upon a unique combination of the temperature gradient vector and primary dendrite arm growth direction. Mechanisms were proposed to explain these solidification phenomena. Mechanical properties of weld metal deposited by consumable guide electroslag welding (ESW) are controlled by the complex macro/microstructural features and impurity segregation developed throughout the weld. In order to understand this behavior, fundamental aspects in the design of high toughness welds in 50 mm and 76 mm thick A36 and A588 structural steels were investigated. Parameters such as voltage, current, gap size, welding speed, guide tube/plate designs, solid/tubular filler metals, flux variables (oxygen potential and basicity), and alloying additions (up to 2% Ni and 0.4% Mo in the filler metal) were examined and found to significantly influence the solidification structures, macro/microstructures, impurity segregation and fracture toughness of ES welds. From these comprehensive studies, five different characteristic grain structures have been developed for electroslag welds, leading to new classifications for these welds. Optimum conditions, based on parametric studies and macro/microstructure analysis in ESW, were determined with a narrow gap (19mm), unalloyed and wing or web guide plates. The optimum conditions provided increased welding efficiency, deposition rate, resistance to hot cracking, and improved microstructures. Dramatically refined weld metal grain size was achieved in the as-welded condition by shielding the consumable guide tube or plate with a fused quartz sleeve. Intense stirring of the weld pool and significantly increased nucleation of polygonal and grain-boundary ferrite (due to excessive a high density of oxide inclusions) were responsible for grain refinement. Charpy V-notch (CVN) tests were carried out for selected optimized and non-optimized ES welds. A substantial increase in CVN toughness was observed in optimized welds. The quartz grain refined welds exhibited identical toughness values throughout the weld.

Identifier

doi:10.6083/M4QF8QT9

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