| Summary: | Construction of reliable, economically feasible and ice-capable vessels and structures for the Arctic has been a subject of great interest in recent years, in spite of the concerns that the undertaking clearly faces profound technical difficulties. One of the technical challenges for modern Arctic shipbuilding is to produce high-quality welds, due to the fact that a weld is usually the weakest part of any structure. Welding is especially difficult for the high strength steels (HSS), which are used in shipbuilding for weight-reduction purposes. The submerged arc welding (SAW) process is one of the most commonly applied welding processes in shipbuilding, because of its high productivity and the outstanding quality of the resultant welds. The objective of the thesis is to explore the usability, development possibilities and parameters of high deposition rate SAW processes and their modifications for welding of thick cold-resistant HSS plates typically used in Arctic shipbuilding applications. Meeting this objective required in-depth understanding of the welding and material science background, which includes the quality requirements of welded joints intended for Arctic service as described in various standards (ISO 19906; ASME 31.3; Norsok M- 001 and Russian Maritime Register of Shipping), properties of cold-resistant HSS and description of testing methods used to validate welding joints for low temperature conditions. The thesis is based on two research methods: literature review and experimental work. The SAW process and welding requirements, cold-resistant HSS and the Arctic operating conditions and challenges from the engineering perspective are first studied. A literature review of materials and welding requirements for Arctic applications according to various standards, experimental testing of advanced SAW processes and assessment of their suitability for application to Arctic conditions, examination of the effects of the SAW process on HSS for Arctic service, and a proposed suitable range of parameters for welding HSS for Arctic conditions was carried out. Based on this analysis, it became possible to design and conduct experiments to present new experimental data that improve understanding of SAW process and its modifications to weld thick quenched and tempered (QT) and thermo-mechanically processed (TMCP) HSS plates. Experiments were conducted to develop high deposition rate SAW welding procedures of modified SAW (conventional single wire, tandem wire welding, and three-wire and four-wire arrangements) to weld several thick (12–35 mm) high strength (580– 650 MPa tensile strength) cold-resistant (intended operational temperature -40°C ~ - 60°C) steel grades, i.e. E500, F500W, X70 and AB2. The welds were evaluated by a wide range of industrial tests: analyses of chemical, microstructural and mechanical properties; hardness tests; and cold resistance evaluation tests: the Charpy V-notch impact test, the Nil-ductility transition temperature (NDT) test, the three-point bending (Tkb) test and the Crack tip opening displacement (CTOD) test. Acceptable welding parameters and recommendations were developed, and the results of the experiments show that highquality welds can be obtained using heat input up to 3.5 kJ/mm. This thesis provides a practical approach and clarifies the difficulties to SAW process parameter selection for welding thick HSS plates intended for Arctic shipbuilding applications. The experimental results can be directly adapted to industrial applications, and the theoretical observations can be used to gain deeper understanding of state-of-theart of SAW processes and its novel developments.
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