The dissertation produces and summarizes results of theoretical and experimental investigations, providing scientific substantiation and a practical solution of an important problem of applied science and engineering, namely devising the principles of design and developing a cost-effective, highly efficient technology for metallurgical production and processing of composite steel ingots for making advanced welding wire grades. Special designs have been developed for ingots with composite inserts varying in design and containing components that stabilize the welding process and enhance the metallurgical treatment of the metal bath. The composite insert may have a sectional design enabling introduction of several materials. Based on the uniform approach to the welding wire as the key component of and the major feeding medium for the metal bath, rational ways have been found to ensure appropriate effects of the various additives introduced into the composite insert for purposes of microalloying, deoxidiz ing and fluxing on welding process and composition and properties of welds. Merits of eliminating the microalloying additions and carbon in the steel bulk and concentrating them in the composite insert, thus preventing any reactions in the process of making the ingot, are substantiated physico-chemically. Thermodynamic calculations have confirmed that, in the welding temperature range, carbon undergoes oxidation first, so that good conditions are established for interactions of the microalloying elements with impurities in the metal and/or for alloying reactions. Physical modeling has revealed features of melt flow and solid growth in the presence of a central macro-size chill during solidification of a composite ingot. A method of analysis has been developed and mathematical modeling carried out for solidification of a composite ingot having an axially symmetric central insert filled with powdered materials whose thermal properties have been determined experimentally. The model adequacy has been supported by close agreement between measured and numerically calculated values of frozen-on layer thickness. Forming of composite sections has been studied at the various stages of working ingots to thin wire. Both metallic and nonmetallic filler powders in the composite insert are shown to flow congruently with the ingot bulk metal. An extensive program of research into making and working composite ingots varying in their compositions and applications has been carried out. The composite inserts under study contained a great variety of powders, namely microalloying additions, such as rare earth metals, zirconium, titanium, molybdenum etc.; fluxing and oxide mixes; and partial or complete sets of alloying additives for the respective wire grades. Feasibility of making composite welding wire based on various types of steel as well as nickel alloys has been investigated. Aside from plain and low-alloy steels and, more specifically, the Sv-08G2S and Sv-08 grades as the candidates for a unified matrix, the studies involved hi gh-alloy and stainless steels. Successful tests have been performed for making and processing of composite ingots with composite insert fillers of diverse powders, both metallic, like ferroalloys and metals, and nonmetallic, e.g. various oxides and carbon. The great potential of the composite method for introduction of numerous additives into steel ingots is demonstrated, thus enabling development of advanced composite welding wire grades of a variety of compositions for diversified applications, using the ingot bulk steel. The basic considerations affecting expediency, rational engineering and cost effectiveness have been determined together with production engineering limitations relating to composite insert geometry and filler materials. Technical feasibility of industrial production of heavy composite ingots involving the Sv-08G2S steel as the unified matrix together with rare earth ferroalloys and zirconium as microalloying additions has been validated. This approach offers great flexibility allowing man ufacture of small lots of diverse wire grades exclusively through variation of insert filler composition. In a program of qualification tests of REM-added composite wire, compliance of the resultant welds to EN and DIN requirements has been confirmed alongside with improvements over conventional wire grades currently in use in the European Union. The high productivity of composite ingot processing and conservation of ferroalloys by their introduction into an isolating composite insert, jointly with enhanced levels of properties, make the composite welding wire a promising and competitive material in the markets of the European Union and the Commonwealth of Independent States.
