High susceptibility to cold cracking induced by diffusible hydrogen and hydrogen embrittlement are major obstacles to greater utilization of underwater wet welding for high-strength steels. The aim of the research was to develop gas–slag systems for flux-cored wires that have high metallurgical activity in removal of hydrogen and hydroxyl groups. Thermodynamic modeling and experimental research confirmed that a decrease in the concentration of diffusible hydrogen can be achieved by reducing the partial pressure of hydrogen and water vapor in the vapor–gas bubble and by increasing the hydroxyl capacity of the slag system in metallurgical reactions leading to hydrogen fluoride formation and ionic dissolution of hydroxyl groups in the basic fluorine-containing slag of a TiO2–CaF2–Na3AlF6 system.

High-strength steel S960 is one of a number of advanced steels able to meet the demands of the shipbuilding, offshore, and construction industries for a favorable good high strength/weight ratio. Gas metal arc welding (GMAW) is commonly used in all structural steel fabrication, and developments in GMAW have removed previous limitations regarding high heat input and have reduced flaws. One solution for controlled heat input while ensuring a stable arc is alloying the welding wire. Usage of nanoparticles as an alloying element in welding wire have shown significant improvements in weld properties. This study investigates an S960QC joint welded with a welding wire having Lanthanum (La) nanoparticles as a coating and examines the influence of La on the welding parameters, arc stability, microstructure formation, and mechanical properties. The results are compared with a weld formed with conventional Union X96 welding wire. The microstructures observed in the weld region were martensite and tempered martensite for both wires. In the heat-affected zone, microstructures of upper bainite, martensite, tempered martensite, and globular bainite were found. The La nanoparticle-coated wire provided a stable arc during welding. However, due to the increase in wire thickness, manual wire feeding was required. The impact toughness was lower in the joint formed with the nanoparticle-coated wire. Additionally, the hardness at the fusion region was higher in the joint welded with the nanoparticle-coated wire.

The paper describes the results of research about orbital hyperbaric FCA-welding of API X70 steel pipes with the use of gas-shielded flux cored wires with the flux of TiO2-CaF2 system with the higher content of fluorides in the flux core. As a result of the research, the dependences of the welding current, arc voltage, metallurgical features and mechanical properties depending the pressure in the hyperbaric chamber are determined. The use of gas-shielded flux cored wires of TiO2-CaF2 system within the gas pressure 0,1–0,6 MPa, for the orbital welding of API X70 steel provides the favourable microstructure of welded joint and mechanical properties of welds according to the Class A per ANSI/AWS D3.6M:2010.

The paper presents research in the field of underwater wet cutting with the use of flux-cored wires in order to improve the quality and performance. The research has resulted into the development of gas and slag systems for flux-cored wires and determination of, optimal parameters for cutting stability and quality. The underwater wet cutting mechanism is a cyclical process with the formation of periodic keyholes in metal, and it consists of operating and idle cycles. Efficiency of the cutting process can be determined by analyzing cycle times, welding current, voltage, power and a number of short circuits. To assess the stability and efficiency of the underwater wet cutting process, the authors have developed the method for analyzing oscillograms to calculate the probability density of current, voltage and power. To determine the quality of cutting, the authors have provided a criterion based on the ratio of the voltage probability density in the idle and operating cycles.

Development of welding consumables with fluorides and borides of rare earth metals is a promising area for improving the weldability of low alloy steels. As lanthanum fluoride and boride dissociate, lanthanum and boron dissolve in the weld pool and the welding arc plasma is saturated with fluorine. As a result of FeO, MnO, SiO2 deoxidation and FeS, MnS desulfurization, refractory lanthanum sulfides and oxides La2O3, La2S3 are formed in the weld pool, which can be the crystallization nuclei in the weld pool and the origin of acicular ferrite nucleation. The paper proposes a model of metallurgical processes in the arc and weld pool, as well as a model of electrochemical adsorption of Ni2+ cations in colloidal electrolytes during electrostatic deposition of nano-dispersed insoluble particles of LaF3 or LaB6 on the surface of wire. The paper discusses the constructional design of the welding wire and the technology for forming electrochemical composite coatings with copper and nickel matrix. The composite wires applied in the welding of low alloy steels make it possible to refine the microstructure, increase the tensile strength by 4% and the impact toughness of welds by 20%.