Features of the Formation of the Structure of Welded Joints of Structural Steels during Underwater Welding with Filler Wire 12X18N9T Using External Electromagnetic Influence

During underwater wet welding, the environment has a corresponding effect on the mechanical properties of the weld metal. The use of external electromagnetic action (EEA) during welding is promising for influencing the formation of welded joints and the structure formation in physically inhomogeneous environments. Experimental studies have demonstrated the effectiveness of EEA application in reducing the tendency of weld metal to form pores, enhancing degassing, and lowering hydrogen content etc. The paper presents a metallographic study of the welded joint metal of structural steel (St3) after underwater welding with 12Kh18N9T filler wire, both without and with the use of EEA. Based on calculation methods and predictive modelling, optimal operating modes of the electromagnetic system for an experimental study of the EEA effect during underwater welding have been established and implemented. It has been established that the weld metal mainly has a ferrite-pearlite structure, while an austenitic structure with elongated grains is formed in the weld metal. When using the EEA, the grain structure of the weld metal is refined by an average of 1.5 times with an insignificant decrease in microhardness. In the heat-affected zone (HAZ), in the areas of large grain (I HAZ), recrystallisation (II HAZ) and incomplete recrystallisation (III HAZ), a bainitic structure is formed in the presence of ferrite layers. Under the influence of the EEA, the grain size is refined by 1.2 times in the I HAZ and II HAZ with a decrease in the thickness of ferrite layers and an increase in microhardness by an average of 7 ... 10%. The formation of such a structure will provide a set of strength properties and toughness of the welded joint metal. Research has proven that the technology of wet welding under water using the EEA allows for the production of high-quality welded joints with a high set of physical and mechanical properties of the metal of both welded joints and the HAZ.