OBRABOTKAMETALLOV technology Vol. 26 No. 2 2024 The discussion of the results It is known that the composition of the coating significantly affects the chemical composition of the weld metal, facilitating the transfer of elements into the welds, and thereby adding alloying elements to the weld zone. The behavior of weld overlaid metal is influenced by some physical-chemical and thermophysical characteristics of the components of the electrode coating [6–8]. The results presented above show that despite the standardization of components in the electrode coating of the well-known brands TMU-21U, TsU-5, the welding engineering properties, depending on the manufacturer, differs significantly; the coating melts unevenly, as shown by experiments to determine the “fingernail” (Figs. 2, 4, 7). The coatings under study belong to the basic type. During the experiments, it was noted that the main violation in stable arcing is associated with the process of droplet formation and its transfer. Due to the short length of the arc, the arc is shunted by a drop and goes out. Visually, this process is fast and difficult to record. The use of a welding process recorder made it possible to establish this effect of arc gap shunting for all studied electrodes. In this regard, in Table 1, in the column of stable arcing, scores are given in the range from 3 to 5 points. TsU-5 ESAB electrodes demonstrated stable arcing. An important physical parameter is the separation of slag from the weld metal. To ensure high welding productivity, good slag release is necessary because no additional time is required for the mechanical removal of slag adhering to the weld metal. In addition, in multi-pass welding, easily removable slag is critical to prevent any contamination of subsequent welding passes due to residual slag added to the weld pool [4, 5, 8]. The main reasons for good slag separation can be the following: providing a large difference in thermal expansion between the solid slag and the weld metal; preventing the formation of excess amounts of refractory phases such as spinel (MgO Al2O3), Cr2O3 or perovskite (CaTiO3); preventing the formation of a chemical bond due to the formation of layers between the slag and the weld metal; ensuring the formation of low-strength slag from molten flux. The coating of the basic type electrode, during melting leads to the release of gases that protect the molten weld pool from environmental impurities and help maintain stable arcing. CaO is added to increase the basicity of the flux and reduce the hydrogen content of the weld metal. It also reduces toughness and improves stable arcing, resulting in improved weld quality and mechanical properties. The main source of CaF2 is calcium fluoride, which reduces the density and melting point of flux mixtures. It also increases the fluidity of the molten metal and removes hydrogen from the molten pool, forming hydrogen fluorides. One of the problems that emerged when testing electrodes is the poor separability of slag from the weld overlaid metal, which seriously affects productivity. The appearance of the weld beads before and after removal of coated slag is shown in Figs. 3, 5, 8. During the experiments, it was seen that the color of the coated slags changed from dark blue to dark brown and then light brown; the surface of the weld changed from small ripples to large ones and the slag detachability became worse. It should also be noted that the dark blue slag tended to flake off in large chunks from the weld metal, while the brown slag would break into small pieces when the weld bead was knocked out and leave some slag stuck to the surface of the weld metal. In appearance, when observing the destroyed slags, it was recorded that the slag of the TsU-5 electrodes produced by CJSC “Elektrodnyi zavod”, was very dense and upon visual inspection there were no visible pores. However, slags of the TsU-5 electrodes from other manufacturers had visually observable porosity. These observations indicated that slag detachability deteriorates as slag porosity increases. It is known [4, 5, 8] that the separability of slag is very closely related to both the physical and chemical properties of the welding flux after melting the electrode coating [4–8]. One of the mechanisms of slag adhesion to the weld metal is a chemical bond due to the formation of a thin layer of oxides of metal phase elements on the surface of the weld metal [6]. This chemical bond can be weakened or eliminated by using a slag system with minimal oxidizing capacity (for example, a basic flux system) [4–6]. The separability of slag is affected by differences between the thermal expansion coefficients of slag and weld metal, as well as phase transformations in the slag during cooling [8].
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