Fig. 5 — Mass transfer coefficients of the two types of electrodes. Fig. 6 — Macrograph of the metal droplets detained in the slag. of CaO and CaF2 leads to a lower viscosity of slag, which results in a reverse effect, i.e., smaller penetration but larger bead width — Fig. 4B. Chemical Composition of the Deposited Metals and Mass Transfer Although the chemical compositions of the deposited metals of the two types of electrodes met the requirements of the AWS A5.11 ENi- CrFe-9 standard (Table 3), some apparent differences in the main alloy elements between the two types could be found. The Mo, W, Cr, Mn, and Si in the deposited metals of the TiO2-SiO2- SrO type electrode were a little higher than those of the CaF2-CaO-SiO2 type. The impurities, S, P, and O, were also higher in the TiO2-SiO2-SrO type. However, the C behaved differently. Considering different flux coating compositions, the real behaviors of each alloying element can be implied by their mass transfer coefficients that were defined by the following equation (Refs. 8, 13): = Wdepo/Wwire + Wcoat (7) where indicates mass transfer coefficient, Wdepo indicates the weight percent of the element in the deposited metal, Wwire indicates the weight percent of the element in the core wire, Wcoat indicates the weight percent of the element in the coating, and is the ratio of the coating weight to the core wire weight. In this study, the average weight of the core wire is about 24.50 g, and the average weight of the coating is about 21.58 g for the CaF2-CaO-SiO2 type and about 22.17 g for the TiO2-SiO2- SrO type. Thus, is 88.07% for the CaF2-CaO-SiO2 type but 90.48% for the TiO2-SiO2-SrO type. The high ratios are because the large amounts of alloying metals were added in the coating. According to the flux coating ingredients (Table 1) and the chemical compositions of the deposited metals (Table 3), the mass transfer coefficients of the main alloying elements could be calculated by using Equation 7 — Fig. 5. It could be found that the mass transfer coefficients of Ni, W, Mo, Fe, Cr, and Mn in the CaF2-CaO-SiO2 type are slightly higher than in the TiO2- SiO2-SrO type. However, that of Nb and Si are in the reverse manner. The higher mass transfer coefficients suggest the smaller loss during welding, while the lower mass transfer coefficients indicate the larger loss via the oxidation, evaporation, spattering, and fumes during welding. According to the data shown in Fig. 5, one could conclude that the loss of Ni, W, Mo, Fe, Cr, and Mn in the CaF2-CaO-SiO2 type was smaller than in the TiO2- SiO2-SrO type. But the loss of Nb and Si in the CaF2-CaO-SiO2 type was larger than in the TiO2-SiO2-SrO type. The loss of metals during welding mainly depends on their oxygen affinity and boiling points. The higher oxygen affinity of metals and a stronger oxidizing environment would lead to forming more oxides that enter into the slag. The lower boiling point of the metals increases evaporation loss (e.g., Mn in this study). The oxygen affinity of the metals was roughly ranked in the order Si > Nb > Mn > Cr > Fe > Mo > W > Ni through all the welding processes (including the droplet reaction, weld pool reaction, and weld metal solidification stages) (Ref. 24). This order just matches the value sequence of the mass transfer coefficients of these elements as shown in Fig. 5. It was noteworthy that the ratio of the coating weight to the core wire weight was very high in this study (much higher than that in the reported research Refs. 7–10, 12) because the alloy elements were added from the coating. When the flux coating contained too many metallic powders, the molten slag became viscous during welding. Some small metal droplets might be enveloped by the viscous slag, and detained in the slag after welding (Refs. 10, 12), resulting in the larger loss of the metals. Such a phenomenon was confirmed by the observation as shown in Fig. 6. The size of the solidified droplets was in the range of 0.1–2.0 mm. One of the droplets was analyzed by EDS analysis, which contained Cr: 26.39, Ni: 21.22, Fe: 18.22, Mo: 13.60, W: 9.27, Nb: 6.25, Mn: 4.36, and Si: 0.69 (wt-%). Obviously, the detained metal particles were part of the metals transferred from the covered electrode. As the amount of the detained metal particles increased, the mass transfer coefficients of these metals were re- WELDING RESEARCH 472-s WELDING JOURNAL / DECEMBER 2016, VOL. 95
Welding Journal | December 2016
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