A C Fig. 8 — Characteristic segregation phenomena across the dendrites: A and C — CaF2CaOSiO2 type; B and D — TiO2CaF2SiO2 type. NiS + (MnO) (or/and CaO) = (MnS) (or/and CaS) + (NiO) (13) 3(CaF2) + 4NiS = 3(CaS) + 4Ni + SF6 ↑ (14) 2Ni3P + 5(NiO) + 3(CaO) = ((CaO)3•P2O5) + 11Ni (15) 2Ni3P + 5(NiO) + 4(CaO) = ((CaO)4•P2O5) + 11Ni (16) The reaction products MnS, CaS, (CaO)3•P2O5, and (CaO)4•P2O5 entered into the slag. Because CaF2 and basic oxides decreased slag viscosity (Ref. 22), they could improve floating of these sulfides and complex oxides in the welding pool. As the reactions progress, the sulfur and phosphorus were expelled from the melted metals. It was reasonable that the higher contents of Mn and basic oxides/fluorides in the flux (Table 1) would eliminate more sulfur and phosphorus, decreasing the sulfur and phosphorus level in B D the deposited metals (Table 3). Since the maximum solubility of oxygen in nickel at 1573 K is 200 ppm (Ref. 26), the oxygen in the deposited metals basically existed in the forms of oxides. In the flux, the deoxidizer (i.e., 45% Si + 55% Fe, 28% Ti + 72% Fe, Mn and 50% Al + 50% Mg, as listed in Table 1) predominantly reacts with oxygen to form oxides in the metallurgical processing. These oxides could float in the weld pool and gather together to form slag. Due to fast solidification of the welding pool melt, there were some residual oxides retained in the weld metal, which were the main source of the oxygen in the deposited metal. Previously, the acidic oxides (e.g., FeO, SiO2, MnO2, TiO2, and Al2O3 are generally the products of the deoxidation reactions) were found to increase oxygen in the deposited metals, but the basic oxide/fluoride (e.g., CaF2 and CaO) decreased oxygen (Refs. 8, 22, 27). Those findings are well consistent with the results of this study, i.e., the CaF2-CaO-SiO2 type flux with higher contents of CaF2 and CaO maintained relatively lower oxygen concentration, while the TiO2- SiO2-SrO type flux with more TiO2 and SiO2 generated a relatively higher level of oxygen (Table 3). The C in the deposited metal was mainly transferred from the core wire and the decomposition of the carbonates in the flux coating during SMAW. According to the previous research (Ref. 11), the increase of carbonates in the flux coating promoted carburetion in the deposited metal. It is obvious that the higher contents of CaCO3 in the CaF2-CaO-SiO2 type flux coating generated a higher C content in the deposited metal (Table 3), but the lower contents of carbonates (i.e., CaCO3 and SrCO3) in the TiO2-SiO2-SrO type flux coating resulted in a lower C content (Table 3). Microstructure Characterization As expected, the as-solidified weld metals exhibited dendritic morphologies that were composed of facecentered cubic-structured nickel solid solution along with some small interdendritic precipitates and grain boundary phases. Under an optical microscope, there was no obvious difference between the deposited metals of the two types of electrodes — Fig. 7. To reveal the distribution of the main alloying metals, some EDS line scanning profiles across dendrites and grain boundary were recorded. It was found that the Fe and Cr preferred to aggregate in the dendrite core, but the Mo and Nb tended to accumulate in the interdendritic zone — Fig. 8. On the grain boundaries, Nb and Mo were obviously aggregated, but Ni and Cr were reasonably depleted — Fig. 9. The equilibrium distribution coefficient, k, which was defined as the ratio of the concentration of the element in the dendrite core to the nominal composition of this element, could indicate segregation behavior. The k values of Fe and Cr were larger than one, suggesting that they aggregate in the dendrite core, while the values of Mo, Mn, and Nb were less than one, indicating that they aggregate in the interdendritic zone. Special emphasis should be put on the Nb, of which the k value WELDING RESEARCH 474-s WELDING JOURNAL / DECEMBER 2016, VOL. 95
Welding Journal | December 2016
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