duced. Due to this reason, some refractory elements, e.g., W, Mo, and Ni, exhibited relatively lower mass transfer coefficients — Fig. 5. In general, the basic oxides (such as CaO and MnO) in the slag preferred to react with the acidic oxides (such as SiO2, TiO2, and Nb2O5) during welding to form complexes (Refs. 12, 13, 17, 18, 20, 25): (CaO) + (SiO2) (or TiO2) = (CaO•SiO2) (or CaO•TiO2) (8) (MnO2) + (SiO2) (or TiO2) = (MnO•SiO2) (or MnO•TO2) (9) For strong basic flux coating, the strong basic oxides were dominant, which consumed more acidic oxides (such as SiO2, TiO2, and Nb2O5) during welding. This tendency drove the following oxidation reaction to the right, and thus consumed more Si and Nb in the deposited metal: M + xO = (MOx) (M = Si, Nb) (10) This is why the mass transfer coefficients of Si and Nb in the CaF2-CaOSiO2 type are smaller than in the TiO2- SiO2-SrO type. For the TiO2-SiO2-SrO type flux coating, the strong acidic oxides were dominant, which consumed more basic oxides (such as CaO, MnO, and FeO) during welding. This drove the following oxidation reaction and led to smaller mass transfer coefficients of these metals — Fig. 5. M + O = (MO) (M = Mn, Fe, etc.) (11) According to the tested results in Fig. 5 and the above discussions, one can immediately conclude that the metals have a larger mass transfer coefficient in the strong basic slag system WELDING RESEARCH if they can form basic oxides (e.g., Mn, Fe). In contrary, the elements have a smaller mass transfer coefficient in the strong basic slag system if they can form acidic oxides (e.g., Si, Nb). In addition, the values of S, P, and O in the deposited metals exhibited similar behaviors, i.e., they all were in a relatively lower level in the CaF2- CaO-SiO2 type, while in a relatively higher level in the TiO2-SiO2-SrO type (Table 3). However, each has itself a mechanism. The desulfurization and dephosphorization during welding were realized by adding Mn and basic oxides/fluorides in the flux. The metallurgical processing involves the following main chemical reactions (Refs. 12, 13, 22): NiS + Mn = (MnS) + Ni (12) DECEMBER 2016 / WELDING JOURNAL 473-s Fig. 7 — Optical micrographs of the deposited metals: A and C — CaF2CaOSiO2 type; B and D — TiO2CaF2SiO2 type. A C B D
Welding Journal | December 2016
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