systems were established in this study. Through a large number of experimental selections, the flux ingredients of these two slag systems were optimized and fixed (as listed in Table 1), which ensured acceptable operationality (stable arc, small spattering and fumes, good slag detachability) and good weld bead quality (good appearance, no porosity, no cracks, appropriate weld geometry). Comparing the two slag systems, the CaF2-CaO-SiO2 type exhibited a larger deposition rate, smaller spatter loss, smaller fumes, and better fluidity of slag during welding, while the TiO2- SiO2-SrO type exhibited better slag detachability. The weldability of these two types of flux-coated electrodes was summarized in Table 4 for comparison. Figure 3 shows the appearance of the weld beads for the two types of electrodes. The slag of the CaF2-CaOSiO2 type was dark brown, but that of the TiO2-SiO2-SrO type was light Fig. 4 — Crosssectional images of the weld metals: A — Sketch map of the weld metal; B — CaF2CaOSiO2 brown — Fig. 3A and C. After removing the slag, both beads exhibited fine ripples on the surface of the welds. Some small burnton slags could be found — Fig. 3B. This indicated that the slag detachability of the CaF2-CaOSiO2 type flux was not as good as that of the TiO2-SiO2-SrO type flux — Fig. 3D. To evaluate the weld geometry, the two weld beads were sectioned — Fig. 4. It was evident that the height and penetration, as defined in Fig. 4A, of the weld bead of the TiO2-SiO2-SrO type were obviously larger than that of the CaF2-CaO-SiO2 type — Fig. 4B and C. The CaF2-CaO-SiO2 flux resulted in a wider weld bead because of its better fluidity of slag during welding (Table 4). Generally, the fluidity of slag depends on its viscosity at the welding temperature, which influences the bead profile and the weld quality via affecting the metallurgical reactions and metal transfer during welding (Ref. 21). The appropriate slag viscosity provides effective protection of the welding pool from atmospheric gases. In the flux ingredients, the titanates, WELDING RESEARCH type; C — TiO2CaF2SiO2 type. silicates, and other acidic oxides are commonly used to enhance the viscosity of slag. For example, the basic unit in the silica network is the silicate tetrahedron made up of a small silicon atom surrounded by four close-packed oxygen atoms. The two- or threedimensional network forms by the close connection between these SiO4– silicate tetrahedrons, which increases the viscosity of slag. On the other hand, the fluoride and the basic oxides (such as CaO) can decrease the viscosity of slag due to the breakdown of the network structure (Ref. 22). Because the atomic size of F– (1.33 Å) is similar to the O2– (1.36 Å), the F– replaces the O2– in the -O-Si-O- to form the -Si-Fstructure. The basic oxides break down the network stemming from the cations (such as the Ca2+ introduced by the CaO) that were inserted into a spatial structure created by the -Si-O- liaison, forming the -Si-O-Ca-O- structure. For the TiO2-SiO2-SrO type, there is a higher level of SiO2 and TiO2 that contribute to a higher viscosity of slag. The network structure built by the SiO2 (and/or TiO2) confines the fluidity of the weld pool and the transverse diffusion of heat, resulting in the deeper penetration and smaller bead width (Refs. 16, 17, 20–23). For the CaF2-CaO-SiO2 type, the higher level 13.60 13.06 3.97 1.39 100% Not good Not good DECEMBER 2016 / WELDING JOURNAL 471-s A C B Electrodes Table 4 — Comparison of Weldability for the Two Types of FluxCoated Type Melting Rate Deposition Rate Coefficient Spatter Loss Slag Fluidity of Slag Fumes (g /amph) (g /amph) of Loss Coefficient (%) Detachability (%) during Welding CaFCaOSiO 2 14.70 14.20 3.40 1.21 95% Better Better 2TiO2SiO 2SrO enough enough
Welding Journal | December 2016
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