A B Fig. 4 — Weight fraction of Mg (A) and Al (B) in the halite phase vs. temperature during solidification of the weld metal, calculated using ThermoCalc. Fig. 5 — SEM micrograph of an Mg- and Al-rich inclusion in weld metal. Thermodyamic Analysis The formation of inclusions in the weld pool was examined by considering the thermodynamic stability of various oxides using ThermoCalc, considering the actual weld metal chemistry and the assumptions for a Scheil solidification plot. The results indicated that the equilibrium phases during solidification of the steel first involves an ionic phase, labeled as ‘Halite’, along with ferrite (BCC_A1) and retained austenite (FCC_A1#1). The halite phase consists mainly of MgO, and is included in the ThermoCalc TCFE6 database within the Fe-Al-Ca-Cr-Mg-Mn- Ni-Si-Ti-C-O system, with possible substitution of Mg for other elements permitted. In the present work, halite begins to precipitate in the melt at just over 2300°C and is initially aluminum rich, and then devoid of aluminum at lower temperatures <1600°C, and this is followed by solidification of ferrite. The calculated content of magnesium and aluminum in the halite during solidification is shown in Fig. 4, with a balance of oxygen, suggesting it would have (Mg,Al)O chemistry. It should be noted that when magnesium is not included in the chemistry, the calculations suggest that Si2O4-Al6O9 phase would solidify first in the melt, followed by MnO-Al2O3, and the halite phase is not formed. Inclusion Analysis Spherical oxide inclusions could be observed in the steel, and measurements indicated they have an average size of 311 ± 120 nm (n = 69). These fine inclusions were found to contain aluminum and magnesium — Fig. 5. Auger electron spectroscopy (AES) was used to map the elemental distribution in these oxides, and a core/shell structure can be observed containing mainly Al oxide in the core, and a shell with Mg oxide — Fig. 6. These observations support the thermodynamic calculations in Fig. 4, which suggest that the inclusions are initially nucleated with a core that is rich in magnesium, aluminum, and oxygen, and then following growth, the outer shell only contains magnesium and oxygen. Some prior research has also shown that halite particles with a MgO stoichiometry are predicted by Thermo- Calc in steels containing low oxygen content and trace amounts of Mg (Ref. 31) However, to the authors’ knowledge, such Mg-Al-O-rich inclusions have never been reported in weld metal and no correlation could be observed between nucleation of ferrite phases and these inclusions. The carbides in the steel were also extracted by dissolving the weld metal in a mixture of HCl and HNO3 acid. The dissolved solution was screened through filter paper in order to capture the solid particles. XRD analysis was used to determine the solid phases recovered following dissolution and filtering. The XRD peaks observed in the residue recovered were identified as ZrC carbide (Ref. 32), and the particles were extracted from the filter paper onto double-sided copper tape for SEM microscopy. This residue is shown in Fig. 7, and consisted mainly of cuboidal particles; however, a small fraction of spherical particles could also be observed, which may correspond with the oxide in- JANUARY 2014, VOL. 93 18-s WELDING RESEARCH Table 3 — Instrumented Charpy Impact Testing Measurements Weld Test Temperature, Dynamic Fracture Total Region °C Toughness J1d, kJ/m2 Energy, J Top 20 246 94 Top –18 245 87 Top –62 280 67 Bottom 20 303 137 Bottom –18 279 139 Bottom –62 294 118
Welding Journal | January 2014
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