A C flux coating could be evaluated by the following equation (Refs. 17, 18) that has been widely used in evaluating the basicity index of flux coating. B.I. = CaF2 + CaO + MgO + BaO + SrO + Na2O + K2O + 0.5 (MnO + FeO) /SiO2 + 0.5 (Al2O3 + TiO2 + ZrO2) (6) where each chemical composition of the slag is expressed in weight percent. When the B.I. for a given flux coating is less than 1.0, the flux coating is regarded as acidic. When the B.I. is between 1.0 and 1.2, the flux coating is considered as neutral. A B.I. greater than 1.2 is regarded as basic. The calculated basicity of CaF2-CaO-SiO2 type is 4.25, which is strong basic. The basicity of TiO2-SiO2-SrO type is 1.59, which is alkalescent. Generally, the higher the basicity, the cleaner the weld metal (i.e., oxygen and other inclusions could be controlled in a low level). However, the slag with the lower basicity, especially containing TiO2, has good detachability and excellent welding processing property (Refs. 13, 18, 20). The weld pad was prepared for the chemical composition analysis according to AWS A5.11/A5.11M. The chemical composition of the deposited metal was analyzed from the undiluted weld metal. The C and S in the deposited metals were measured by using the high-frequency infrared ray carbon sulfur analyzer (CS-206, Baoying Photoelectric Technology Co., Ltd., Shanghai). The O was measured by a LECO TCH-600 analyzer, and the other elements were analyzed by using a Thermo Scientific Inductively Coupled Plasma Emission Spectrometer (iCAP 6300 ICP, Thermo Fisher Scientific, Inc., Waltham, Mass.). The analyzed results of the chemical compositions of the deposited metals are given in Table 3. The specimens for the microstructure observation were prepared by lightly grinding and polishing using diamond pastes of 5, 2.5, and 1 m. The final polishing was done with 0.5- m colloidal silica. Then electrolytic etching was performed with a 7% aqueous chromic acid solution at 0.2–0.22 A (DC) for 30–40 s. The macroscopic morphologies of the welding joint were observed by the stereomicroscope (Olympus SZ61). The microstructure was observed by using JSM-7600F SEM equipped with an energy dispersive x-ray spectrometer (EDS) at 15 kV. The chemical composition of the subgrain structures and the precipitations were analyzed by EDS. The appearance of the transverse weld perpendicular to the welding direction is shown in Fig. 1C. The picture showed the deposited metal without any defects. The specimens for the tensile and impact tests were machined from the deposited metal according to the AWS A5.11/A5.11M standard. All-weld metal tensile specimens were prepared according to ASTM E8. The tensile tests were carried out by using a Zwick/Roell-Z100 testing machine (Zwick GmbH & Co. KG, Ulm) at room temperature. Three Charpy V-Notch impact test specimens with dimensions of 55 10 10 mm were prepared according to ASTM E23, on which a V-type 45-deg notch with a 2 mm depth and root radius of 0.25 mm at the center of the specimen was machined. The impact tests were conducted by using a pendulum impact testing machine (PTM2200-D1, Suns Co., Ltd. Stock Technology, Shenzhen) at the liquid nitrogen temperature. Results and Discussion Assessment of the Weldability and Weld Quality The weldability and weld bead quality, including arc stability, deposition rate, spattering, fumes, slag detachability, penetration, and bead geometry, are especially influenced by the flux composition of the covered electrodes. Considering that clean weld metals (i.e., ultra low hydrogen, low oxygen, low sulphur, and phosphorus) are extremely important for the cryogenic application, an alkalescent TiO2-SiO2-SrO type and a strong basic CaF2-CaO-SiO2 type slag WELDING RESEARCH 470-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 B D Fig. 3 — Macro morphologies of the welds with and without slag: A and B — CaF2CaOSiO2 type; C and D — TiO2CaF2SiO2 type. Table 3 — Chemical Compositions of the Deposited Metals for the NickelBased Alloy Covered Electrodes (wt%) Type Ni Cr Fe Mo Mn Nb W Si C S P O CaFCaOSiO 2 Bal. 14.10 5.15 3.24 3.30 1.58 0.67 0.51 0.05 0.0070 0.0057 0.0293 2 TiOSiO SrO Bal. 14.43 5.20 3.63 3.50 1.57 0.81 0.67 0.04 0.0081 0.0060 0.0359 22AWS A5.11 ENiCrFe9 ≥ 55 12–17 12 2.5–5.5 1.0–4.5 0.5–3.0 1.5 0.75 0.15 0.015 0.02 —
Welding Journal | December 2016
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