The notched specimens were prepared by placing the V-notch on weld metal and base metal to evaluate notched tensile properties. All the tensile tests were conducted in 100 kN, electromechanical controlled universal testing machine (Make: INSTRON LIMITED, England; Model: INSTRON-8862) as per ASTM E8M-04 guidelines. An average of three readings was reported. Charpy Impact Testing Subsize Charpy impact specimens, due to smaller plate thickness, were prepared as shown in Fig. 3C to evaluate the impact toughness of the weld metal. Impact testing was performed at room temperature using a pendulum type impact testing machine as per ASTM E23-07 and an average of four readings was reported. Scanning Electron Microscopy The fractured surfaces of impact tested specimens were examined under scanning electron microscope (SEM) (Make: JEOL Ltd., Japan; Model: LSM-6360) to understand the micromechanism in fracture. Results and Discussion Effect of CO2 Content on Variation of Weld Profile Shielding gas compositions offer different physical and chemical properties, such as thermal conductivity, ionization energy, and chemical activity, which affect the arc behavior, and consequently the weld bead profiles as presented in Table 3. It can be observed from Table 3 that the bead height and toe angle decrease and bead width increases with an increase in CO2 content. The reinforcement height, width, and toe angle are significantly influenced by the Marangoni force (Refs. 14, 15) due to more negative surface tension temperature gradient (dg/dT) with an increase in oxygen potential (OP) of the shielding gas mixtures (Ref. 9). Oxygen potential of the shielding gas mixtures can be derived using the equation OP = O2 + μCO2, where μ is the oxidizing factor and taken as 0.7 as per existing literature B Fig. 8 — Optical micrograph of welds. A — J1; B — J2; C — J3; D — J4 shows different morphologies of g phase. (Ref. 16). Therefore, an increase in CO2 content increases the OP and governs the formation of extremely volatile oxide films, which have lower surface tension compared to the base metal (Refs. 9, 17). Hence, more negative dg/dT pushes the molten fluid in the outward direction (away from the weld pool center) as schematically shown in Fig. 4A, which ultimately leads to lower bead height and wider bead width. This is also the possible reason why lower toe angles are obtained with higher CO2 content. Again, the arc force was found to be related to the arc length and defined by the following equation (Ref. 26): (8) where Farc is the arc force, I is the mean current, and larc is the arc length. It is generally accepted that the thinner isothermal distribution, lower thermal conductivity with less heat flow associated with the pure Ar due to higher ionization potential when compared to binary mixtures, leads to a higher arc length and consequently to a lower arc force (Refs. 9, 17). On the contrary, due to higher heat flow associated with gas mixtures containing higher CO2, the radial distribution of the arc temperature is more uniform and its length should be shorter for the same heat intensity (Ref. 17). Hence, mixtures with higher amounts of CO2 will lead to higher arc force and consequently deeper lateral penetration. Prediction of Weld Metal Microstructure The DL% is calculated from the geometrical characteristics of the welded joint as schematically shown in Fig. 2. The estimation of AWD, ABF, and DL% are F I l arc –5 arc = 3.57×10 × 2 1/2 WELDING RESEARCH APRIL 2015 / WELDING JOURNAL 105-s Table 4 — Dilution Calculations for Different Welds Sample Specification ABF (mm2) AWD (mm2) DL (%) J1 22.43 ±5.0 48.5 ±8.0 46.24 ±3.0 J2 16.65 ±6.4 35.46 ±9.0 46.95 ±4.8 J3 17.33 ±6.6 36.62 ±8.4 47.32 ±5.8 J4 21.27 ±7.4 44.81 ±8.2 47.46 ±6.6 A C D
Welding Journal | April 2015
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