TAPS) to provide the electron flow with an amperage I1 and a constant current power (separated power source or SPS) to provide the separated electron flow I2. Their equivalences in Fig. 1 are power source A and B, respectively. The negative terminals of both power sources were connected together to the plasma torch. The positive terminal of the TAPS was connected to the base metal and that of the SPS was connected to the guidance terminal. In this way, the total current through the PAW torch was the sum of the currents: I = I1 + I2. Thus, the SPS controls the electron flow that was separated from the arc plasma. This is similar to the double-electrode GMAW (DE-GMAW) (Refs. 29–32) and the arcing wire GTAW (Refs. 33, 34), but the SPTAWS separated the electron flow from the stiff arc plasma while in the two comparative processes, the arc plasma was distributed and no electron flow was separated from the distributed arc plasma. Experimental Procedure Plasma transferred arc weld surfacing (PTAWS) is a standard technology used for hardfacing. Surfacing experiments were thus conducted using both the SPTAWS process and the PTAWS to compare. Grade D steel plates were used as the base metal with dimensions 250 ¥ 100 ¥ 10 mm. The high-chromium, iron-based alloy powders were used for cladding. The particle size was between 53 and 150 mm. The guidance terminal was replaced by a tungsten Fig. 13 — Bead cross section made using conventional PTAWS. Fig. 14 — Illustration of zones in base metal after surfacing. electrode. 99.99% pure argon was used as the shielding gas, plasma gas, and powder feeding gas. Their pressures were 800, 400, and 400 kPa, respectively. The plasma torch traveled at 0.08 m/min and oscillated with a fixed magnitude of 16 mm. The arc length was 15 mm. Results and Discussion Formation and Macro Appearance of the Deposited Bead The bead shown in Fig. 11 is the appearance of the deposited bead produced by conventional PTAWS, using 130 A. Figure 12 shows the SPTAWS deposited bead made using 11 = 110 A and 12 = 20 A, and the sum of currents is 130 A. Adjusting these two currents while maintaining their sum constant, the deposited bead appearance was not significantly changed because of the torch oscillation. Thus, the separation did not significantly control the deposited bead appearance, while the oscillation did. Macro metallurgical graphs of the bead cross section made using the conventional PTAWS are shown in Fig. 13. The base metal after surfacing can be divided into three zones as illustrated in Fig. 14, where Zones I and II belong to the heat-affected zone (HAZ) and Zone III is the unaffected base metal. In Zone I, the size of the grains were reduced WELDING RESEARCH when away from the weld interface experiencing a change from the overheated subzone where the grains were coarse toward the less heated subzone, where grains were finer. In Zone II, the annealing was incomplete and finer grains were mixed with unaffected base metal grains. It can be further observed from Fig. 13 that as the base metal heat input decreases, the thickness of Zones I and II are both reduced. In particular, for A with 150-A current, Zone I approximately extends over 70% of the base metal and Zone II reaches the bottom of the base metal. In B with 130 A current, Zone I approximately reaches 60% of the base metal and Zone II also reaches the bottom of the base metal, but the extent is less than that of A. When the current was further reduced to 110 A in C, Zones I and II only reached 25 and 35% of the base metal, respectively. This suggests that adjusting the heat input may provide an effective way to control the microstructure of the deposited beads. Unfortunately, for conventional PTAWS, the needed current was dictated by the needed melting rate of the powders and was thus not allowed to be freely adjusted to control the microstructures. In Fig. 13A–C, the deposition is apparently different. On the other hand, for the proposed SPTAWS, while the sum of the total currents can be determined JUNE 2016 / WELDING JOURNAL 225-s A C B
Welding Journal | June 2016
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