based on the needed melting rate of powders to produce the desired deposited bead appearance, the heat input can be adjusted by the degree of the separation (CP-CG rate) to achieve the desired microstructures. As such, SPTAWS experiments were conducted with different degrees of separation, thus different heat inputs. Macro metallurgical graphs of the bead cross section made using the SPTAWS process are shown in Fig. 15. In all experiments, the total current flowing through the torch remained 130 A. The heat input of the base metal was controlled through CP-CG rate. Figure 15 clearly suggests that as the CG (electron flow separated) increased, Zones I and II both became thinner. In particular, for A without the separation, Zone I approximately extended to 60% of the base metal and Zone II reached the bottom of the base metal. In B with 20 A of separated electron flow, Zone I approximately extended to 50% of the base metal; Zone II also reached the bottom of the base metal but the extent was less than that in A. In C with 40 A of separated electron flow, Zone I apparently did not reach half of the base metal and Zone II apparently did not reach the bottom of the base metal, and only extended to 90% of the base metal. In the meantime, from the cross sections of all these deposited beads, the amounts of the melted powders are approximately the same. The same deposited bead appearance and powder melting were thus achieved using different heat inputs. Of course, the microstructures changed accordingly and the separation was determined to achieve the desired microstructures without affecting the deposited bead appearance and powder melting. Microstructure of the Bead Cross Section Figure 15 suggests that the separation with 40 A of electron flow produces the most desirable results for hard surfacing, which prefers minimal heat input. This bead cross section was thus compared with the one made using conventional PTAWS, i.e., the one given in Fig. 15A. To examine the micrographs, samples were cut, prepared, and etched following standard procedures for being examined by a confocal laser scanning microscope. The micrographs for the bead cross sections shown in Fig. 15A and C are given in Figs. 16 and 17, respectively. That is, Fig. 16 is the microstructure of the bead cross section made using the conventional PTAWS process with 130 A current and Fig. 17 is the microstructure with a separated electron flow of 40 A. In Figs. 16 and 17, the dark organization represents pearlite and the light represents ferrite (Ref. 35). The microstructure characteristics changed obviously from A to C when away from the weld interface. Comparison between Fig. 16A and Fig. 17A clearly suggests that the grain-size distribution in Fig. 17A by the proposed method is more uniform than that in Fig. 16A by the conventional method. The comparison between Figs. 16B and 17B and the comparison between Figs. 16C and 17C also show the same trend, especially in the fine-grain zone. The peak temperature was one of the key factors that controlled the microstructure of bead cross section, and the coarse grains formed because of high temperatures. The microstructure of bead cross section was also affected by the heating speed. As the heating speed increased, the temperature of Ac3 and Ac1 (which can be found in the iron carbon metal alloy equilibrium diagram) increased, the tendency of element migrating decreased, and the degree of homogenization decreased (Ref. 36). The grain-size distribution was nonuniform. As the separated electron flow was 40 A, the heat input decreased, and the microstructure of bead cross section was favorably modified. The microstructure of SPTAWS bead cross section was better than that of the conventional PTAWS bead cross section. Conclusions In this study, the property of arc separability was analyzed by measurement. It was verified that the pressure distribution of arc plasma was affected by the ratio of the CP and the CG (the electron flow separated from the arc plasma), arc length, and the plasma gas flow rate, but the ratio of the currents was the main factor and the effects from the arc length and the plasma gas flow rate were insignificant. The heat applied into the workpiece through the electron flow was less than that from the arc plasma. Based on the measurement and the property of the arc separability, the separated plasma transferred arc weld surfacing (SPTAWS) process is proposed and was tested on Grade D steel for surfacing. The influence of current on the macro profile and mi- WELDING RESEARCH 226-s WELDING JOURNAL / JUNE 2016, VOL. 95 Fig. 15 — Bead cross section made using SPTAWS. A C B
Welding Journal | June 2016
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