WELDING RESEARCH merged arc welding, the main sources of oxygen are the decomposed flux constituents, which contaminate the droplet (Ref. 23). In the determination of oxygen contents, significant changes were detected between the observed processes in droplet and weld joint. The highest variations were measured between different settings of polarity (DCEP, DCEN, and AC). Figure 11 shows the transient amount of oxygen in the wire, droplet, and solid-weld joint stage within varying processes. As can be seen, the amount of oxygen rises in the droplet in all specimens. The value of oxygen content depends mainly on the polarity but also on the current. It is highest in DCEP compared to DCEN, and slightly increases with higher currents. After passing over into the weld pool and solidifying, the oxygen content drops to a lower level in all investigated processes. This can be explained by two separate effects. First, the droplet and the oxygen it contains are “dissolved” in the weld pool. Second, the thermochemical deoxidation effects in the weld pool reduce the oxygen content of the weld metal further. This happens through the oxidation of alloying elements with a high affinity to oxygen, like silicon or aluminum. These oxidized compounds are transferred from the weld metal through the slag-metal interface into the slag. However, the level of the final oxygen content in the solid weld joint seems to be determined by the oxygen content in the droplet stage. It is known that a lower oxygen content in weldments can be achieved by DCEN polarity due to electrochemical reactions (Refs. 13–15). Supposedly, this is one of the main reasons for the final amount of oxygen in SAW. Other investigations show that the final oxygen content depends on the weld solidification time as well (Ref. 23). Referring to the high-speed images, the oxygen content also seems to be influenced by the droplet-flux interaction and arc length, which are determined by the chosen polarity. As can be seen in Ref. 16 (SOM1) (DCEP) and described in an earlier subsection, the flux merges directly into the droplet. The molten droplet is situated in the upper part of the cavern and in direct contact with the cavern wall, respectively, Fig. 11 — Oxygen content in the wire, droplets, and weld joint. with the flux. At this point, the flux, consisting of a wide range of oxides, raises the oxygen content of the droplet due to its absorption. Within the DCEN process (Ref. 16, SOM6), the arc is much shorter and the droplet is closer to the weld pool. Therefore, the droplet transfer is not clearly visible most of the observed time. There is only a slight dropletflux interaction visible, as opposed to the DCEP process. This could lead to a lower amount of oxygen in the droplet due to less absorption of flux with its oxidic compounds. The AC process shows characteristics of both DCEP and DCEN in the highspeed images (Ref. 16, SOM8). It has a medium oxygen content in droplet and solid weld joint, which is between the DCEP and DCEN processes. The highest amount of oxygen was detected in the DCEP process with high current (I = 1000 A). Referring to the high-speed images (Ref. 16, SOM7) and the explanations in an earlier subsection, the flux is completely molten because of the high energy input and heat in the cavern. This leads to an increased reactivity of flux compounds, 498-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 and most likely an increased oxygen range in the cavern. These reactions could result in a higher oxygen content in the droplet. Conclusion In the presented investigations, a combined and synchronized method for high-speed imaging and spatialresolved spectroscopy in submerged arc welding (SAW) was introduced. It was shown that there is a minimum invasive influence on the process achieved by using this method. Based on the high-speed images, detailed descriptions of the process were made concerning the nature, behavior, and size of cavern, droplet, and arc in different polarities (DCEN, DCEP, and AC) and welding currents (600 and 1000 A) in submerged arc single-wire welding. Through observation of the physical state of the flux and slag inside the cavern, estimations of cavern temperatures could be made. Based on this and in combination with the results of the spectroscopy, the main components of the cavern atmosphere are
Welding Journal | December 2016
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