Observation of Phenomena in the Slag Bath during Electroslag Welding

In ESW process, welding phenomena and mechanism have not been understood enough. In this study, phenomena in the slag bath during ESW were observed by using X-ray transmission imaging system. By changing welding conditions, various droplet transfer mode similar to GMAW were observed in the slag bath. According to the temperature measurement result in the slag bath, upper and middle part was around 1700-1900 K that was near the melting point of iron, and bottom part was more than 2300 K. Therefore, it is presumed that most of welding current is concentrated at the bottom of the slag bath and spreads radially from the wire tip, and then the electromagnetic pinch force acts effective in the droplet detachment. In addition, in the bottom of slag bath, high-speed rotating convection was observed. It is estimated that penetration depth is governed by this convective heat transport. These observations indicate that wire tip position and temperature distribution in the slag bath are determined by the setting of welding conditions, then the welding current path in the slag bath changes by these acting mutually. It is suggested that droplet transfer mode and convection phenomenon are also determined by the current path in the slag bath. Then the effects of wire extension length and slag bath depth on wire melting rate were also investigated. In the case of the same wire extension length and the same welding current, the wire melting rate of ESW was lower than that of EGW. Furthermore, it was clarified that Joule heat at the wire extension and wire melting rate varied with the slag bath depth also, despite the same wire extension length. Therefore, the slag bath depth control method should be developed to expand the application of ESW process in the future.