Studies on the Electroslag Welding (Report 1)

Penetration Mechanism of Base Metal and Influence of Polarity Effect

Abstract

The authors have investigated the penetration mechanism of base metal and the polarity effect in electroslag welding. The paper discusses the penetration mechanism of base metal considering the electric conductivity of molten slag, the temperature and current distribution in slag pool under the welding conditions shown in Table 1.
In general, the metal is deeply penetrated only at the lower part of slag pool as is shown in Fig. 1. The temperature distribution of slag pool is shown in Fig. 2. The temperature at the bottom of slag pool is considerably high, i.e., about 1700-2000°C. On the other hand, it is low, i.e., about 1400-1500°C at the upper part. The relation between the electric conductivity and the temperature of molten slag is shown in Fig. 3, where one notices that the electric conductivity remarkably increases as the temperature rises above 1400°C. The temperature distribution of slag pool (Fig. 2) and the characteristic of electric conductivity (Fig. 3) lead one to the presumption that the most part of welding current concentrates on the bottom of slag pool. This was experimentally verified as shown in Fig. 4. The current distribution shown in Fig. 5 causes a difference of electromagnetic pressure between the wire end and the bottom of slag pool, and consequently it generates the convection of molten slag. (Fig. 5)
The lower part of base metal, therefore, is effectively melted.
When direct current is used, the polarity effect on the penetration of base metal is as follows;
In straight polarity (wire cathode), the metal is penetrated not only at the lower part but also at the upper part of slag pool. In this case, the melting speed of wire is less than that in reverse polarity (wire anode). As to the melting speed of wire in R.P. and S.P., a model experiment was conducted as shown in Fig. 9 and it was found that the melting speed of R.P. was greater than that of S.P. It was experimentally proved that the potential difference at anode side was higher than that at cathode side as shown in Fig. 12.