Electric slag welding (Electroslag welding) is a fundamentally new method of permanently joining metals. It has been developed and put to practical use by the Paton Electric Welding Institute of the Ukrainian Academy of Sciences in collaboration with the Engineering Works at Novo-Kramatorsk and the Krasny Kotelshchik Boiler Making Factory at Taganrog, both of which are leading plants in the field.

As distinct from other fusion welding methods, the electric slag process depends on the heat generated by the passage of an electric current from the welding rod (electrode) to the work piece through the molten pool of a high-resistance conductive flux, or slag. Hence its name electric slag process.

Submerged arc welding has proved less efficient on thicknesses over 50 or 60 mm than on lighter sections. This is because of the difficulty and, at limes, impossibility of making well-shaped welds with strong arcs in the down hand position in a single pass. Therefore, heavy-gauge plate has to be bevelled prior to welding and welded in many passes which is out of pace with modern heavy engineering practice.

Electric slag welding (Electroslag welding) is a big step forward, as this process, coupled with weld moulding, has rendered possible the single-pass welding of plate of practically unlimited thickness.

The electric slag process will inevitably bring about sweeping changes in the fabrication of large-size structures, unique machines, heavy presses, large shafts, etc. With electric slag welding, all-cast and forged parts may be replaced with cast-welded, forged -welded, and rolled-welded parts, thus reducing the burden on the foundries and forging shops of engineering works and stepping up production without having to expand floor space.

Electric slag welding (Electroslag welding) has opened up new possibilities for the manufacture of composite and compound metals, automatic hard-facing and repair.

In recent years, further headway has been made in the science and art of electric slag welding, which has found many applications in heavy engineering and many other industries.

With Soviet help, the electric slag process is now being employed in Czecho slovakia, the German Democratic Republic, Poland and People's China.

Equipment for electric slag welding evoked keen interest at the World Fair in Brussels in 1958, where the electric slag process won a Grand Prix, and at Soviet exhibitions in New York, Marseille and Helsinki in 1959. Licences for the process and equipment have been bought by the ESAB of Sweden.

The obvious advantages, both technical and economic, that the electric slag process possesses in comparison with other methods and processes of fusion welding of heavy sections have appealed to welding people in some Western countries. In 1959, equipment for electric slag welding went into production in Britain and West Germany.

Recently, new modifications of the process have been developed welding with large-size electrodes, welding with consumable electrode guides, and resistance slag welding. Electric slag processes have found applications in metal making for resmelting alloy steels and alloys, burning off risers and gates from odd- shaped castings, and repair of ingots.

Furthermore, new data have been obtained as to the heat and metallurgical phenomena accompanying electric slag welding, primary crystallization in the weld pool, and the strength of electric slag welds. Welding procedures have been developed for nigh-alloy steels and alloys, titanium alloys, non-ferrous metals, and cast iron, and put to commercial use. Electric slag welding techniques have been perfected to include straight butt-weld seams in sections up to two metres thick and circumferential seams on vessels and shafts with a wall thickness of up to 400 mm.

Electric slag welding (Electroslag welding) is a process in which welding heat is produced by the passage of an electric current through a pool of molten flux, or slag. The process is shown schematically in Fig. 1. The pool of molten slag 3 is formed between the edges of the parts to be welded 1 and the travelling moulding shoes 2. The metal welding rod or electrode 4 is dipped into the molten slag. Traversing the distance between the electrode and the parent metal, the current heats the molten slag, thus maintaining its elevated temperature and electric conductivity. The temperature of the slag pool should be above the melting point of both the parent and electrode metal. The slag melts the electrode and the edges. The molten parent metal and the electrode metal settle on the bottom of the slag pool to form a metal pool 5 which solidifies to give a weld 6. The electrode is fed into the welding zone as it melts.

The best conditions for the parent metal to melt and for a deep slag pool to be obtained exist when the joint runs vertically. For this reason, the weld pool has to be retained by moulding shoes. In the downhand position the electric slag process is less convenient and has not found any appreciable use.

The moulding of the weld consists in cooling the surface of the metal pool. Fig. 2 shows how the direction of heat abstraction affects the shape of the pool. Fig. 2a shows a hypothetical case with no heat transmitted through the unrestricted surface of the metal pool. In Fig. 2b, the metal pool is heated by molten slag, as in the submerged arc process. If the direction of heat abstraction is reversed, as in Fig. 2c, the pool will be dish-shaped. In this case the electric slag process can be applied to vertical joints. The heat is usually abstracted by copper travelling shoes or stationary back-up strips, which in turn are cooled by running water.

The main purpose of the slag in the electric slag process is to convert electric energy into heat. Accordingly, the principal properties of a slag are its electric conductivity and temperature coefficient of resistance.

Presuming we had a slag that would not change its conductivity with temperature, we would have no problems to tackle when employing it for welding purposes. After all, it is always possible to adjust the voltage impressed across a constant resistance so that the required amount of power is dissipated in the resistance and the desired temperature maintained. In practice, however, the conductivity of molten slag rises sharply with temperature, while below a certain point slag is non-conducting for all practical purposes. This complicates the stabilization of welding.

Some slags containing titanium dioxide are good conductors even when in a solid state at room temperature. Such slags show electron conductivity rather than the ion conductivity of molten slags.

As distinct from submerged arc welding, nearly all the electric power in the electric slag process is imparted to the slag pool which transfers it to the electrode and the parent metal. Therefore, if welding is to proceed steadily, the temperature of the slag pool must be constant, i. e., it must give as much heat as it receives.

The shape of the heat abstraction curve depends on the geometrical dimensions of the space in which welding takes place, notably on the surface area of the slag pool; rate of heat abstraction from the slag to the parent metal and to the moulding shoes; depth of the slag pool; molting of the parent metal; heat loss due to the evaporation of volatile components in the slag; amount of filler metal fed into the welding zone; and some other factors.

The factors affecting the shape of the heat input curve are the temperature coefficient of resistance of the slag; volt-ampere characteristic of the power sources depth of immersion of the electrode in the slag; and depth of the slag pool. If the two curves do not intersect, the electric slag process is not applicable at all (curve a). If the curves intersect at one point (curve 6), welding will not be steady; the temperature and power will decrease continuously to the left of the point of intersection (point A), meanwhile they will be constantly increasing to the right of the point. In case the two curves intersect at one more point (point B on the curve c), welding will be steady.


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