Introduction
Electroslag welding is not an arc process, as the arc is only used to start the welding process. ESW is the acronym for Electro Slag Welding.
In electroslag welding, a molten slag (temperature of approximately 1700°C) melts the filler metal and the base metal. The slag bath formed superimposes the molten pool, protecting it during welding.
The process begins by opening the electric arc between the electrode and an appendage placed at the base of the joint. Grainy flux is added and melted by the heat of the arc. When a layer of slag forms, all arc action ceases, and the welding current passes from the electrode to the base metal through the slag by electrical conduit.
How the ESW process works
The resistance of the molten slag to the passage of the welding current is sufficient to generate the heat necessary for welding (joule effect), which is sufficient to melt the electrode and the bevel faces. The molten electrode (and guide tube, if used) and molten base metal form the weld below the molten slag bath. The following figure schematically demonstrates this process.
Welding Equipment
The equipment used in electroslag welding consists of the following components:- Power source.
- Arame feeder and oscillator
- Guide tube and electrode
- Displacer (in case the guide is not consumable)
- Retaining shoe (mounting shoe)
- Control system.
- Cables and electrical connection
- Insulators
Power sources for the electroslag welding process are of the constant voltage transformer / rectifier type, which operate in the range of 450 to 1000 A. They are similar to those used in the submerged arc welding process. The minimum open circuit voltage of the power source must be 60 V. A separate power source is required for each electrode.
The figure below schematically shows a typical electroslag welding installation.
Welding current and electrode feed rate can be treated as a single variable because one varies depending on the other. If the electrode feed speed is increased, the welding current and deposition rate are also increased. As the welding current is increased, the depth of the weld pool is also increased.
Welding voltage is another variable that needs to be taken into account. Stress has a major effect on the depth of fusion in the base metal and also on the operating stability of the process.
Increasing the stress increases the depth of fusion and the width of the weld pool and also increases the form factor (width/depth ratio) and, as a result, the possibility of cracking is lower.
If the voltage is low, a short circuit can occur between the electrode and the weld pool. If the voltage is too high, weld spatter or arcing may occur on top of the molten slag.
Types and functions of consumables – electrodes and fluxes
The filler material must have a similar chemical composition to the base material; eventually, addition materials with different characteristics can be used, but it is preferable that the chemical compositions are compatible, because if a thermal treatment is necessary, the adequacy between this, the base material and the cast material will be guaranteed.The consumables used in electroslag welding are solid wire, accompanied by flux, and tubular wire, when alloying elements are added.
The composition of the stream is also important, as it determines the smooth operation of the process. The fluxes can be made from various materials such as complex oxides of silicon, manganese, titanium, calcium, magnesium and aluminum. The special characteristics desired for the solder are achieved by changing or varying the composition of the flux.
The normal functions of flows are:
- Conduction of welding current.
- Supply heat to melt the electrode and base metal.
- Enables stable operation.
- Protection of molten metal from the atmosphere.
Features and uses
The electroslag welding process has limited application, it is used to make vertical welds in medium thicknesses of carbon, low alloy, high strength, medium carbon, and some stainless steels. The process is best applied to thicknesses above 200mm, for maximum thicknesses there are practically no limitations. Although manual skill is not required, knowledge of technique is necessary to operate the equipment.The electroslag welding process has many applications, mainly due to the high deposition rates, making the process economically viable. Among the advantages of using the electroslag process we can mention:
- High deposition rate and good weld quality with respect to non-destructive testing makes this process desirable for thick sections found in numerous industrial applications such as heavy machinery, pressure vessels, ships and large castings.
- Requires little adjustment and joint preparation (usually joints without chamfer).
- Welds thick materials in one pass, with a single adjustment.
- It is a mechanized process with minimal material handling. Once the process starts, it continues to completion.
- Requires minimal soldering time and has minimal distortion.
- There is no visible welding arc and no arc flash.
The fragility of the weld thus obtained needs, to be corrected, a post-welding heat treatment – normalization. Of course, this is a generalization because the engineer must assess this need after testing and qualifying the welding procedure.
Process-induced discontinuities
Welds made with the electroslag welding process under proper operating conditions are of high quality and free of discontinuities. Discontinuities may appear, however, if a proper welding procedure is not followed.The welding inspector should be aware of some discontinuities that may result from this process are:
Lack of Fusion
Thick plate welds, in which heat is distributed by electrode oscillation, may lack fusion in the center or near the shoes.The cooling effect of the shoes can prevent the base metal from melting close to the surface the shoe is resting on.
The resulting indication resembles a bite.
They can also occur at the beginning of welding with a temperature below the necessary one.
Inclusions
They are uncommon, but they can happen. This is the case where pieces of wire are introduced into the puddle too quickly by the wire feed unit and do not melt.Rods and even parts of welding equipment, such as the end of the electrode tube guide, have also been found in the molten zone.
Slag Inclusions
Can occur if welding is nearly stopped and restarted. The welding process requires a slag pool heated to approximately 1700°C. Improper welding restart may not fuse the metal perfectly, resulting in weld slag.Overlap
It can occur if the shoes are not well adjusted to the plates, allowing the leakage of molten material.Porosity
When it occurs, it is coarse and worm-like, and may be caused by a piece of wet asbestos used as a seal between the retaining shoe and the part to be welded, contaminated or wet flux, electrode, guide tube or material to start wet welding.Special cracks
It has not been observed in the electroslag welding of butt joints because stresses are not recorded in the direction of the thickness of the base metal plates.Cracks
Cold cracks are not found in electroslag welding. This is due to the slow process of heating and cooling the joint inherent in the process. Hot cracks are common in electroslag welding, especially in the case of welds with a high degree of restriction, due to the coarse granulation of the welded joint. These cracks propagate along the grain boundaries.Laminations
They do not constitute major inconveniences for electroslag welding. The molten slag draws out any existing inclusions in the double lamination and seals the double lamination along the weld. Similarly, chips and bends are absorbed by electroslag welding.Learn Welding
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