Electroslag Welding Operation with Equipment Diagram

Electroslag welding (ESW) is a highly productive, single-pass welding process for thick Sections, typically greater than 25 mm to 760mm, in a vertical or near-vertical position. Electroslag welding (ESW) does not actually qualify as arc welding, considering an arc is only utilised to initiate the process. It is commonly used to make high-strength welds with high integrity in low-carbon alloy steels.

What is Electroslag Welding? Principle of Operation

The electroslag process resembles submerged arc welding (SAW) since both utilise slag for protection. However, SAW constantly generates heat from an electric arc, whereas ESW only initially utilises an arc to initiate the process, after that producing heat via the electrical resistance of the slag bath itself. In both methods, the molten slag refines the weld and protects it from atmospheric exposure.

In ESW, the arc is initially struck between an electrode and the bottom of the joint, as in traditional arc welding processes. Before welding, welding flux is filled in the gap between the two workpieces. The arc then melts a tubular flux to generate a conductive slag floating above the arc. In the electroslag welding of steel, the slag bath heats up to an approximate temperature of 1930°C. Once there is adequate slag, the arc is extinguished. The molten slag pool then generates the necessary heat to melt the electrode and workpieces while protecting the weld from contamination. The heat from the electric current keeps the poorly conductive slag in a molten state. Wire feeders feed metal as the electrode melts into the advancing slag pool, which ultimately joins the workpieces. An oscillator moves the electrodes across the joint to ensure an even distribution of filler metal for a completed joint.

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A persistent slag pool is maintained via two water-cooled copper retaining shoes on the joint faces that travel along the weld. Multiple solid or flux-cored wires may be utilised to accelerate the welding of thick plates. Once finished, surplus slag can be taken off from the finished joint.

Equipment of Electroslag Welding

The equipment for ESW includes a welding head, DC power supply unit, One or more electrode wires and guide tubes, flux feed system, One or more wire feeders and oscillators, Copper retaining shoes (or moulds)

DC Power Supply Unit: A prevalent choice for power in welding is the Constant Arc Voltage (CAV) power source, typically ranging from 950 to 1050 A at a 100% duty cycle.

Electrode/Filler Metal: The filler metal, melting at the weld joint, plays a crucial role in uniting the workpieces. Electrode diameter usually hovers around 3.0 mm, with nearly 100% efficiency in metal transfer. For a 3.0 mm diameter, the recommended electrode speed is between 20 and 160 mm/s. Electrode oscillation is activated only when the plate thickness exceeds 80 mm.

Electrode Guide Tube: To ensure precise positioning during welding, the electrode guide tube is employed to direct the electrode wire to the desired location.

Flux feed system supplies a metered flow of flux to the joint: The flux feed system ensures a regulated supply of flux to the joint. Flux converting electrical energy into heat is essential for the formation of molten slag. Utilising flux in granular form serves as the primary source for generating the slag.

Copper Retaining Shoes (or Molds): Employed on both sides of the weld joint, water-cooled copper retaining shoes effectively confine the molten liquid metal and slag. These shoes, designed for vertical movement, remain connected to the welding machine, either manually operated by the welder or automatically. Crafted from high-conductivity copper, the shoes contribute to efficient heat dissipation. The copper guide tubes, featuring high-strength Cu-2% Be alloy, boast a diameter of approximately 13.0 mm.

Electroslag Welding Process

The ESW process involves the following procedural steps:

The process begins with introducing flux into the cavity, and the power supply is started. When the filler wire makes contact with the base plate through the flux, an arc is triggered by a short circuit between the filler electrode and the base plate. The heat generated by this arc is adequate to melt the flux. The welding process progresses upward once the arc is established and the flux is molten.

Additional flux is introduced from above, initiating heat generation through the electrical resistance posed by the molten flux. This process resembles resistance welding. The cooled copper shoe solidifies the filler, solidifying the filler metal within the weld cavity and preventing molten weld metal's escape beyond the designated area. This facilitates the fusion of the workpiece walls and the consumable wire, ultimately forming a weld between the two workpieces. The weld is a product of the molten filler wire, molten flux, and the fusion of the workpiece walls.

After the filler metal undergoes solidification, the current continues to flow through it, generating heat through electrical resistance. This heat is effectively utilised to sustain the ongoing melting of the filler metal within the weld cavity, effectively regenerating heat and minimising energy wastage.  A roller arrangement ensures a continuous supply of filler metal. Throughout the welding process, the upward movement of the copper shoe and the feed mechanism continue until the entire weld cavity is formed.

Electroslag weld-metal is deposited in a single pass, with temperatures significantly lower than those observed in traditional arc welding processes. In conventional arc welding processes, a weld of comparable thickness is deposited using a pointed heat source with a much higher temperature and in multiple passes. The electroslag process allows the weld metal to remain molten for a sufficiently prolonged period, enabling slag-refining action that releases dissolved gases and transfers non-metallic inclusions to the slag bath.

Applications of Electroslag Welding

• Due to its high deposition rates and deep penetration capabilities, ESW sees heavy usage in the following applications:
• Welding of heavy-duty structural components in the construction of bridges and infrastructure projects.
• Electroslag welding (ESW) is widely applied in the construction of boiler components for thermal plants and in heavy construction projects.
• Thick steel plate welding.
• Fabrication of pressure vessels and boilers.
• Suitable for fabrication of thick plates for ship hulls, storage tanks, and bridges.
• The process lends itself well to automated and mechanised operation. When performed manually, it requires specially skilled welders. High-quality, reproducible welds can be obtained via ESW with the right techniques and joint preparation.

Advantages And Disadvantages of Electroslag Welding

Here is a summary of the key advantages and disadvantages of electroslag welding:

Advantages of Electroslag Welding

• High deposition rates - can weld thicker sections in a single pass than other welding processes.
• The weld undergoes a very slow cooling; thus, there is no chance of cold cracking.
• There is no possibility of slag inclusions and porosities.
• This welding process is characterised by its safety and cleanliness, with no occurrence of arc flashing or sparking. It produces minimal spatter and ensures a clean deposit. 
• Butt joints formed using ESW exhibit no angular deformation.
• There is minimal transverse shrinkage.
• Minimum slag: slag deposition is less than 5% in electroslag welding.
• Deep weld penetration even without joint preparation, Low cost for joint preparation.
• Low hydrogen weld deposits reduce the risk of hydrogen embrittlement
• Slag protects molten metal, preventing porosity and oxidation defects.
• The process is easily adaptable to mechanisation, reducing the demand for highly skilled manual welders.

• The use of electroslag welding (ESW) is restricted to vertical welding positions due to the presence of substantial molten metal pools and slag.
• It is not a suitable method for joining thinner materials.
• The resultant weld often exhibits low toughness.
• The slow cooling rate in ESW may result in a coarse grain structure forming within the weld, consequently leading to decreased toughness.
• Managing the high temperatures associated with ESW might require additional cooling measures to ensure the attainment of high-quality welds.
• Hot cracks at the centre of the welding joint interface are a potential concern.
• Compared to Submerged Arc Welding (SAW), ESW is less flexible.