Electro Slag Welding | Flux Core Welding
Electro slag welding is a welding process of heavy plates in the vertical position. Wherein coalescence is produced by molten slag which melts the filler metal and the surfaces of the work. It’s an arc-less process that utilizes resistance heating of the slag pool covering the molten steel.
Electro Slag Welding Process
Parts to be joined are positioned approximately an inch apart, and an electrode (weld wire) guide tube is positioned between the parts. Copper cooling shoes are clamped to the sides, bottom, and top of the joints.
After the components are assembled, power is applied and the wire is fed through the guide tube. When the wire reaches the start block, there is momentary arcing which melts the granulated flux, forms the slag pool, and extinguishes the arc. The process is initiated by filling the joint with the flux and starting an arc by short-circuiting. The consumable guide tube directs the electrode (welding wire) and conducts the welding current to the molten slag pool. The electrical resistance of the slag pool generates heat which melts the wire, the guide tube, and the edges of the two components to be joined. The temperature obtained is approximately 1800 degrees Celsius at the surface and 1930 degrees Celsius under the surface. This much heat is sufficient to fuse the edges of the workpieces and the welding electrode.
Electro Slag Welding Gap
In ESW, wires and guide tubes are melted by the flux. Then the liquid metal sinks through the slag to the metal pool below and solidifies. Since the slag is less dense than liquid steel, it floats to the top and protects the metal from exposure to air. With the continuing addition of weld wire, the molten steel fills the gap, solidifies, and fuses the two components. The weld terminated when it reaches the top of the run-out cooling shoes above the rail running surface. Unnecessary weld reinforcements are removed immediately, while the weld is hot.
A DC current of 750–1000 A is applied from a DC generator with a flat volt-ampere. Load voltages generally range from 30 to 55 V, so the minimum open circuit voltage of the power source should be 60 V. The speed range of electro slag welding is 17 to 150 mm/s.
History of Electro slag welding:
Heavy plates require Single pass welding than multi-pass welding techniques. In the early 1950s, Russian scientists announced the single-pass vertical weld by the principle of electrically conductive slag. In 1959, electro slag welding was introduced in the United States.
Electro slag welding Applications:
- ESW is often used in structural box columns and wide flanges.
- Manufacture of large Presses and machine tools work with large heavy plates.
- Other machinery applications include kilns, gear blanks, motor frames, press frames, turbine rings, shrink rings, crusher bodies, rebuilding metal mill rolls and rims for road rollers
- Pressure vessels for the chemical, petroleum, marine, and power generating industries
Advantages of electro slag welding:
- Electro slag welding can have extremely high deposition rates, but only one single pass is required no matter how thick the workpiece is.
- Unlike SAW or other arc welding processes, there is no angular distortion in ESW because the weld is symmetrical with respect to its axis.
- High welding speed and good stress distribution across the weld.
- Joint preparation is often much simpler than other arc welding processes.
- Residual stresses and distortions produced are low.
- The flux composition as compared to submerged arc welding (SAW) is very low.
Disadvantages of electro slag welding:
- When the heat input is very high and the weld quality can be rather poor, including low toughness caused by the coarse grains in the fusion zone and the heat-affected zone.
- In Electro slag welding, there is some tendency toward hot cracking and notch sensitivity in the heat-affected zone.
- ESW is restricted to vertical position welding, because of large molten metal pools and slag.
- It is difficult to close cylindrical welds.
- Electro slag welding tends to produce large grain sizes.
- Submerged Arc Welding is more economical than electro slag welding for joints below 60 mm.