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 to be welded. It’s an arc less process that utilizes
resistance heating of the slag pool covering the molten steel as the weld’s
heat source.
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 joint and contain the molten slag and metal
during the weld.
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 degree Celsius at the surface and 1930 degree Celsius inside
under the surface. This much heat is sufficient to fuse the edges of the work
pieces and the welding electrode.
As the wire
and guide tube are melted by the flux 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 continuing addition of weld wire the molten steel fills the gap,
solidifies and fuses the two components. The weld is terminated when it reaches
the top of the run-out cooling shoes above the rail running surface.
Unnecessary weld reinforcement is removed immediately while the weld is hot.
A DC current
of 750 – 1000 A is applied from a DC generator with flat volt-ampere. Load
voltages generally range from 30 to 55 V, therefore the minimum open circuit
voltage of the power source should be 60 V. Speed range of Electro slag welding
are 17 to 150 mm/s.
History:
Single pass
welding of heavy plates are desired one to avoid multi pass welding techniques.
In the early 1950’s Russian scientists announced the single pass vertical welds
by the principle of electrically conductive slag. In 1959 Electro slag welding
was introduced in United states.
Applications:
- ESW is often used to weld stiffeners’ 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:
- 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 distortion produced are low
- Flux composition as
compared to submerged arc welding (SAW) is very low.
Disadvantages:
- However, 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.
- Electro slag welding is restricted to vertical position welding because of the very large pools of the molten metal 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.
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