Flux Cored Arc Welding (fcaw)
Electro-slag welding is most commonly used to weld very thick components or plates (up to 40 to 500 mm thick) where the weld junction is vertical.
The welded components are positioned vertically with the needed distance between the butted edges, and a backing plate is tacked to the bottom.
Copper shoes that are water-cooled and can travel along the joint are originally placed in the lowest position.
These shoes fill the gap between the weldable components, forming a V-shaped starting block that keeps slag and molten metal from leaking out of the pool.
An arc is formed between the tip of the consumable electrodes and the bottom plate to begin the welding operation, and when granular flux is introduced into the joint, a 3-4 mm thick layer of molten slag begins to float at the top of the weld metal pool.
The arc is extinguished as the molten slag approaches the electrode’s tip, and current is transmitted straight from the electrode wire to the base metal through the conductive slag.
The high electrical resistance of the slag then generates the majority of the welding heating, i.e. melting the wire electrode and the workpiece metal.
Single or multiple electrode wires, coupled with flux, are constantly supplied into the molten slag pool enclosed between the copper shoes as the welding advances.
Because slag is lighter than molten metal, it rises to the top and protects the weld metal pool.
The shoes are designed to travel upwards along the joint at a rate regulated by the melting rate of the electrode and work material at the joint.
As heat is transported away by the copper shoes and the work material, the lower half of the weld-metal bath solidifies.
Welding is done at 40-50 V, with current requirements of 500-600 amps, while larger currents are employed for very thick plates.
Advantages
- Welding thick metals in a single pass is possible.
- There is almost no cooperative preparation necessary.
- Because the heating is homogeneous, the stress distribution across the weld is good, and deformation is minimal.
- The weld quality is good because the weld metal is kept clean at all times.
- The weld speed is excellent, ranging from 15 to 40 mm/min.
- The process is nearly automatic; once started, it continues until the work is finished.
Limitations
- The scale obstructs heat conduction to the copper shoes and can hinder a good fit-up of the joint, resulting in some weld metal running out.
- Welding plates with a thickness of less than 20 mm is uneconomical.
Applications
- For welding plates up to 1 metre thick in one pass.
- Large structural steel components, such as heavy machinery and nuclear reactor vessels, require welding.
- Hot-rolled carbon steels, low-alloy steels, and quenched-and-tempered steels are all suitable for welding.
