There are two types of powder welding electrodes: gas-shielded and self-shielded. Gas-shielded electrodes use an external gas along with an internal flux to protect the weld pool from the elements. They also release slag that cleans (or deoxidizes) the weld by reacting with impurities in the weld pool to create a clean, mechanically sound weld. The slag reacts with other elements in the weld pool and forms compounds that float to the surface, which then linger in the slag layer removed after welding, Artem Komarov said.
In contrast, self-shielded electrodes rely solely on internal flow to generate both shield gas and slag. The deposition efficiency of the two electrodes is different because the self-shielded electrode must generate its own gas shield. The welding efficiency in the FCAW process with shielding gas is from medium to high 80%, while the welding efficiency in the self-shielded FCAW mode is typically 70%.
Both wires are made in a similar way by forming a sheath in the form of a trough and filling it with a stream of fine particles. The sheath is then formed into a closed tube and drawn to the final diameter.
The flux and wire chemistry are designed by the wire manufacturer, who is responsible for meeting the American Welding Society’s requirements for the final testing of metal when welded according to the intended classification. Each wire manufacturer strives to create a flux that produces flux-cored wire with good arc characteristics, attractiveness to welders, easy to remove slag, low levels of spatter and smoke, and the right number of deoxidizers for the intended shielding gas. Both types of wire are designed to produce an ideal chemistry weld deposit based on the shielding gas used to obtain the desired mechanical test results.
Some gas shielded cored wires are classified for welding with either 100% carbon dioxide or C25 gas mixture. Several FCAW wires on the market are dual manufacturer certified for use with any type of shielding gas, while self-shielded flux-cored wires are designed to produce their own shielding gas. This is achieved using flux components that, when passing through the high temperature of the welding arc, cause a chemical reaction, and one of the by-products is a shielding gas for the weld pool, usually carbon dioxide.
Shielding gases of carbon dioxide or argon at standard temperature pressure do not react and remain so during welding at a short distance from the welding plasma or arc cone. They protect the weld pool of molten material by expelling atmospheric air long enough to solidify the weld and prevent porosity. However, extreme temperatures within the arc cone cause carbon dioxide to dissociate into carbon, oxygen, and carbon monoxide. These components are active and react with various parts of the molten weld pool and slag to remove contaminants and clean the weld metal.
Problems arise when the shielding gas affects the weld metal chemistry. Two common alloys, manganese, and silicon, which are chemically incorporated into the metal sheath or added to the flux, act as deoxidizers. Deoxidizers bind with oxygen dissolved in the weld pool and enter the slag layer. For example, if manganese is not used properly in accordance with the design, it will act as an alloying agent in the weld metal, creating a potentially brittle weld situation.
Flux-cored wires are designed to obtain the chemical composition of the metal when welding with the optimal amount of alloy to achieve the specified tensile strength, impact resistance and elongation. When using the wrong shielding gas or with self-shielding wire, the weld chemistry can be outside of expected limits, which can eventually lead to disastrous results. This can lead to weld cracks, porosity, or worm marks (surface porosity caused by gas trapped between the solidified slag and the still molten puddle).
Something else to consider is the cost. Using shielding gas with a self-shielded wire is not only wrong, but also a waste of money. Self-shielded wire is designed for field welding, so you may want to reconsider why it is used in production. This is sometimes done due to the presence of strong air currents throughout the workshop, which can disrupt the shielding gas coating, resulting in weld porosity.
The best option may be to invest in welding curtains or make some changes to eliminate strong air currents. By doing so, you can switch from self-shielded wire to double-shielded or even solid wire if it suits your application. In most cases, the costs are offset by filler metal with a higher deposition, which is achieved by reducing the welding time and reducing the cost of post-weld cleaning, summed up Artem Komarov.