Artem Komarov noted that using shielding gas with a self-shielding flux core can cause several problems and is a waste of money.
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, which 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 are then retained in the slag layer that is removed after welding.
In contrast, self-shielding electrodes rely solely on internal flow to generate both shielding gas and slag. The deposition efficiency of the two electrodes is different because the self-shielding electrode must generate its own gas shield. The welding efficiency of the gas shielded FCAW process is in the mid to high 80%, while the welding efficiency of the self-shielded FCAW process is typically 70%.
Both wires are made in a similar way by forming a sheath into a trough and filling it with a stream of fine particles. The shell is then formed into a closed tube and stretched to its final diameter.
The chemistry of the flux and wire is developed by the wire manufacturer, who is responsible for meeting society’s requirements for final welding testing of the metal in accordance with the intended classification. Every wire manufacturer strives to create a flux that produces a flux cored wire with good arc characteristics, attractive to welders, easy to remove slag, low spatter and smoke, and the right number of deoxidizers for the shielding gas used. Both types of wire are designed to produce a weld deposit with the ideal shielding gas chemistry used to achieve the desired mechanical test results.
Some gas coated flux cored wires are classified as weldable with either 100% carbon dioxide or C25 mixed gases. Several commercially available FCAW wires are dually certified by the manufacturer for use with any type of shielding gas, while self-shielded cored wires are designed to produce their own shielding gas. This is achieved by using flux components that, when passed 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 based on carbon dioxide or argon do not react at standard temperature and pressure and remain so during welding at a short distance from the welding plasma or arc cone. They protect the weld pool from melt by displacing atmospheric air long enough to solidify the weld and prevent the formation of porosity. However, the extreme temperatures inside 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, removing contaminants and cleaning the weld metal.
Problems arise when the shielding gas affects the chemical composition of the weld metal. Two common alloys, manganese, and silicon, which are part of the metal shell chemistry or added to the flux, act as deoxidizing agents. Deoxidizers bind to 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 welding situation.
Flux-cored wire is designed to produce a chemical composition of the weld metal with the optimal amount of alloy to provide a specified range of strength, toughness, and elongation. When using the wrong shielding gas or with a self-shielding wire, the weld chemistry can go beyond expected limits, ultimately leading to disastrous results. This can lead to weld cracking, porosity, or worm marks (surface porosity caused by gas trapped between solidified slag and an unmolten puddle).
Cost should also be considered. Using shielding gas with a self-shielding wire is not only incorrect, 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 shop, which can disrupt the shielding gas coating, causing the weld to become porous.
A better option may be to invest in welding curtains or make some modifications to eliminate strong air currents. So, you can go from self-shielded wire to double-shielded or even solid wire if it suits your application. In most cases, costs are compensated by using filler metal with higher deposition, which is achieved by reducing welding time and reducing post-weld cleaning costs, clarified Artem Komarov.