Getting the 4-1-1 on flux-cored electrodes
March 13, 2007
Becoming knowledgeable about the different types of flux-cored stainless steel electrodes, what their capabilities are, how they are classified, and how to handle and maintain them safely can offer welders many benefits.
To get the most benefit from flux-cored stainless steel electrodes, you first must know what they are, what types are available, how they work, when they are used, and how to maintain them.
A thin stainless steel strip about 1/2 inch wide is formed into a U shape and passed under a belt that feeds a stream of powder that fills the U shape to a set level. Rolls close the U to form an O shape. The round wire is drawn through a series of dies until it reaches its final size, typically 0.045 in. or 1/16 in. The final product is a thin stainless steel outer shell with a powder filling. The structure, combined with the ingredients in the powder fill, is responsible for the properties of flux-cored electrodes.
Welding current is conducted only by the thin sheath of stainless steel, not the core. Because the cross-sectional area carrying the current is small, the wire heats up rapidly and melts at a faster rate than a solid wire of the same diameter. The result is that for a given current level, a flux-cored wire will deposit more pounds of weld metal per hour. For this reason, flux-cored stainless steel electrodes are used on thick materials, because they can fill joints and make fillets rapidly.
Most of the common stainless steel compositions are available as flux-cored electrodes. This includes the austenitic grades (300 series), martensitic grades (410, 410NiMo, and 420), and some duplex grades (2209). The alloy powders added to the core can be adjusted to produce almost any desired chemistry. This can be done in small batches, making it easier for you to purchase unique or difficult-to-find compositions.
As mentioned, the fill in the interior of the wire consists of alloy powders and flux ingredients. The flux ingredients melt to form a slag that helps protect the weld metal from the atmosphere, as well as support and shape the weld bead. Electrodes designed for out-of-position welding form a quick-freezing slag shelf that can support a relatively large amount of molten weld metal and give you more control of the puddle.
Flux-cored electrodes are available as gas-shielded or self-shielded (gasless) types. Gas-shielded electrodes are designated for flat and horizontal use and for all-position use. Self-shielded electrodes are for flat and horizontal use only. The American Welding Society (AWS) uses a classification system to designate the various types.
Gas-shielded electrodes include flat and horizontal (T0) and all-position types (T1). As its name suggests, the T1 can weld in all positions and has a quick-freezing slag system that helps to hold the molten weld puddle in position. It deposits up to 10 pounds per hour in the vertical position using conventional DC power sources (see Figures 1and 2). Although the flat and horizontal beads produced by the all-position electrodes generally are acceptable, some prefer the superior bead appearance that the flat and horizontal electrodes can provide.
The slag system of the T0 is more fluid, and the weld metal flows out to a shiny and finely rippled bead (see Figure 3). The slag system also makes this electrode difficult to use out of position. Both the T1 and T0 are available in diameters of 0.045 and 1/16 in. The 0.035-in. diameter may be available in some grades, but anything larger than 1/16 in. is uncommon.
Gas-shielded, flux-cored wires are suitable for use with either CO2or 75 percent to 80 percent argon with a balance of CO2, but you should check the manufacturer's literature for any limitations.
The use of gas blends with more than 75 percent to 80 percent argon generally is not recommended because it could result in porosity, excessive alloy recovery, or loss of arc control.
Shielding Gas. Carbon dioxide is relatively low in cost, provides good penetration characteristics, and has low weld metal hydrogen levels, decreasing the chance of cracking in the hard, martensitic grades, such as 410, 410NiMo, and 420. An argon with balance CO2blend is smooth with a spraylike arc, generates little spatter and fumes, and provides strong weld deposits because less alloy is lost during welding.
It is important to set shielding gas flow rates at 40 to 50 cubic feet per hour. A low flow rate or disruption of the protective gas cover (from wind or strong drafts) may result in porosity. Even if porosity does not occur, there still may be excessive pickup of nitrogen from the air, which can deteriorate the surface appearance of the weld and make slag removal more difficult. Excessive nitrogen in the weld metal will also reduce the ferrite content of the weld metal, making it more susceptible to hot cracking.
Excessively high flow rates, which cause turbulence in the shielding gas cover, can have the same effect. The machine settings used for CO2and 75 percent to 80 percent argon, balance CO2shielding gases are similar, except that the voltage should be set about 2 volts higher for CO2.
Power. Constant-voltage DC power sources operating on direct current electrode positive (DCEP) are recommended for welding gas-shielded, flux-cored electrodes. Using pulsed-arc power sources in the pulsed mode is unnecessary and may actually hurt electrode performance.
Current levels used are similar to those used with carbon steel flux-cored wires. Consult the manufacturer's literature for recommended current and voltage settings.
Technique. Maintain a distance of 3/4 in. between the contact tip and the work area for 0.045- and 1/16-in.-dia. electrodes. Excessively short electrode extensions may lead to wormtrack porosity, while excessively long electrode extensions may reduce penetration and hurt shielding gas coverage. A backhand (dragging) technique without weaving is recommended for welding stainless steel horizontal fillets to help ensure good penetration and a flat to slightly convex bead shape, resulting in less spatter than a forehand (pushing) technique. Small, vertical-up fillets can be made without weaving. Large, vertical fillets may require some side-to-side motion to allow the bead edges to wet out.
A slightly convex bead shape is preferable to a flat or concave bead, because this will reduce the tendency to shape centerline hot cracks. Remember, cleaning stainless steel weld beads should be done only with clean stainless steel brushes. Residue from carbon steel can lead to discoloration of the stainless steel weld bead.
The self-shielded, flux-cored electrodes (T0-3) are designed to be used without external shielding gas. They are used in areas where gas is not available or where winds and drafts would disrupt the shielding gas coverage. Because the penetration and the resultant dilution are low, they often are used for cladding operations.
Power. Most self-shielded electrodes are used with constant-voltage DC power sources operating on DCEP.
Excessively high voltage will lead to surface porosity, while excessively low voltage will result in a ropey bead.
Technique. A backhand (dragging) technique is recommended. The arc should be kept at the front edge of the puddle. To maintain a constant travel speed, keep a constant distance between the wire and the molten slag behind the wire. If the slag runs ahead and interferes with the arc, increase the drag angle, but not excessively. Adjust the voltage to obtain a well-shaped, porosity-free bead.
Because the atmosphere is in direct contact with the weld puddle during open-arc welding, nitrogen is more readily absorbed into the weld puddle. Nitrogen has a strong effect on the properties of stainless steel weld metal. For this reason it is essential to follow the manufacturer's instructions closely.
Electrode diameters are available from 0.045 to 3/32 in. or larger. The larger-diameter electrodes are used for heavy fillets or to fill large joints quickly. Operating parameters and welding techniques are more sensitive for self-shielded wires. For this reason, it is essential to follow the manufacturer's instructions closely. The fume level from self-shielded wires is high, which is why these types are often confined to open-air usage.
Because flux-cored electrodes can operate at such high deposition rates, they can generate a large amount of fumes per unit of time. The Occupational Safety and Health Administration has issued a new regulation for exposure to hexavalent chromium, a known human carcinogen and a byproduct of welding stainless steels.
The new permissible exposure level (PEL) is approximately 10 times lower than it was previously. The new regulation requires employers to determine the exposure level of workers who are likely to deal with hexavalent chromium. If exposure levels are found to be in excess of the PEL, engineering and workplace controls must be instituted. This can include enclosing the source of exposure, placing a barrier between employees and the source of exposure, or using ventilation and exhaust systems to reduce the exposure. If these fail to reduce exposure sufficiently, then respiratory equipment must be used until engineering controls are installed.
Stainless steel flux-cored electrodes can pick up moisture if left unprotected. This can lead to wormtrack porosity. Most of these products are packaged in vacuum-sealed bags. They should remain usable for several years if they are stored in a dry location in their original, unopened package. Once opened, the wires should not be left exposed for extended periods of time. Highly humid environments or condensed moisture produced by cool evening temperatures can result in moisture pickup. When not in use, the wires should be returned to their original packaging, sealed, and stored in a dry area.