December 11, 2003
The demand for flux cored arc welding (FCAW) has grown significantly over the past 10 years. Manufacturers who weld carbon, stainless, low-alloy, and high-alloy steel are turning to this process primarily because:
Before you can obtain the best results with gas-shielded FCAW, you must understand several variables of the process.
Gas-shielded FCAW wires range in diameter from 0.035 to 1/8 inch, with 0.045 in. being the most commonly used.
Many welders mistakenly believe that a larger-diameter wire corresponds to a higher deposition rate with the same current settings. Actually, deposition rate is governed by the current density rather than the overall wire diameter.
For instance, 0.045-in. wire has a higher current density than a 1/16-in. wire, and when compared at the same current levels, the 0.045-in. wire delivers a higher deposition rate. At 250 amps, for example, you can obtain about 11 lbs. of weld per hour with the 0.045-in. wire and about 8 lbs. of weld per hour at 250 amps with 1/16-in.-dia. wire.
The advantage of using a larger-diameter wire comes from being able to operate at higher current levels. Many welders use lower-than-optimum parameters for larger-diameter wires and don't benefit from the intended deposition rates. But if you use the manufacturer's recommended parameters for larger-diameter electrodes, you can achieve excellent deposition rates.
The increased current density of the FCAW process is the reason for its increased deposition rates over gas metal arc welding (GMAW) and shielded metal arc welding (SMAW).
Stick-out is a common term defined as the length of the unmelted electrode extending beyond the end of the welding tip. The stick-out portion of the wire is important because it transmits the current from the welding tip to the welding arc. Generally, the welding tip and gas nozzle are aligned so that the stick-out can be controlled accurately.
While the arc length remains constant when you use a constant-voltage machine, the current you use will fluctuate depending on your skill. The longer the stick-out length becomes, the lower the current output will be. Likewise, if you use a short stick-out, current levels will be higher. You must have consistent stick-out length for a good-looking weld.
Most manufacturers recommend a minimum of 1/2 in. stick-out for 0.045-in. and larger wires. The larger wire diameters require a greater amount of stick-out.
A minimum stick-out is beneficial because the wire can be preheated in a short amount of time. This preheat can help you vaporize any moisture that has gathered in the core of the wire and prepare the wire to be transferred to a molten state as it reaches the arc.
Another factor to consider regarding stick-out is its effect on the gas shielding of the puddle. Excessive stick-out can create a lack of shielding gas covering the weld zone.
FCAW creates a very fluid puddle with a slag system to support it. Because of the supporting slag system, various torch angles can be used to manipulate the puddle. Experimentation can help you determine what is best for a particular procedure.
Most common with FCAW is a backhand or drag technique. This method allows for more penetration, as the arc force holds back the molten puddle. The backhand technique also helps the slag system to provide more consistent coverage on the weld bead. An angle of 10 to 20 degrees off-center is sufficient for a consistent bead with good penetration.
The forehand or push method is another technique. This can be a more comfortable method if you also weld with GMAW. The forehand technique creates a nice bead profile that is flat to slightly concave. Again, 10 to 20 degrees is sufficient to provide a consistent-looking weld.
With both the forehand and backhand techniques, don't use a sharp torch angle. A torch angle that is too large can reduce the effectiveness of the shielding gas, especially those containing lighter gases like argon.
Many variables are involved in producing FCAW wires that can be used and stored, if necessary, with a minimum amount of moisture pickup in the core.
A metal sheath is drawn around metal powder and other flux elements to form the finished FCAW wire. An important part of this procedure is the seam left after the drawing process. This seam should be structurally sound so that it will support the core throughout the welding process.
This seam also is a barrier against unwanted moisture pickup in the core material. If a wire has been stored in an improper environment, moisture pickup can create porosity such as a pit or blowhole in the weld bead. Excessive moisture also can create rust on the wire's sheath.
Ideally, use the wire within a couple of days after being opened. If this is not possible, store the wire on a rack or shelf away from the floor. Also, wrap the wire in a plastic bag to eliminate any further unwanted moisture.
The power source for gas-shielded FCAW should be a constant-voltage machine and generally direct current electrode positive (DCEP). It should provide higher current ranges. With a 0.045-in.-dia. wire, typical current settings are more than 180 amps. With 1/16-in.-dia. wire, current settings often are more than 300 amps. Most high-end GMAW power source-wire feeder combinations can provide substantial power to be successful with FCAW.
When you're buying a feeder, you'll need to consider how you want the wire to be packaged. Because FCAW is a high-deposition process, most FCAW wire manufacturers package their product on full-size spools or coils.
With its cored structure, flux-cored wire is softer than solid wire. Because of this, you should take extra care in setting up the feeding process. Always use the proper size drive rolls with required pressure to feed the wire to the workplace. Also, U-groove drive rolls apply even pressure, which can help make feeding smooth and extend liner life.
As with other welding processes, it's helpful if you can keep the liner from the feeder to the workplace as straight as possible. Any variation causes a strain on a push system. Once you start welding, make sure to have consistent wire feed for a nice looking weld.
While self-shielded FCAW still is used for field fabrication, gas-shielded FCAW is used predominantly for in-house fabrications. The quality of the weld, higher efficiency, and welder appeal can make up the cost of shielding gas.The two most common shielding gases for FCAW are 100 percent CO2and a mix of 75 percent argon and 25 percent CO2. Gas companies also are marketing other blends made for lower spatter and fume levels.CO2gas is decomposed to two active gases: oxygen and carbon monoxide. Under the heat of a welding arc, these active gases react with alloys in the molten weld metal, such as manganese and silicon, leading to the loss of these elements in the solidified weld.Because of these types of reactions, varying the shielding gas can produce varying chemical compositions. Therefore, you should follow your welding wire manufacturer's guidelines regarding what shielding gas can be used with each formulation. Failure to do this can give you unexpected results with chemical analysis, tensile strengths, impact strengths, and crack resistance.
In many weld procedures, heat input is a crucial factor to obtaining the desired results. Heat input is related directly to three factors:
While travel speed stays constant, you can expect increased heat input with increased current or voltage. On the other hand, if travel speed slows, heat input will increase if current and voltage remain constant.
Improper heat input can cause problems that can't be detected visually. Heat input can affect the chemical compositions and even the microstructure of the weld. The cooling rate of the weld zone also can affect the weld's microstructure. Cooling rate depends on many factors, such as heat input, interpass temperature, thickness, sizes of work, and the surrounding environment.
Controlling heat input and temperature of the work within the desired value can ensure sound weld quality.
Tex Ikeda is product manager and David Haynie is senior welding engineer for Kobelco Welding of America Inc., 4755 Alpine 250, Stafford, TX 77477, 281-240-5600, fax 281-240-5625, firstname.lastname@example.org email@example.com, www.kobelcowelding.com.
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