Flux Core Information
Flux Core, Core Flux Welding, Flux Cored, FCAW
FCAW, Flux Core Flux-cored, tubular electrode welding has evolved from the MIG welding process to improve arc action, metal transfer, weld metal properties, and weld appearance.
It is an arc welding process in which the heat for welding is provided by an arc between a continuously fed tubular electrode wire and the workpiece.
Shielding is obtained by a flux contained within the tubular electrode wire or by the flux and an externally supplied shielding gas. A diagram of the process is shown in figure 10-55.
Flux Core, Core Flux Welding, Flux Cored, FCAW
The flux-cored wire used for this process gives it different characteristics.
Flux-cored arc welding is widely used for welding ferrous metals and is particularly good for applications in which high deposition rates are needed.
At high welding currents, the arc is smooth and more manageable when compared in using large diameter gas metal arc welding electrodes with carbon dioxide.
The arc and weld pool are clearly visible to the welder. A slag coating is left on the surface of the weld bead, which must be removed.
Since the filler metal transfers across the arc, some spatter is created and some smoke produced.
Flux Core, Core Flux Welding, Flux Cored, FCAW
b. FCAW, Flux Core Equipment.
(1) The FCAW, Flux Core equipment used for flux-cored arc welding is similar to that used for gas metal arc welding. The basic arc welding equipment consists of a power source, controls, wire feeder, welding gun, and welding cables.
A major difference between the gas shielded electrodes and the self-shielded electrodes is that the gas shielded wires also require a gas shielding system.
This may also have an effect on the type of welding gun used.
Fume extractors are often used with this process.
For machines and automatic welding, several items, such as seam followers and motion devices, are added to the basic equipment.
Figure 10-56 shows a diagram of the equipment used for semiautomatic flux-cored arc welding.
Most power sources operate on 230 or 460 volt input power, but machines that operate on 200 or 575 volt input are also available.
Power sources may operate on either single phase or three-phase input with a frequency of 50 to 60 hertz.
Most power sources used for flux-cored arc welding have a duty cycle of 100 percent, which indicates they can be used to weld continuously.
Some machines used for this process have duty cycles of 60 percent, which means that they can be used to weld 6 of every 10 minutes.
The power sources generally recommended for flux-cored arc welding are direct current constant voltage type.
Both rotating (generator) and static (single or three-phase transformer-rectifiers) are used.
The same power sources used with gas metal arc welding are used with flux-cored arc welding.
Flux-cored arc welding generally uses higher welding currents than gas metal arc welding, which sometimes requires a larger power source.
It is important to use a power source that is capable of producing the maximum current level required for an application.
(3) FCAW, Flux Core Flux-cored arc welding uses direct current. Direct current can be either reverse or straight polarity.
Flux-cored electrode wires are designed to operate on either DCEP or DCEN.
The wires designed for use with an external gas shielding system are generally designed for use with DCEP.
Some self-shielding flux-cored ties are used with DCEP while others are developed for use with DCEN.
Electrode positive current gives better penetration into the weld joint.
Electrode negative current gives lighter penetration and is used for welding thinner metal or metals where there is poor fit-up.
The weld created by DCEN is wider and shallower than the weld produced by DCEP.
(4) The FCAW, Flux Core generator welding machines used for this process can be powered by an electric rotor for shop use, or by an internal combustion engine for field applications.
The gasoline or diesel engine-driven welding machines have either liquid or air-cooled engines.
Motor-driven generators produce a very stable arc, but are noisier, more expensive, consume more power, and require more maintenance than transformer-rectifier machines.
(5) A wire feed motor provides power for driving the electrode through the cable and gun to the work.
There are several different wire feeding systems available. FCAW, Flux Core System selection depends upon the application.
Most of the wire feed systems used for flux-cored arc welding are the constant speed type, which are used with constant voltage power sources.
With a variable speed wire feeder, a voltage sensing circuit is used to maintain the desired arc length by varying the wire feed speed.
Variations in the arc length increase or decrease the wire feed speed.
A wire feeder consists of an electrical rotor connected to a gear box containing drive rolls.
The gear box and wire feed motor shown in figure 10-57 have form feed rolls in the gear box.
Air-cooled guns are cooled primarily by the surrounding air, but a shielding gas, when used, provides additional cooling effects.
A water-cooled gun has ducts to permit water to circulate around the contact tube and nozzle.
Water-cooled guns permit more efficient cooling of the gun.
Water-cooled guns are recommended for use with welding currents greater than 600 amperes, and are preferred for many applications using 500 amperes.
Welding guns are rated at the maximum current capacity for continuous operation.
Air-cooled guns are preferred for most applications less than 500 amperes, although water-cooled guns may also be used. Air-cooled guns are lighter and easier to manipulate.
(7) FCAW, Flux Core Shielding gas equipment and electrodes.
(a) FCAW, Flux Core Shielding gas equipment used for gas shielded flux-cored wires consists of a gas supply hose, a gas regulator, control valves, and supply hose to the welding gun.
(b) The FCAW, Flux Core shielding gases are supplied in liquid form when they are in storage tanks with vaporizers, or in a gas form in high pressure cylinders. An exception to this is carbon dioxide. When put in high pressure cylinders, it exists in both liquid and gas forms.
(c) The primary purpose of theFCAW, Flux Core shielding gas is to protect the arc and weld puddle from contaminating effects of the atmosphere.
The nitrogen and oxygen of the atmosphere, if allowed to come in contact with the molten weld metal, cause porosity and brittleness.
In flux-cored arc welding, shielding is accomplished by the decomposition of the electrode core or by a combination of this and surrounding the arc with a shielding gas supplied from an external source.
A shielding gas displaces air in the arc area. Welding is accomplished under a blanket of shielding gas.
Inert and active gases may both be used for flux-cored arc welding. Active gases such as carbon dioxide, argon-oxygen mixture, and argon-carbon dioxide mixtures are used for almost all applications.
Carbon dioxide is the most common.
The choice of the proper shielding gas for a specific application is based on the type of metal to be welded, arc characteristics and metal transfer, availability, cost of the gas, mechanical property requirements, and penetration and weld bead shape. The various shielding gases are summarized below.
1. Carbon dioxide.
Carbon dioxide is manufactured from fuel gases which are given off by the burning of natural gas, fuel oil, or coke.
It is also obtained as a by-product of calcining operation in lime kilns, from the manufacturing of ammonia and from the fermentation of alcohol, which is almost 100 percent pure.
Carbon dioxide is made available to the user in either cylinder or bulk containers.
The cylinder is more common.
With the bulk system, carbon dioxide is usually drawn off as a liquid and heated to the gas state before going to the welding torch.
The bulk system is normally only used when supplying a large number of welding stations.
In the cylinder, the carbon dioxide is in both a liquid and a vapor form with the liquid carbon dioxide occupying approximately two thirds of the space in the cylinder.
By weight, this is approximately 90 percent of the content of the cylinder. Above the liquid, it exists as a vapor gas.
As carbon dioxide is drawn from the cylinder, it is replaced with carbon dioxide that vaporizes from the liquid in the cylinder and therefore the overall pressure will be indicated by the pressure gauge.
When the pressure in the cylinder has dropped to 200 psi (1379 kPa), the cylinder should be replaced with a new cylinder.
A positive pressure should always be left in the cylinder in order to prevent moisture and other contaminants from backing up into the cylinder.
The normal discharge rate of the CO2 cylinder is about 10 to 50 cu ft per hr (4.7 to 24 liters per min).
However, a maximum discharge rate of 25 cu ft per hr (12 liters per min is recommended when welding using a single cylinder.
As the vapor pressure drops from the cylinder pressure to discharge pressure through the CO2 regulator, it absorbs a great deal of heat.
If flow rates are set too high, this absorption of heat can lead to freezing of the regulator and flowmeter which interrupts the shielding gas flow.
When flow rate higher than 25 cu ft per hr (12 liters per min) is required, normal practice is to manifold two CO2 cylinders in parallel or to place a heater between the cylinder and gas regulator, pressure regulator, and flowmeter.
Excessive flow rates can also result in drawing liquid from the cylinder.
Carbon dioxide is the most widely used shielding gas for flux-cored arc welding.
Most active gases cannot be used for shielding, but carbon dioxide provides several advantages for use in welding steel.
These are deep penetration and low cost. Carbon dioxide promotes a globular transfer.
The carbon dioxide shielding gas breaks down into components such as carbon monoxide and oxygen.
Because carbon dioxide is an oxidizing gas, deoxidizing elements are added to the core of the electrode wire to remove oxygen. The oxides formed by the deoxidizing elements float to the surface of the weld and become part of the slag covering.
Some of the carbon dioxide gas will break down to carbon and oxygen.
If the carbon content of the weld pool is below about 0.05 percent, carbon dioxide shielding will tend to increase the carbon content of the weld metal.
Carbon, which can reduce the corrosion resistance of some stainless steels, is a problem for critical corrosion application.
Extra carbon can also reduce the toughness and ductility of some low alloy steels. If the carbon content in the weld metal is greater than about 0.10 percent, carbon dioxide shielding will tend to reduce the carbon content. This loss of carbon can be attributed to the formation of carbon monoxide, which can be trapped in the weld as porosity deoxidizing elements in the flux core reducing the effects of carbon monoxide formation.
2. Argon-carbon dioxide mixtures. Argon and carbon dioxide are sometimes mixed for use with flux-cored arc welding. A high percentage of argon gas in the mixture tends to promote a higher deposition efficiency due to the creation of less spatter. The most commonly used gas mixture in flux-cored arc welding is a 75 percent argon-25 percent carbon dioxide mixture. The gas mixture produces a fine globular metal transfer that approaches a spray. It also reduces the amount of oxidation that occurs, compared to pure carbon dioxide. The weld deposited in an argon-carbon dioxide shield generally has higher tensile and yield strengths. Argon-carbon dioxide mixtures are often used for out-of-position welding, achieving better arc characteristics. These mixtures are often used on low alloy steels and stainless steels. Electrodes that are designed for use with CO2 may cause an excessive buildup of manganese, silicon, and other deoxidizing elements if they are used with shielding gas mixtures containing a high percentage of argon. This will have an effect on the mechanical properties of the weld.
3. Argon-oxygen mixtures. Argon-oxygen mixtures containing 1 or 2 percent oxygen are used for some applications. Argon-oxygen mixtures tend to promote a spray transfer which reduces the amount of spatter produced. A major application of these mixtures is the welding of stainless steel where carbon dioxide can cause corrosion problems.
(d) The electrodes used for flux-cored arc welding provide the filler metal to the weld puddle and shielding for the arc. Shielding is required for sane electrode types. The purpose of the shielding gas is to provide protection from the atmosphere to the arc and molten weld puddle. The chemical composition of the electrode wire and flux core, in combination with the shielding gas, will determine the weld metal composition and mechanical properties of the weld. The electrodes for flux-cored arc welding consist of a metal shield surrounding a core of fluxing and/or alloying compounds as shown in figure 10-58. The cores of carbon steel and low alloy electrodes contain primarily fluxing compounds. Some of the low alloy steel electrode cores contain high amounts of alloying compounds with a low flux content. Most low alloy steel electrodes require gas shielding. The sheath comprises approximately 75 to 90 percent of the weight of the electrode. Self-shielded electrodes contain more fluxing compounds than gas shielded electrodes. The compounds contained in the electrode perform basically the same functions as the coating of a covered electrode used in shielded metal arc welding. These functions are:
1. To form a slag coating that floats on the surface of the weld metal and protects it during solidification.
2. To provide deoxidizers and scavengers which help purify and produce solid weld-metal.
3. To provide arc stabilizers which produce a smooth welding arc and keep spatter to a minimum.
4. To add alloying elements to the weld metal which will increase the strength and improve other properties in the weld metal.
5. To provide shielding gas. Gas shielded wires require an external supply of shielding gas to supplement that produced by the core of the electrode.
(e) The classification system used for tubular wire electrodes was devised by the American Welding Society. Carbon and low alloy steels are classified on the basis of the following items:
1. Mechanical properties of the weld metal.
2. Welding position.
3. Chemical composition of the weld metal.
4. Type of welding current.
5. Whether or not a CO2 shielding gas is used.
An example of a carbon steel electrode classification is E70T-4 where:
1. The "E" indicates an electrode.
2. The second digit or "7" indicates the minimum tensile strength in units of 10,000 psi (69 MPa). Table 10-12, below, shows the mechanical property requirements for the various carbon steel electrodes.
4. The "T" stands for a tubular or flux cored wire classification.
5. The suffix "4" gives the performance and usability capabilities as shown in table 10-13. When a "G" classification is used, no specific performance and usability requirements are indicated. This classification is intended for electrodes not covered by another classification. The chemical composition requirements of the deposited weld metal for carbon steel electrodes are shown in table 10-14. Single pass electrodes do not have chemical composition requirements because checking the chemistry of undiluted weld metal does not give the true results of normal single pass weld chemistry.
The classification of low alloy steel electrodes is similar to the classification of carbon steel electrodes. An example of a low alloy steel classification is E81T1-NI2 where:
1. The "E" indicates electrode.
2. The second digit or "8" indicates the minimum tensile in strength in units of 10,000 psi (69 MPa). In this case it is 80,000 psi (552 MPa). The mechanical property requirements for low alloy steel electrodes are shown in table 10-15. Impact strength requirements are shown in table 10-16.
3. The third digit or "1" indicates the welding position capabilities of the electrode. A "1" indicates all positions and an "0" flat and horizontal position only.
4. The "T" indicates a tubular or flux-cored electrode used in flux cored arc welding.
5. The fifth digit or "1" describes the usability and performance characteristics of the electrode. These digits are the same as used in carbon steel electrode classification but only EXXT1-X, EXXT4-X, EXXT5-X and EXXT8-X are used with low alloy steel flux-cored electrode classifications.
6. The suffix or "Ni2" tells the chemical composition of the deposited weld metal as shown in table 10-17 below.
The classification system for stainless steel electrodes is based on the chemical composition of the weld metal and the type of shielding to be employed during welding. An example of a stainless steel electrode classification is E308T-1 where:
1. The "E" indicates the electrode.
2. The digits between the "E" and the "T" indicates the chemical composition of the weld as shown in table 10-18 below.
3. The "T" designates a tubular or flux cored electrode wire.
4. The suffix of "1" indicates the type of shielding to be used as shown in table 10-19 below.
(a) The welding cables and connectors are used to connect the power source to the welding gun and to the work. These cables are normally made of copper. The cable consists of hundreds of wires that are enclosed in an insulated casing of natural or synthetic rubber. The cable that connects the power source to the welding gun is called the electrode lead. In semiautomatic welding, this cable is often part of the cable assembly, which also includes the shielding gas hose and the conduit that the electrode wire is fed through. For machine or automatic welding, the electrode lead is normally separate. The cable that connects the work to the power source is called the work lead. The work leads are usually connected to the work by pinchers, clamps, or a bolt.
(b) The size of the welding cables used depends on the output capacity of the welding machine, the duty cycle of the machine, and the distance between the welding machine and the work. Cable sizes range from the smallest AWG No 8 to AWG No 4/0 with amperage ratings of 75 amperes on up. Table 10-20 shows recommended cable sizes for use with different welding currents and cable lengths. A cable that is too small may become too hot during welding.
c. Advantages. The major advantages of flux-cored welding are reduced cost and higher deposition rates than either SMAW or solid wire GMAW. The cost is less for flux-cored electrodes because the alloying agents are in the flux, not in the steel filler wire as they are with solid electrodes. Flux-cored welding is ideal where bead appearance is important and no machining of the weld is required. Flux-cored welding without carbon dioxide shielding can be used for most mild steel construction applications. The resulting welds have higher strength but less ductility than those for which carbon dioxide shielding is used. There is less porosity and greater penetration of the weld with carbon dioxide shielding. The flux-cored process has increased tolerances for scale and dirt. There is less weld spatter than with solid-wire MIG welding. It has a high deposition rate, and faster travel speeds are often used. Using small diameter electrode wires, welding can be done in all positions. Some flux-cored wires do not need an external supply of shielding gas, which simplifies the equipment. The electrode wire is fed continuously so there is very little time spent on changing electrodes. A higher percentage of the filler metal is deposited when compared to shield metal arc welding. Finally, better penetration is obtained than from shielded metal arc welding.
d. Disadvantages. Most low-alloy or mild-steel electrodes of the flux-cored type are more sensitive to changes in welding conditions than are SMAW electrodes. This sensitivity, called voltage tolerance, can be decreased if a shielding gas is used, or if the slag-forming components of the core material are increased. A constant-potential power source and constant-speed electrode feeder are needed to maintain a constant arc voltage.
e. Process Principles. The flux-cored welding wire, or electrode, is a hollow tube filled with a mixture of deoxidizers, fluxing agents, metal powders, and ferro-alloys. The closure seam, which appears as a fine line, is the only visible difference between flux-cored wires and solid cold-drawn wire. Flux-cored electrode welding can be done in two ways: carbon dioxide gas can be used with the flux to provide additional shielding, or the flux core alone can provide all the shielding gas and slagging materials. The carbon dioxide gas shield produces a deeply penetrating arc and usually provides better weld than is possible without an external gas shield. Although flux-cored arc welding may be applied semiautomatically, by machine, or automatically, the process is usually applied semiautomatically. In semiautomatic welding, the wire feeder feeds the electrode wire and the power source maintains the arc length. The welder manipulates the welding gun and adjusts the welding parameters. Flux-cored arc welding is also used in machine welding where, in addition to feeding the wire and maintaining the arc length, the machinery also provides the joint travel. The welding operator continuously monitors the welding and makes adjustments in the welding parameters. Automatic welding is used in high production applications.