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Flux-Cored Arc Welding: A. Process Principles

Flux-cored arc welding (FCAW) is an arc welding process that uses a continuous tubular electrode fed from a coil or spool into a welding gun. The electrode core contains alloy additions, de-oxidizers and flux materials that protect the weld from atmospheric gases as it cools. Shielding is provided either by decomposition of the core in self-shielded electrodes or an externally supplied gas. FCAW allows for higher deposition rates than gas metal arc welding due to higher currents and minimized electrode heating. While more expensive initially than gas metal arc welding, higher productivity offsets this cost. FCAW is capable of all-position welding but large diameter electrodes are restricted to flat positions.

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265 views

Flux-Cored Arc Welding: A. Process Principles

Flux-cored arc welding (FCAW) is an arc welding process that uses a continuous tubular electrode fed from a coil or spool into a welding gun. The electrode core contains alloy additions, de-oxidizers and flux materials that protect the weld from atmospheric gases as it cools. Shielding is provided either by decomposition of the core in self-shielded electrodes or an externally supplied gas. FCAW allows for higher deposition rates than gas metal arc welding due to higher currents and minimized electrode heating. While more expensive initially than gas metal arc welding, higher productivity offsets this cost. FCAW is capable of all-position welding but large diameter electrodes are restricted to flat positions.

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sn200709
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© © All Rights Reserved
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FLUX-CORED ARC WELDING

a. Process Principles
Flux cored arc welding (FCAW) is an arc welding process that uses a continuous tubular electrode fed from a coil or spool into a
welding "gun". The electrode core contains alloy additions, de-oxidizers and flux materials. The heat of the arc causes the base metal,
tubular electrode wire and core materials to melt. The flux materials bind impurities, rise to the top of the molten weld, and protect the
cooling weld from atmospheric nitrogen or oxygen. Shielding of the exposed arc is provided either by the decomposition of the core in
self-shielded electrodes, designated FCAW-S, or by an externally supplied gas or gas mixture, designated FCAW-G.

(1) With FCAW-G, carbon dioxide (CO2) or a mixture of argon (Ar) of 75 to 90% and of CO2 10 to25% is used in addition to the
gas provided by the flux core. The shielding gas selection may affect the mechanical properties (yield and tensile strength,
elongation, and notch toughness) of the weld. Carbon dioxide, as a reactive gas, may cause some of the alloys in the electrode
to become oxidized, and therefore less alloy is transferred to the weld deposit. When an inert gas such as argon is substituted
forCO2, alloy transfer typically increases. With more alloy in the weld deposit, higher yield and tensile strengths and reduced
　
ductility is expected. The notch toughness of the weld deposit may increase or decrease, depending on the alloys affected.
(2) The power source is usually the constant voltage type, using either direct current electrode positive or electrode negative
polarity. A separate wire feeder sends wire into the welding gun at a preset rate. The Welding Procedure Specification (WPS)
provides the appropriate voltage, wire feed speed, electrode extension, and travel speed. For a given wire feed speed and
electrode extension, a specific current (amperage) will be provided. As the wire feed speed is increased, the current is likewise
increased. The WPS should, preferably, state the wire feed speed to be used because electrode extension, polarity and
electrode diameter also affect current. Shorter electrical stickout results in higher current for a given wire feed speed. If current
is used in the WPS, an inaccurate electrode extension may go undetected.
(3) FCAW is most commonly used as "semi-automatic", wire fed but with the welding gun manipulated by the welder. It may also
be used as automatic, but the intensity of arc rays from the high current arc, and the significant volume of smoke generated,
makes Submerged Arc Welding (SAW) more desirable for automatic welding.

b. Filler Metal Designation, Specification and Certification

　 FCAW electrodes are specified in AWS filler metal specifications AWS A5.20 and A5.29. AWS A5.20 is applicable to carbon
steelelectrodes, and AWS A5.29 is applicable to low alloy steel electrodes.
　
(1) (1) All FCAW electrodes are considered low hydrogen.
Self-shielded FCAW electrodes are limited to 550 MPa (80 ksi) tensile strength of less, but higher strengths are available from
gas-shielded FCAW electrodes. AWS A5.20 electrodes EXXT-2, -3, -10, -13, -14, and -GS electrodes are not permitted by
AWS D1.1 because they are limited to single pass welds. AWS A5.20 electrodes EXXT-3, EXXT-11, andEXXT-14 are for
limited thickness applications only, and the manufacturer's recommendations should be consulted.
(2) Tables C-8 and C-9 provide additional information regarding electrode limitations, usage and toughness properties for
electrodes permitted by AWS D1.1 for classification strengths of 550 MPa (80ksi) and lower. For higher strength and other
electrodes, the AWS A5.20 and A5.29 specifications should be consulted.

c. Advantages, Disadvantages and Limitations


　The Flux Cored Arc Welding (FCAW) process offers several advantages over Shielded Metal Arc Welding (SMAW), but also has
a fewdisadvantages and limitations.
(1) The FCAW electrode is continuous, eliminating the numerous starts and stops necessary with SMAW on longer and larger
welds.
(2) Increased deposition rates are possible with FCAW because the current can be higher than with SMAW. SMAW currents are
limited by rod heating and coating breakdown concerns. With FCAW, the electrode is passed through a contact tip usually 20
to 25 mm (3/4 to 1 in.) from the end of the electrode, minimizing the buildup of heat from electrical resistance. This electrode
extension distance, commonly called "stickout," varies for each WPS, and may be considerably higher. Both factors
provideFCAW an economic advantage over SMAW.
(3) The number of arc starts and stops, a potential source of weld discontinuities, is also reduced.
(4) The equipment required for FCAW is more expensive and complicated than SMAW, and more difficult to maintain. The higher
productivity levels achieved using FCAW offset this increased cost compared to SMAW.
(5) FCAW electrode wires do not need heated holding ovens for ordinary applications, but caution should be
used when FCAW wires are exposed to the elements for extended periods of time. For critical welds requiring
very low hydrogen deposits, more restrictive storage requirements may be warranted.
(6) FCAW is capable of all-position welding when using small diameter electrodes. Large diameter electrodes, using higher
electrical currents, are restricted to the flat and horizontal positions.
(7) There are several advantages to using FCAW-S (self-shielded) rather than FCAW-G (gas-shielded). The FCAW-S welding gun
assembly does not require a gas nozzle, also called a gas cup; therefore access into smaller areas is possible,
significant whenwelding in tight locations such as weld access holes in beam-to-column connections. The welder is also better
able to see the arc and weld puddle because the gas cup is not present.
(8) A second advantage to FCAW-S over FCAW-G is its ability to make quality welds under field conditions involving wind. For
FCAW-G, it is necessary to erect protective shielding from wind to maintain the shielding gas around the molten weld puddle.
Such shielding may be expensive, time-consuming, require additional ventilation for the welder, and constitute a fire hazard.
FCAW-S eliminates the handling of high-pressure gas cylinders, theft of cylinders, protection of gas distribution hosing under
field conditions, and the cost of the shielding gas. For shop fabrication, wind is less of a problem than under field conditions.
However, drafts from doorways and windows, fans used to cool personnel and provide ventilation, and welding fume exhaust
equipment can create unacceptable wind speeds that degrade weld quality.
(9) FCAW-G "operator appeal" is usually higher than with FCAW-S because of better arc control and less fume generation.
FCAW-G is less sensitive to variations in electrode extension and arc voltage than FCAW-S. The range of suitable applications
for a single size and classification of FCAW-G electrodes is generally broader than for FCAW-S electrodes.
(10) FCAW-S procedures must be closely controlled to ensure the required level of weld quality and mechanical properties.
Because of the high deposition rates possible, travel speeds and technique must be monitored to ensure that excessively large
bead sizes are not produced. Large bead size, because of the high heat input and excessively slow cooling rates, may reduce
notch toughness, reduce weld soundness, decrease heat affected zone toughness, and decrease the weld metal yield and
tensile strengths.

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