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Structural Welding

Why is FCAW-S considered taboo? by CWIC
Posted: 14 Feb 03 (Edited 1 Sep 03)

It is not a problematic welding process, but a misunderstood process. Many SE's designing under the AWS D1.1 code prohibit FCAW-S or specify testing for this prequalified process and joints. Often the results of mechanical and nondestructive testing confirm what is already "public knowledge" of the process. It is not uncommon these days to find the only prequalified welding process for seismic applications is SMAW (stick welding).

A better understanding of the process (and the advantages and limitations of other arc welding processes) may help to mitigate testing that has been performed to prequalify these processes, materials and conditions.

DESCRIPTION: FCAW-S is an open arc process which utilizes a continuously-fed consumable bare metal tubular electrode that is self-shielded (does not require external shielding gas), or, it may be gas-shielded (FCAW-G), depending on the electrode used. This process is similar in operation to the GMAW process. FCAW feeds the electrode through a low voltage motor driven feeder which sometimes employs a wire straightener in the form of an additional set of drive rolls. At the front end of the feeder is a cable assembly that contains the power cable, electrode conduit, control wiring, gas hose (for FCAW-G), and water cooling hoses if so equipped. At the front end of the cable assembly is the gun or torch, depending on the process control. The gun contains a switch that the welder activates to energize the the contactor (if so equipped) at the power source which energizes the welding, gas/water solenoid(s) and wire feeder circuits. A contact tip at the front end of the gun/torch is the final electrical contact.

POWER SOURCES: Power supplies are similar to those used for GMAW. As with the GMAW process, amperage is determined by the wire feed speed control. The FCAW process primarily uses a constant voltage (CV) or constant potential type of power source with either positive or negative electrode polarity. When using a constant current (CC) type of power source, changes in the arc length (arc voltage) result in an increase in welding current. This type of power supply requires a voltage-sensing or voltage-controlled wire feeding system that compensates for changes in the arc voltage by changing the wire feed speed during operation. It should be noted that when using a CC power source it is difficult to maintain welding parameter tolerances specified in the AWS D1.1 Structural Welding Code-Steel. The FCAW tolerance for arc voltage is +/- 7%.

ELECTRODES: The electrodes used for this process have an advantage over the GMAW solid electrodes in that they benefit from from the flux which has the following functions:
1.) Arc stabilization.
2.) Shielding of the molten weld pool from the atmosphere.
3.) Supports the molten weld pool until solidification occurs and aids in bead shaping.
4.) Scavengers which aid in the the removal of impurities from the molten metal.
5.) Elements in the flux determine the mechanical, chemical and physical properties of the weld metal deposit.
Because of the electrodes tubular shape and relative ductility, knurled drive rolls are used to push the electrode through the gun/torch cable assembly.
Electrode diameters range from 0.035" to 5/32" as well as custom sizes determined by the manufacturer and the end user. The AWS has two specifications for welding steels:
ANSI/AWS A5.20-XX Specification of Carbon Steel Electrodes for Flux Cored Arc Welding
ANSI/AWS A5.29-XX Specification for Low-Alloy Steel Electrodes for Flux Cored Arc Welding

APPLICATIONS: There are many applications for this process, but FCAW is primarily used because of the high deposition rates compared to that of the SMAW process. This is typically the process of choice for field welding because it does not require gas-shielding and the relative simplicity of the equipment.
*Gas-shielded electrodes are used for weldments that require deep penetration with narrow grooves or cross sections such as those joints used in pipe welds. Shielding is usually 100% CO2 or a CO2/Ar mix.
*One parameter applicable to the FCAW process is the electrode stick-out (electrode extension.) With FCAW, the welder may wish to increase or decrease the stick-out to change the arc voltage during welding. This feature is advantageous when there is an uneven or inconsistent joint fit-up condition.
*FCAW can join most ferrous metals and alloys that can be joined with SMAW or GMAW. Process control may be semiautomatic, automatic, mechanized, machine or robotic.
*Self shielded electrodes are often used for structural steel erection due to its portability and ease of use.
*Many FCAW electrodes with identical AWS classifications may operate differently. Some electrodes are more "user friendly" than others and are easier to use for out of position welding.

(Excerpt from Section 2 of the CWIC Structural Fabricator Workshop training manual.)   
   

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