Tubular or Flux-Cored Electrodes

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Tubular or Flux-Cored Electrodes

Summary:

The flux-cored arc welding process (FCAW or FCA) is made possible by the design of
the electrode. This inside-outside electrode consists of a metal sheath
surrounding a core of fluxing and alloying compounds. The compounds
contained in the electrode perform essentially the same functions as the
coating on a covered electrode, i.e., deoxidizers, slag formers, arc
stabilizers, alloying elements, and may provide shielding gas. There are
three reasons why cored wires are developed to supplement solid
electrode wires of the same or similar analysis.

There is an
economic advantage. Solid wires are drawn from steel billets of the
specified analyses. These billets are not readily available and are
expensive. A single billet might also provide more solid electrode wire
than needed. In the case of cored wires, the special alloying elements
are introduced in the core material to provide the proper deposit
analysis.

Tubular wire production method provides versatility of
composition and is not limited to the analysis of available steel
billets.

Tubular electrode wires are easier for the welder to
use than solid wires of the same deposit analysis, especially for
welding pipe in the fixed position.

Flux Cored Welding Basics Video

Flux-Cored Electrode Design

The sheath or steel portion of the flux-cored wire comprises 75 to 90
percent of the weight of the electrode, and the core material
represents 10 to 25 percent of the weight of the electrode.

For a covered electrode, the steel represents 75 percent of the
weight and the flux 25 percent. This is shown in more detail below:

More flux is used on covered electrodes than in a flux-cored wire to do
the same job. This is because the covered electrode coating contains
binders to keep the coating intact and also contains agents to allow the
coating to be extruded.

Self-Shielding Flux-Cored Electrodes

The self-shielding type
flux-cored electrode wires include additional gas forming elements in
the core. These are necessary to prohibit the oxygen and nitrogen of the
air from contacting the metal transferring across the arc and the
molten weld puddle. Self-shielding electrodes also include extra
deoxidizing and denigrating elements to compensate for oxygen and
nitrogen which may contact the molten metal. Self-shielding electrodes
are usually more voltage-sensitive and require electrical stickout for
smooth operation. The properties of the weld metal deposited by the
self-shielding wires are sometimes inferior to those produced by the
externally shielded electrode wires because of the extra amount of
deoxidizers included. It is possible for these elements to build up in
multipass welds, lower the ductility, and reduce the impact values of
the deposit. Some codes prohibit the use of self-shielding wires on
steels with yield strength exceeding 42,000 psi (289,590 kPa). Other
codes prohibit the self-shielding wires from being used on dynamically
loaded structures.

Metal Transfer

Metal transfer from consumable electrodes across
an arc has been classified into three general modes. These are:

  • spray
    transfer
  • globular transfer
  • short circuiting transfer

The metal
transfer of flux-cored electrodes resembles a fine globular transfer. On
cored electrodes in a carbon dioxide shielding atmosphere, the molten
droplets build up around the outer sheath of the electrode. The core
material appears to transfer independently to the surface of the weld
puddle.

At low currents, the droplets tend to be larger than when the
current density is increased. Transfer is more frequent with smaller
drops when the current is increased. The larger droplets at the lower
currents cause a certain amount of splashing action when they enter the
weld puddle. This action decreases with the smaller droplet size. This
explains why there is less visible spatter, the arc appears smoother to
the welder, and the deposition efficiency is higher when the electrode
is used at high current rather than at the low end of its current range.

Mild Steel Electrodes

Carbon steel electrodes are classified by
the American Welding Society specification, "Carbon Steel Electrodes for
Flux-cored-Arc Welding". This specification includes electrodes having
no appreciable alloy content for welding mild and low alloy steels. The
system for identifying flux-cored electrodes follows the same pattern as
electrodes for gas metal arc welding, but is specific for tubular
electrodes. For example, in E70T-1, the E indicates an electrode; 70
indicates the required minimum as-welded tensile strength in thousands
of pounds per square inch (psi); T indicates tubular, fabricated, or
flux-cored electrode; and 1 indicates the chemistry of the deposited
weld metal, gas type, and usability factor.

Classification of Flux-Cored Electrodes

  1. E60T-7 electrode classification. Electrodes of this
    classification are used without externally applied gas shielding and may
    be used for single-and multiple-pass applications in the flat and
    horizontal positions. Due to low penetration and to other properties,
    the weld deposits have a low sensitivity to cracking.
  2. E60T-8 electrode classifications. Electrodes of this
    classification are used without externally applied gas shielding and may
    be used for single-and multiple-pass applications in the flat and
    horizontal positions. Due to low penetration and to other properties,
    the weld deposits have a low sensitivity to cracking.
  3. E70T-1 electrode classification. Electrodes of this
    classification are designed to be used with carbon dioxide shielding gas
    for single-and multiple-pass welding in the flat position and for
    horizontal fillets. A quiet arc, high-deposition rate, low spatter loss,
    flat-to-slightly convex bead configuration, and easily controlled and
    removed slag are characteristics of this class.
  4. E70T-2 electrode classification. Electrodes of this
    classification are used with carbon dioxide shielding gas and are
    designed primarily for single-pass welding in the flat position and for
    horizontal fillets. However, multiple-pass welds can be made when the
    weld beads are heavy and an appreciable amount of mixture of the base
    and filler metals occurs.
  5. E70T-3 electrode classification. Electrodes of this
    classification are used without externally applied gas shielding and are
    intended primarily for depositing single-pass, high-speed welds in the
    flat and horizontal positions on light plate and gauge thickness base
    metals. They should not be used on heavy sections or for multiple-pass
    applications.
  6. E70T-4 electrode classification. Electrodes of this
    classification are used without externally applied gas shielding and may
    be used for single-and multiple-pass applications in the flat and
    horizontal positions. Due to low penetration, and to other properties,
    the weld deposits have a low sensitivity to cracking.
  7. E70T-5 electrode classification. This classification
    covers electrodes primarily designed for flat fillet or groove welds
    with or without externally applied shielding gas. Welds made
    using-carbon dioxide shielding gas have better quality than those made
    with no shielding gas. These electrodes have a globular transfer, low
    penetration, slightly convex bead configuration, and a thin, easily
    removed slag.
  8. E70T-6 electrode classification. Electrodes of this
    classification are similar to those of the E70T-5 classification, but
    are designed for use without an externally applied shielding gas
  9. E70T-G electrode classification. This classification
    includes those composite electrodes that are not included in the
    preceding classes. They may be used with or without gas shielding and
    may be used for multiple-pass work or may be limited to single-pass
    applications. The E70T-G electrodes are not required to meet chemical,
    radiographic, bend test, or impact requirements; however, they are
    required to meet tension test requirements. Welding current type is not
    specified.

The flux-cored electrode wires are considered to be low hydrogen,
since the materials used in the core do not contain hydrogen. However,
some of these materials are hydroscopic and thus tend to absorb moisture
when exposed to a high-humidity atmosphere. Electrode wires are
packaged in special containers to prevent this. These electrode wires
must be stored in a dry room.

Stainless Steel Tubular Wires

Flux-cored tubular electrode
wires are available which deposit stainless steel weld metal
corresponding to the A.I.S.I. compositions. These electrodes are covered
by the A.W.S specification, "Flux-Cored Corrosion Resisting Chromium
and Chromium-Nickel Steel Electrodes." These electrodes are identified
by the prefix E followed by the standard A.I.S.I. code number. This is
followed by the letter T indicating a tubular electrode. Following this
and a dash are four-possible suffixes as follows:

  • 1 indicates the use of C02 (carbon dioxide) gas for shielding and DCEP.
  • 2 indicates the use of argon plus 2 percent oxygen for shielding and DCEP.
  • 3 indicates no external gas shielding and DCEP.
  • G indicates that gas shielding and polarity are not specified.

Tubular or flux-cored electrode wires are also used for surfacing and submerged arc welding applications.

Deposition Rates and Weld Quality

The deposition rates for flux-cored
electrodes are shown in figure 8-2. These curves show deposition rates
when welding with mild and low-alloy steel using direct current
electrode positive.

Two type of of covered electrodes are shown for
comparison. Deposition rates of the smaller size flux-cored wires exceed
that of the covered electrodes. The metal utilization of the flux-cored
electrode is higher. Flux-cored electrodes have a much broader current
range than covered electrodes, which increases the flexibility of the
process. The quality of the deposited weld metal produced by the
flux-cored arc welding process depends primarily on the flux-cored
electrode wire that is used. It can be expected that the deposited weld
metal will match or exceed the properties shown for the electrode used.
This assures the proper matching of base metal, flux-cored electrode
type and shielding gas.

Quality depends on the efficiency of the gas
shielding envelope, on the joint detail, on the cleanliness of the
joint, and on the skill of the welder. The quality level of of weld
metal deposited by the self-shielding type electrode wires is usually
lower than that produced by electrodes that utilize external gas
shielding.

For Additional Reading on Flux Cored Electrodes

Usability Designators

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