A composite tubular filler metal electrode consisting of a metal sheath and a core of various powdered materials, producing an extensive slag cover on the face of a weld bead. External shielding may be required.

A flux cored electrode consists of a mild steel sheath surrounded by a core of flux or alloying compound, or both. The compounds contained in flux core make up about 15%to 20% of the weight of the electrode and serve the following functions:

(1) Act as a deoxidizer or scavenger, helping to purify the metal and produce solid weld metal.

(2) Form slag to float on the molten weld pool and protect it from the atmosphere during solidification.

(3) Act as an arc stabilizer, which produces a smooth welding arc and reduces spatter.

(4) Add alloying elements to the weld metal to increase strength and provide other desirable weld metal properties.

(5)Provide for shielding gas. However, to assure weld quality, externally supplied shielding gas is often used to supplement and guarantee weld metal shielding.

Metal Transfer Characteristics. Metal transfer from flux cored electrodes is in globular form. Molten droplets appear to form on the sheath of the electrode; as one is transferred, another droplet forms in another location on the sheath. The flux material appears to be transported to the weld deposit independent of the metal transfer. The droplets are larger at low current intensity. This means there is less visible spatter, the arc appears smoother and deposition efficiency is higher when welding is done at high current intensities.

The flux content of a flux cored electrode is less than that for a coated stick electrode of comparable size, because the flux covering on a stick electrode must have a binder to make it adhere to the electrode wire and must also contain materials that aid in the extrusion process. As a result, the fluxing material on a coated stick electrode is about 24% of its weight, compared to a flux content of 15% in flux cored wire.

Advantages

Among the advantages of flux cored welding is that flux cored wires can be used at high current densities, which achieves high deposition rates and good weldability. The efficiency of flux cored welding is greater than stick electrode welding. For coated electrodes, the process efficiency is 65% to 70%, and for flux cored wires, nearly 85%.

Flux cored wire is also good in welding conditions which include narrow grooves. The minimum angle of preparation is 40 to 45″, so joints can be welded with about half of the amount of weld metal in less time. The lower total heat input also minimizes distortion.

Manufacture. Flux cored wire manufacturing is a highly specialized and precise operation. Most flux cored electrode wire is made by passing low-carbon steel strip through a contour forming roll which bends the strip into a U-shape cross section. This cross section is filled with a measured amount of granular flux cored material, after which the U-shape section passes through closing rolls that form it into a tube with tightly compressed core material. This tube, which may have assumed a variety of interior shapes, is then pulled through drawing dies to reduce its diameter and further compress the core material. After the wire has been reduced to the specified diameter, it may or may not be baked, depending on the flux content. It is then wound on 10 or 20 kg (25 or 50 lb) spools or 30 kg (60 lb) coils.

Classification of Electrodes

Figure F-11 illustrates the identification system for mild steel FCAW electrodes. Most mild steel FCAW electrodes are classified according to the requirements of ANSIIAWS A5.20, latest edition, Specification for Carbon Steel Electrodes for Flux Cored Arc Welding. Table F-2 explains the significance of the last digit of the FCAW designations.

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