Manufacturing Processes phần 4 pdf

wire feed speed (WFS), since electrode extension, polarity, and electrode

diameter will also affect amperage. For a fixed wire feed speed, a shorter

electrical stick-out will result in higher amperages. If procedures are set

based on the wire feed speed, the resulting amperage verifies that proper

electrode extensions are being used. If amperage is used to set welding

procedures, an inaccurate electrode extension may go undetected.

Self-Shielded and Gas-Shielded FCAW Within the category of

FCAW, there are two specific subsets: self-shielded flux core arc welding

(FCAW-S) (Fig. 13.3.4) and gas-shielded flux core arc welding (FCAW-G)

(Fig. 13.3.5). Self-shielded flux cored electrodes require no external

shielding gas. The entire shielding system results from the flux ingredients

contained in the tubular electrode. The gas-shielded variety of flux cored

electrode utilizes, in addition to the flux core, an externally supplied shield￾ing gas. Often, CO2 is used, although other mixtures may be used.

Both these subsets of FCAW are capable of delivering weld deposits

featuring consistency, high quality, and excellent mechanical proper￾ties. Self-shielded flux cored electrodes are ideal for field welding oper￾ations, for since no externally supplied shielding gas is required, the

process may be used in high winds without adversely affecting the qual￾ity of the weld metal deposited. With any gas-shielded processes, wind

shields must be erected to preclude wind interference with the gas

shield. Many fabricators with large shops have found that self-shielded

flux core welding offers advantages when the shop door can be left

open or fans are used to improve ventilation.

Gas-shielded flux cored electrodes tend to be more versatile than

self-shielded flux cored electrodes and, in general, provide better arc

action. Operator acceptance is usually higher. The gas shield must be

protected from winds and drafts, but this is not difficult for most shop

fabrication. Weld appearance is very good, and quality is outstanding.

Higher-strength gas-shielded FCAW electrodes are available, but cur￾rent practice limits self-shielded FCAW deposits to a tensile strength of

80 ksi or less.

Submerged Arc Welding (SAW)

Submerged arc welding differs from other arc welding processes in that

a blanket of fusible granular flux is used to shield the arc and molten

metal (Fig. 13.3.6). The arc is struck between the workpiece and a bare￾wire electrode, the tip of which is submerged in the flux. The arc is

completely covered by the flux and it is not visible; thus the weld is

made without the flash, spatter, and sparks that characterize the open-arc

processes. The flux used develops very little smoke or visible fumes.

ARC WELDING 13-31

Fig. 13.3.3 FCAW and GMAW equipment.

Fig. 13.3.4 Self-shielded FCAW.

Fig. 13.3.5 Gas-shielded FCAW.

Fig. 13.3.6 SAW process.

Typically, the process is operated fully automatically, although semi￾automatic operation is possible. The electrode is fed mechanically to the

welding gun, head, or heads. In semiautomatic welding, the welder moves

the gun, usually equipped with a flux-feeding device, along the joint.

Flux may be fed by gravity flow from a small hopper atop the torch and

then through a nozzle concentric with the electrode, or through a nozzle

tube connected to an air-pressurized flux tank. Flux may also be applied

in advance of the welding operation or ahead of the arc from a hopper run

along the joint. Many fully automatic installations are equipped with a

vacuum system to capture unfused flux left after welding; the captured,

unused flux is recycled for reuse.

During welding, arc heat melts some of the flux along with the tip

of the electrode. The electrode tip and the welding zone are always

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