Welding design, procedures and inspection

This chapter contains general requirements for welding processes that may be used for the applications covered in paragraph 1-2.

3-2. Processes

a. The welding. processes covered by this design manual are as follows:

(1) Shielded metal-arc (SMAW)
(2) Gas metal-arc (GMAW)
    (a) Free-flight transfer
    (b) Pulsed-current out-of-position welding
    (c) Short circuiting
(3) Flux-cored arc welding (FCAW)
(4) Gas tungsten-arc (GTAW)
(5) Submerged-arc (SAW)
(6) Exothermic (Thermit)
(7) Arc stud (STUD)

b. Basically, in the electric welding processes, an arc is produced between an electrode and the work - piece (base metal). The arc is formed by passing a current from the electrode to the work piece through a gap. The current melts the base metal and the electrode if it is a consumable type, creating a molten pool. On solidifying, the weld is formed. An alternate method employs a nonconsumable electrode such as a tungsten rod. In this case, the weld is formed by melting and solidifying the base metal at the joint. In some instances, additional metal is required and is added to the molten pool from a filler rod.

c. Electrodes which become the deposited weld metal are available in various diameters and lengths. In welding, the molten pool must be protected from the ambient atmosphere to prevent contamination. There are three ways to do this. Two involve a flux; in one, the flux is part of the electrode, either as a coating on the wire or as the core of a hollow wire. The second method uses a granulated flux that is applied separately before welding. The third method involves a gas such as helium, argon, or carbon dioxide. In addition to shielding, the flux may function as a deoxidizer to purify the deposited metal or to form slag to protect the weld metal from oxidation. The flux may contain ionizing elements to provide smoother operation, alloying elements to provide higher strength, and iron powder to increase production rates. The selection of electrodes for a specific job can be based on the following eight factors:

(1) Base metal strength properties
(2) Base metal composition
(3) Welding position
(4) Welding current
(5) Joint design and fit-up
(6) Thickness and shape of base metal
(7) Service conditions and/or specifications
(8) Production efficiency and job conditions

The AWS publishes a group of specifications for filler metals (electrodes) and recommends the welding process for which they are to be used. In addition, AWS D1.1 describes the welding procedures to be used with the various welding processes.

d. Welding material — electrodes, welding wire, and fluxes — must produce satisfactory welds when used by a qualified welder or welding operator using qualified welding procedures. Welding materials must comply with the applicable requirements of AWS D1.1, ASME Boiler and Pressure Vessel Code, Section II, or other requirements in the contract specifications.


3-3. Shielded metal-arc (SMAW)

This is the most widely used method for general  welding application; it may also be referred to as metallic-arc, manual metal-arc, or stick-electrode welding.
a. Advantages. The SMAW process can be used for welding most structural and alloy steels. These
include low-carbon or mild steels; low-alloy, heattreatable steels; and high-alloy steels such as stainless steels. SMAW is used for joining common nickel alloys and can be used for copper and aluminum alloys. This welding process can be used in all positions — flat, vertical, horizontal, or overhead — and requires only the simplest equipment. Thus, SMAW lends itself very well to field work (fig 3-l).

b. Disadvantages. SMAW is clearly inferior to GMAW if one compares the cost of the time and materials needed to deposit the weld metal. SMAW deposits the weld more slowly than does GMAW. In addition, slag removal, unused electrode stubs, and spatter add a lot to the cost of SMAW; the latter two items account for about 44 percent of the consumed electrodes. Another potential cost is the entrapment of slag in the form of inclusions which may have to be removed.

c. Process principles. The SMAW process produces an arc between the base metal and the electrode. The electrode, put in a hand-held clamp, is struck against the base metal and withdrawn to create a gap. The molten portion of the electrode fuses into the molten pool of the base metal, producing the weld (fig 3-2). Since SMAW is a manual process,
the operator is primarily responsible for quality of the weld. Most of the melted electrode metal is transferred to the work piece; the rest is thrown free of the weld as spatter or is vaporized. Of that vaporized, some escapes into the surrounding air, becomes oxidized, and appears as smoke or fumes. The electrode used for SMAW has a special covering which serves several purposes. Part of the covering contains gas-producing compounds that, when heated, produce a gaseous envelope around the arc that displaces air and stabilizes the arc. The covering also protects the molten weld metal from contamination by air. Without this stabilization, the arc would be erratic, would often short out, and generally would be hard to control. Different gas-producing compounds are used in the coating, depending on the type of current — alternating (AC) or direct (DC). The covering also contains slag-forming materials that mix with the molten weld metal and pick up impurities from the weld metal. This cleaning action improves the quality of the weld. Most of the electrode coating does not become vaporized but instead is melted by the arc heat and forms a molten slag cover over the top of the weld bead. This molten slag cover helps to control the shape of the weld bead. It also helps to hold the molten weld metal in place during out-of-position welding (i.e., welding in the overhead, vertical, or horizontal positions). Chapters 4 and 5 discuss the numbering system, color coding, flux composition, and other data concerning welding electrodes.


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