Shielded metal arc welding (SMAW)
produces coalescence, or fusion, by the heat of an electric arc
struck between a coated metal electrode and the material being
joined, or base metal. The electrode supplies filler metal for making
the weld, gas for shielding the molten metal, and flux for refining
this metal.
This process is commonly known also as
manual, hand, or stick welding. Pressure is not used on the parts to
be joined. When an arc is struck between the electrode and the base
metal, the intense heat forms a small molten pool on the surface of
the base metal.
The arc also decomposes the electrode
coating and melts the metal at the tip of the electrode. The electron
stream carries this metal in the form of fine globules across the gap
and deposits and mixes it into the molten pool on the surface of the
base metal. (Since deposition of electrode material does not depend
on gravity, arc welding is feasible in various positions, including
overhead.)
The decomposed coating of the electrode
forms a gas shield around the molten metal that prevents contact with
the air and absorption of impurities. In addition, the electrode
coating promotes electrical conduction across the arc, helps
stabilize the arc, adds flux, slag-forming materials, to the molten
pool to refine the metal, and provides materials for controlling the
shape of the weld.
In some cases, the coating also adds
alloying elements. As the arc moves along, the molten metal left
behind solidifies in a homogeneous deposit, or weld. The electric
power used with shielded metal arc welding may be direct or
alternating current. With direct current, either straight or reverse
polarity may be used.
For straight polarity, the base metal
is the positive pole and the electrode is the negative pole of the
welding arc. For reverse polarity, the base metal is the negative
pole and the electrode is the positive\ pole.
Electrical equipment with a
welding-current rating of 400 to 500 A is usually used for structural
steel fabrication. The power source may be portable, but the need for
moving it is minimized by connecting it to the electrode holder with
relatively long cables.
The size of electrode (core wire
diameter) depends primarily on joint detail and welding position.
Electrode sizes of 1⁄8, 5⁄32, 3⁄16, 7⁄32, 1⁄4, and 5⁄16
in are commonly used. Small-size electrodes are 14 in long, and the
larger sizes are 18 in long.
Deposition rate of the weld metal
depends primarily on welding current. Hence use of the largest
electrode and welding current consistent with good practice is
advantageous.
About 57 to 68% of the gross weight of
the welding electrodes results in weld metal. The remainder is
attributed to spatter, coating, and stub-end losses.
Shielded metal arc welding is widely
used for manual welding of low-carbon steels, such as A36, and HSLA
steels, such as A572 and A588. Though stainless steels, high-alloy
steels, and nonferrous metals can be welded with this process, they
are more readily welded with the gas metal arc process.
1 comments:
Shielded metal arc welding (SMAW), also known as manual metal arc (MMA) welding, flux shielded arc welding[1] or informally as stick welding, is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld. An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined.
Perth Electrical.
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