Page 49 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch02.qxd 9/29/05 5:01 PM Page 2.11

FABRICATION AND ERECTION*

FABRICATION AND ERECTION 2.11

In the gas-shielded process, additional shielding is provided by an externally supplied shielding gas
fed through the electrode gun. The flux performs functions similar to the electrode coatings used for
SMAW. The self-shielded process is particularly attractive for field welding because the shielding
produced by the cored wire does not blow off in normal ambient conditions and heavy gas supply
bottles do not have to be moved around the site.
Electroslag welding (ESW) produces fusion with a molten slag that melts filler metal and the
surfaces of the base metal. The weld pool is shielded by this molten slag, which moves along the
entire cross section of the joint as welding progresses. The electrically conductive slag is maintained
in a molten condition by its resistance to an electric current that flows between the electrode and the
base metal.
The process is started much like the submerged arc process by striking an electric arc beneath a
layer of granular flux. When a sufficiently thick layer of hot molten slag is formed, arc action stops.
The current then passes from the electrode to the base metal through the conductive slag. At this
point, the process ceases to be an arc welding process and becomes the electroslag process. Heat gen-
erated by resistance to flow of current through the molten slag and weld puddle is sufficient to melt
the edges at the joint and the tip of the welding electrode. The temperature of the molten metal is in
the range of 3500°F. The liquid metal coming from the filler wire and the molten base metal collect
in a pool beneath the slag and slowly solidify to form the weld. During welding, since no arc exists,
no spattering or intense arc flash occurs.
Because of the large volume of molten slag and weld metal produced in electroslag welding, the
process is generally used for welding in the vertical position. The parts to be welded are assembled
1
with a gap 1 to 1 / 4 in wide. Edges of the joint need only be cut squarely, by either machine or
flame.
Water-cooled copper shoes are attached on each side of the joint to retain the molten metal and
slag pool and to act as a mold to cool and shape the weld surfaces. The copper shoes automatically
slide upward on the base-metal surfaces as welding progresses.
Preheating of the base metal is usually not necessary in the ordinary sense. Since the major por-
tion of the heat of welding is transferred into the joint base metal, preheating is accomplished with-
out additional effort.
1
The electroslag process can be used to join plates from 1 / 4 to 18 in thick. The process cannot be
used on heat-treated steels without subsequent heat treatment. AWS and other specifications prohibit
the use of ESW for welding quenched-and-tempered steel or for welding dynamically loaded
structural members subject to tensile stresses or to reversal of stress. However, research results cur-
rently being introduced on joints with narrower gaps should lead to acceptance in cyclically loaded
structures.
Electrogas welding (EGW) is similar to electroslag welding in that both are automatic processes
suitable only for welding in the vertical position. Both utilize vertically traveling, water-cooled
shoes to contain and shape the weld surface. The electrogas process differs in that once an arc is
established between the electrode and the base metal, it is continuously maintained. The shielding
function is performed by helium, argon, carbon dioxide, or mixtures of these gases continuously fed
into the weld area. The flux core of the electrode provides deoxidizing and slagging materials for
cleansing the weld metal. The surfaces to be joined, preheated by the shielding gas, are brought to
the proper temperature for complete fusion by contact with the molten slag. The molten slag flows
toward the copper shoes and forms a protective coating between the shoes and the faces of the weld.
As weld metal is deposited, the copper shoes, forming a weld pocket of uniform depth, are carried
continuously upward.
1
The electrogas process can be used for joining material from / 2 to more than 2 in thick. The
process cannot be used on heat-treated material without subsequent heat treatment. AWS and other
specifications prohibit the use of EGW for welding quenched-and-tempered steel or for welding
dynamically loaded structural members subject to tensile stresses or to reversal of stress.
Stud welding produces coalescence by the heat of an electric arc drawn between a metal stud or
similar part and another work part. When the surfaces to be joined are properly heated, they are
brought together under pressure. Partial shielding of the weld may be obtained by surrounding the
stud with a ceramic ferrule at the weld location.

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