Use of welding in fabrication of structural steel for buildings and bridges is governed by one or more of the following: American Welding Society Specifications Dl.1, Structural Welding Code, and D1.5, Bridge Welding Code, and the AISC Specification for Structural Steel Buildings, both ASD and LRFD. In addition to these specifications, welding may be governed by individual project specifications or standard specifications of agencies or groups, such as state departments of transportation.
Steels to be welded should be of a weldable grade, such as A36, A572, A588, A514, A709, A852, A913, or A992. Such steels may be welded by any of several welding processes:
shielded metal arc, submerged arc, gas metal arc, flux-cored arc, electroslag, electrogas, and stud welding. Some processes, however, are preferred for certain grades and some are excluded, as indicated in the following.
AWS Structural Welding Code and other specifications require the use of written, qualified procedures, qualified welders, the use of certain base and filler metals, and inspection.
The AWS Dl.1 code exempts from tests and qualification most of the common welded joints used in steel structures which are considered prequalified. The details of such prequalified joints are shown in AWS Dl.1 and in the AISC Steel Construction Manual ASD and Steel Construction Manual LRFD. It is advantageous to use these joints where applicable to avoid costs for additional qualification tests.
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 1â„8, 5â„32, 3â„16, 7â„32, 1â„4, and 5â„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.
Submerged-arc welding (SAW) produces coalescence by the heat of an electric arc struck between a bare metal electrode and the base metal. The weld is shielded by flux, a blanket of granular fusible material placed over the joint. Pressure is not used on the parts to be joined. Filler metal is obtained either from the electrode or from a supplementary welding rod.
The electrode is pushed through the flux to strike an arc. The heat produced by the arc melts adjoining base metal and flux. As welding progresses, the molten flux forms a protective shield above the molten metal. On cooling, this flux solidifies under the unfused flux as a brittle slag that can be removed easily. Unfused flux is recovered for future use. About 1.5 lb of flux is used for each pound of weld wire melted.
Submerged-arc welding requires high currents. The current for a given cross-sectional area of electrode often is as much as 10 times as great as that used for manual welding.
Consequently, the deposition rate and welding speeds are greater than for manual welding.
Also, deep weld penetration results. Consequently, less edge preparation of the material to be joined is required for submerged-arc welding than for manual welding. For example, material up to 3â„8 in thick can be groove-welded, without any preparation or root opening, with two passes, one from each side of the joint. Complete fusion of the joint results.
Submerged-arc welding may be done with direct or alternating current. Conventional welding power units are used but with larger capacity than those used for manual welding.
Equipment with current ratings up to 4000 A is used.
The process may be completely automatic or semiautomatic. In the semiautomatic process, the arc is moved manually. One-, two-, or three-wire electrodes can be used in automatic operation, two being the most common. Only one electrode is used in semiautomatic operation.
Submerged-arc welding is widely used for welding low-carbon steels and HSLA steels.
Though stainless steels, high-alloy steels, and nonferrous metals can be welded with this process, they are generally more readily welded with the gas-shielded metal-arc process.
Gas metal arc welding (GMAW) produces coalescence by the heat of an electric arc struck between a filler-metal electrode and base metal. Shielding is obtained from a gas or gas mixture (which may contain an inert gas) or a mixture of a gas and flux.
This process is used with direct or alternating current. Either straight or reverse polarity may be employed with direct current. Operation may be automatic or semiautomatic. In the semiautomatic process, the arc is moved manually.
As in the submerged-arc process, high current densities are used, and deep weld penetration results. Electrodes range from 0.020 to 1â„8 in diameter, with corresponding welding currents of about 75 to 650 A.
Practically all metals can be welded with this process. It is superior to other presently available processes for welding stainless steels and nonferrous metals. For these metals, argon, helium, or a mixture of the two gases is generally used for the shielding gas. For welding of carbon steels, the shielding gas may be argon, argon with oxygen, or carbon dioxide. Gas flow is regulated by a flowmeter. A rate of 25 to 50 ft3 /hr of arc time is normally used.
Flux-cored arc welding (FCAW) is similar to the GMAW process except that a fluxcontaining tubular wire is used instead of a solid wire. The process is classified into two sub-processes self-shielded and gas-shielded. Shielding is provided by decomposition of the flux material in the wire. 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 generated 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 3500F. 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 with a gap 1 to 11â„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 portion of the heat of welding is transferred into the joint base metal, preheating is accomplished without additional effort.
The electroslag process can be used to join plates from 11â„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 currently 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.
The electrogas process can be used for joining material from 1â„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.
Stud welding usually is done with a device, or gun, for establishing and controlling the arc. The operator places the stud in the chuck of the gun with the flux end protruding. Then the operator places the ceramic ferrule over this end of the stud. With timing and weldingcurrent controls set, the operator holds the gun in the welding position, with the stud pressed firmly against the welding surface, and presses the trigger. This starts the welding cycle by closing the welding-current contactor. A coil is activated to lift the stud enough to establish an arc between the stud and the welding surface. The heat melts the end of the stud and the welding surface. After the desired arc time, a control releases a spring that plunges the stud into the molten pool.
Direct current is used for stud welding. A high current is required for a very short time.
For example, welding currents up to 2500 A are used with arc time of less than 1 sec for studs up to 1 in diameter.
(O. W. Blodgett, Design of Welded Structures, The James F. Lincoln Arc Welding Foundation, Cleveland, Ohio.) See also Arts. 5.15 to 5.23.
