Section III. ARC WELDING EQUIPMENT AND ACCESSORIES
In electric welding processes, an arc is produced between an electrode and the work piece (base metal). The arc is formed by passing a current between the electrode and the workpiece across the gap. The current melts the base metal and the electrode (if the electrode is a consumable type), creating a molten pool. On solidifying, the weld is formal. An alternate method employs a non-consumable 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.
Electrical equipment required for arc welding depends on the source from which the electric power is obtained. If the power is obtained from public utility lines, one or more of the following devices are required: transformers (of which there are several types), rectifiers, motor generators, and control equipment. If public utility power is not available, portable generators driven by gasoline or diesel engines are used.
5-19. DIRECT CURRENT ARC WELDING MACHINES
a. The direct current welding machine has a heavy duty direct current generator (fig. 5-17). The generators are made in six standardized ratings for general purposes as described below:
(1) The machines rated 150 and 200 amperes, 30 volts, are used for light shielded metal-arc welding and for gas metal-arc welding. They are also used for general purpose job shop work.
(2) The machines rated 200, 300, and 400 amperes, 40 volts, used for general welding purposes by machine or manual application.
(3) Machines rated 600 amperes, 40 volts, are used for submerged arc welding and for carbon-arc welding.
b. The electric motors must commonly used to drive the welding generators are 220/440 volts, 3 phase, 60 cycle. The gasoline and diesel engines should have a rated horsepower in excess of the rated output of the generator. This will allow for the rated overload capacity of the generator and for the power required to operate the accessories of the engine. The simple equation HP = 1.25P/746 can be used; HP is the engine horsepower and P is the generator rating in watts. For example, a 20 horsepower engine would be used to drive a welding generator with a rated 12 kilowatt output.
c. In most direct current welding machines, the generator is of the variable voltage type, and is arranged so that the voltage is automatically adjusted to the demands of the arc. However, the voltage may be set manually with a rheostat.
d. The welding current amperage is also manually adjustable, and is set by means of a selector switch or series of plug receptacles. In either case, the desired amperage is obtained by tapping into the generator field coils. When both voltage and amperage of the welding machine are adjustable, the machine is known as dual control type. Welding machines are also manufactured in which current controls are maintained by movement of the brush assembly.
e. A direct current welding machine is described in TM 5-3431-221-15, and is illustrated in figure 5-18.
f. A maintenance schedule should be set up to keep the welding machine in good operating condition. The machine should be thoroughly inspected every 3 months and blown free of dust with clean, dry, compressed air. At least once each year, the contacts of the motor starter switches and the rheostat should be cleaned and replaced if necessary. Brushes should be inspected frequently to see if they are making proper contact on the commutator, and that they move freely in the brush holders. Clean and true the commutator with sandpaper or a commutator stone if it is burned or roughened. Check the bearings twice a year. Remove all the old grease and replace it with new grease.
g. Direct current rectifier type welding machines have been designed with copper oxide, silicon, or selenium dry plates. These machines usually consist of a transform to reduce the power line voltage to the required 220/440 volts, 3 phase, 60 cycle input current; a reactor for adjustment of the current; and a rectifier to change the alternating current to direct current. Sometimes another reactor is used to reduce ripple in the output current.
5-20. ALTERNATING CURRENT ARC WELDING MACHINES
a. Most of the alternating current arc welding machines in use are of the single operator, static transformer type (fig. 5-19). For manual operation in industrial applications, machines having 200, 300, and 400 amphere ratings are the sizes in general use. Machines with 150 ampere ratings are sometimes used in light industrial, garage and job shop welding.
b. The transformers are generally equipped with arc stabilizing capacitors. Current control is provided in several ways. One such method is by means of an adjustable reactor in the output circuit of the transformer. In other types, internal reactions of the transformer are adjustable. A handwheel, usually installed on the front or the top of the machine, makes continuous adjustment of the output current, without steps, possible.
c. The screws and bearings on machines with screw type adjustments should be lubricated every 3 months. The same lubrication schedule applies to chain drives. Contacts, switches, relays, and plug and jack connections should be inspected every 3 months and cleaned or replaced as required. The primary input current at no load should be measured and checked once a year to ensure the power factor connecting capacitors are working, and that input current is as specified on the nameplate or in the manufacturer’s instruction book.
5-21. GAS TUNGSTEN-ARC WELDING (GTAW) EQUIPMENT (TIG)
a. General. In tungsten inert gas (TIG) welding, (also known as GTAW), an arc is struck between a virtually nonconsumable tungsten electrode and the workpiece. The heat of the arc causes the edges of the work to melt and flow together. Filler rod is often required to fill the joint. During the welding operation, the weld area is shielded from the atmosphere by a blanket of inert argon gas. A steady stream of argon passes through the torch, which pushes the air away from the welding area and prevents oxidation of the electrode, weld puddle, and heat affected zone.
(1) The basic equipment requirements for manual TIG welding are shown in figure 5-20. Equipment consists of the welding torch plus additional apparatus to supply electrical power, shielding gas, and a water inlet and outlet. Also, personal protective equipment should be worn to protect the operator from the arc rays during welding operations.
Different types of TIG welding equipment are available through normal supply channels. Water-cooled torches and air-cooled torches are both available. Each type carries different amperage ratings. Consult the appropriate manual covering the type torch used.
(2) Argon is supplied in steel cylinders containing approximately 330 cu ft at a pressure to 2000 psi (13,790 kPa). A single or two stage regulator may be used to control the gas flow. A specially designed regulator containing a flowmeter, as shown in figure 5-21, may be used. The flowmeter provides better adjustment via flow control than the single or two stage regulator and is calibrated in cubic feet per hour (cfh). The correct flow of argon to the torch is set by turning the adjusting screw on the regulator. The rate of flow depends on the kind and thickness of the metal to be welded.
(3) Blanketing of the weld area is provided by a steady flow of argon gas through the welding torch (fig. 5-22). Since argon is slightly more than 1-1/3 times as heavy as air, it pushes the lighter air molecules aside, effectively preventing oxidation of the welding electrode, the molten weld puddle, and the heat affected zone adjacent to the weld bead.
(4) The tremendous heat of the arc and the high current often used usually necessitate water cooling of the torch and power cable (fig. 5-22). The cooling water must be clean; otherwise, restricted or blocked passages may cause excessive overheating and damage to the equipment. It is advisable to use a suitable water strainer or filter at the water supply source. If a self-contained unit is used, such as the one used in the field (surge tank) where the cooling water is recirculated through a pump, antifreeze is required if the unit is to be used outdoors during the winter months or freezing weather. Some TIG welding torches require less than 55 psi (379 kPa) water pressure and will require a water regulator of some type. Check the operating manual for this information.
c. Nomenclature of Torch (fig. 5-22).
(1) Cap. Prevents the escape of gas from the top of the torch and locks the electrode in place.
(2) Collet. Made of copper; the electrode fits inside and when the cap is tightened, it squeezes against the electrode and leeks it in place.
(3) Gas orifice nut. Allows the gas to escape.
(4) Gas nozzle. Directs the flew of shielding gas onto the weld puddle. Two types of nozzles are used; the one for light duty welding is made of a ceramic material, and the one for heavy duty welding is a copper water-cooled nozzle.
(5) Hoses. Three plastic hoses, connected inside the torch handle, carry water, gas, and the electrode power cable.
5-22. GAS METAL-ARC WELDING (GMAW) EQUIPMENT
a. General. GMAW is most commonly referred to as “MIG” welding, and the following text will use “MIG” or “MIG welding” when referring to GMAW. MIG welding is a process in which a consumable, bare wire electrode is fed into a weld at a controlled rate of speed, while a blanket of inert argon gas shields the weld zone from atmospheric contamination. In addition to the three basic types of metal transfer which characterize the GMAW process, there are several variations of significance.
(1) Pulsed spray welding. Pulsed spray welding is a variation of the MIG welding process that is capable of all–position welding at higher energy levels than short circuiting arc welding. The power source provides two current levels; a steady “background” level, which is too low to produce spray transfer; and a “pulsed peak” current, which is superimposed upon the background current at a regulated interval. The pulse peak is well above the transition current, and usually one drop is transferred during each pulse. The combination of the two levels of current produces a steady arc with axial spray transfer at effective welding currents below those required for conventional spray arc welding. Because the heat input is lower, this variation in operation is capable of welding thinner sections than are practical with the conventional spray transfer.
(2) Arc spot welding. Gas metal arc spot welding is a method of joining similar to resistance spot welding and riveting. A variation of continuous gas metal arc welding, the process fuses two pieces of sheet metal together by penetrating entirely through one piece into the other. No joint preparation is required other than cleaning of the overlap areas. The welding gun remains stationary while a spot weld is being made. Mild steel, stainless steel, and aluminum are commonly joined by this method.
(3) Electrogas welding. The electrogas (EG) variation of the MIG welding process is a fully automatic, high deposition rate method for the welding of butt, corner, and T-joints in the vertical position. The eletrogas variation essentially combines the mechanical features of electroslag welding (ESW) with the MIG welding process. Water-coded copper shoes span the gap between the pieces being welded to form a cavity for the molten metal. A carriage is mounted on a vertical column; this combination provides both vertical and horizontal movement. Welding head, controls, and electrode spools are mounted on the carriage. Both the carriage and the copper shoes move vertically upwards as welding progresses. The welding head may also be oscillated to provide uniform distribution of heat and filler metal. This method is capable of welding metal sections of from 1/2 in. (13 mm) to more than 2 in. (5.08 an) in thickness in a single pass. Deposition rates of 35 to 46 lb (16 to 21 kg) per hour per electrode can be achieved.
b. MIG Equipment.
Different types of MIG welding equipment are available through normal supply channels. Manuals for each type must be consulted prior to welding operations.
(1) The MIG welding unit is designed for manual welding with small diameter wire electrodes, using a spool-on-gun torch. The unit consists of a torch (fig. 5-23), a voltage control box, and a welding contractor (fig. 5-24). The torch handle contains a complete motor and gear reduction unit that pulls the welding wire electrode from a 4 in. (102 mm) diameter spool containing 1 lb (0.5 kg) of wire electrode mounted in the rear of the torch.
(2) Three basic sizes of wire electrode maybe used: 3/32 in. (2.38 mm), 3/64 in. (1.19 mm), and 1/16 in. (1.59 mm). Many types of metal may be welded provided the welding wire electrode is of the same composition as the base metal.
(3) The unit is designed for use with an ac-dc conventional, constant-current welding power supply. Gasoline engine-driven arc welding machines issued to field units may be used as both a power source and a welding source.
c. Nomenclature of Torch.
(1) Contact tube (fig. 5-23). This tube is made of copper and has a hole in the center of the tube that is from 0.01 to 0.02 in. (0.25 to 0.51 mm) larger than the size of the wire electrode being used. The contact tube and the inlet and outlet guide bushings must be charged when the size of the wire electrode is changed. The contact tube transfers power from the electrode cable to the welding wire electrode. An insulated lock screw is provided which secures the contact tube in the torch.
(2) Nozzle and holder (fig. 5-23). The nozzle is made of copper to dissipate heat and is chrome-plated to reflect the heat. The holder is made of stainless steel and is connected to an insulating material which prevents an arc from being drawn between the nozzle and the ground in case the gun canes in contact with the work.
(3) Inlet and outlet guide bushings (fig. 5-23). The bushings are made of nylon for long wear. They must be changed to suit the wire electrode size when the electrode wire is changed.
(4) Pressure roll assembly (fig. 5-23). This is a smooth roller, under spring tension, which pushes the wire electrode against the feed roll and allows the wire to be pulled from the spool. A thumbscrew applies tension as required.
(5) Motor (fig. 5-23). When the inch button is depressed, the current for running the motor comes from the 110 V ac-dc source, and the rotor pulls the wire electrode from the spool before starting the welding operation. When the trigger is depressed, the actual welding operation starts and the motor pulls the electrode from the spool at the required rate of feed. The current for this rotor is supplied by the welding generator.
(6) Spool enclosure assembly (fig. 5-23). This assembly is made of plastic which prevents arc spatter from jamming the wire electrode on the spool. A small window allows the operator to visually check the amount of wire electrode remaining on the spool.
If for any reason the wire electrode stops feeding, a burn-back will result. With the trigger depressed, the welding contactor is closed, thereby allowing the welding current to flow through the contact tube. As long as the wire electrode advances through the tube, an arc will be drawn at the end of the wire electrode. Should the wire electrode stop feeding while the trigger is still being depressed, the arc will then form at the end of the contact tube, causing it to melt off. This is called burn-back.
(7) Welding contactor (fig. 5-24). The positive cable from the dc welding generator is connected to a cable coming out of the welding contactor, and the ground cable is connected to the workpiece. The electrode cable and the welding contactor cable are connected between the welding contactor and voltage control box as shown.
(8) Argon gas hose (fig. 5-24). This hose is connected from the voltage control box to the argon gas regulator on the argon cylinder.
(9) Electrode cable (fig. 5-24). The electrode cable enters through the welding current relay and connects into the argon supply line. Both then go out of the voltage control box and into the torch in one line.
(10) Voltage pickup cable (fig. 5-24). This cable must be attached to the ground cable at the workpiece. This supplies the current to the motor during welding when the trigger is depressed.
(11) Torch switch and grounding cables (fig. 5-24). The torch switch cable is connected into the voltage control box, and the torch grounding cable is connected to the case of the voltage control box.
5-23. OPERATING THE MIG
a. Starting to Weld.
(1) Press the inch button and allow enough wire electrode to emerge from the nozzle until 1/2 in. (13 mm) protrudes beyond the end of the nozzle. With the main line switch “ON” and the argon gas and power sources adjusted properly, the operator may begin to weld.
(2) When welding in the open air, a protective shield must be installed to prevent the argon gas from being blown away from the weld zone and allowing the weld to become contaminate.
(3) Press the torch trigger. This sends current down the torch switch cable and through the contactor cable, closing the contactor.
(4) When the contactor closes, the welding circuit from the generator to the welding torch is completed.
(5) At the same time the contactor closes, the argon gas solenoid valve opens, allowing a flow of argon gas to pass out of the nozzle to shield the weld zone.
(6) Lower the welding helmet and touch the end of the wire electrode to the workpiece. The gun is held at a 90 degree angle to the work but pointed at a 10 degree angle toward the line of travel.
To prevent overloading the torch motor when stopping the arc, release the trigger; never snap the arc out by raising the torch without first releasing the trigger.
(7) Welding will continue as long as the arc is maintained and the trigger is depressed.
b. Setting the Wire Electrode Feed.
(1) A dial on the front of the voltage control box, labeled WELDING CONTROL, is used to regulate the speed of the wire electrode feed.
(2) To increase the speed of the wire electrode being fed from the spool, turn the dial counterclockwise. This decreases the amount of resistance across the arc and allows the motor to turn faster. Turning the dial clockwise will increase the amount of resistance, thereby decreasing the speed of wire electrode being fed from the spool.
(3) At the instant that the wire electrode touches the work, between 50 and volts dc is generated. This voltage is picked up by the voltage pickup cable shunted back through the voltage control box into a resistor. There it is reduced to the correct voltage (24 V dc) and sent to the torch motor.
(1) Two 10-ampere fuses, located at the front of the voltage control box, protect and control the electrical circuit within the voltage control box.
(2) A 1-ampere fuse, located on the front of the voltage control box, protects and controls the torch motor.
d. Installing the Wire Electrode.
(1) Open the spool enclosure cover assembly, brake, and pressure roll assembly (fig. 5-23).
(2) Unroll the straighten 6 in. (152 mm) of wire electrode from the top of the spool.
(3) Feed this straightened end of the wire electrode into the inlet and outlet bushings; then place spool onto the mounting shaft.
(4) Close the pressure roller and secure it in place. Press the inch button, feeding the wire electrode until there is 1/2 in. (13 mm) protruding beyond the end of the nozzle.
e. Setting the Argon Gas Pressure.
(1) Flip the argon switch on the front of the voltage control panel to the MANUAL position.
(2) Turn on the argon gas cylinder valve and set the pressure on the regulator.
(3) When the proper pressure is set on the regulator, flip the argon switch to the AUTOMATIC POSITION.
(4) When in the MANUAL position, the argon gas continues to flow. When in the AUTOMATIC position, the argon gas flows only when the torch trigger is depressed, and stops flowing when the torch trigger is released.
f. Generator Polarity. The generator is set on reverse polarity. When set on straight polarity, the torch motor will run in reverse, withdrawing the wire electrode and causing a severe burn-back.
g. Reclaiming Burned-Back Contact Tubes. When the contact tubes are new, they are 5-3/8 in. (137 mm) long. When burn-backs occur, a maximum of 3/8 in. (9.5 mm) may be filed off. File a flat spot on top of the guide tube, place a drill pilot on the contact tube, then drill out the contact tube. For a 3/64 in. (1.2 mm) contact tube, use a No. 46 or 47 drill bit.
h. Preventive Maintenance.
(1) Keep all weld spatter cleaned out of the inside of the torch. Welding in the vertical or overhead positions will cause spatter to fall down inside the torch nozzle holder and restrict the passage of the argon gas. Keep all hose connections tight.
(2) To replace the feed roll, remove the nameplate on top of the torch, the flathead screw and retainer from the feed roll mounting shaft, and the contact ring and feed roll. Place a new feed roll on the feed roll mounting shaft, making certain that the pins protruding from the shaft engage the slots in the feed roll. Reassemble the contact ring and nameplate.
5-24. OTHER WELDING EQUIPMENT
a. Cables. Two welding cables of sufficient current carrying capacity with heavy, tough, resilient rubber jackets are required. One of the cables should be composed of fine copper strands to permit as much flexibility as the size of the cable will allow. One end of the less flexible cable is attached to the ground lug or positive side of the direct current welding machine; the other end to the work table or other suitable ground. One end of the flexible cable is attached to the electrode holder and the other end to the negative side of a direct current welding machine for straight polarity. Most machines are equipped with a polarity switch which is used to change the polarity without interchanging the welding cables at the terminals of the machine. For those machines not equipped with polarity switches, for reverse polarity, the cables are reversed at the machine.
b. Electrode Holders. An electrode holder is an insulated clamping device for holding the electrode during the welding operation. The design of the holder depends on the welding process for which it is used, as explained below.
(1) Metal-arc electrode holder. This is an insulated clamp in which a metal electrode can be held at any desired angle. The jaws can be opened by means of a lever held in place by a spring (fig. 5-25).
(2) Atomic hydrogen torch. This electrode holder or torch consists of two tubes in an insulated handle, through which both hydrogen gas and electric current flow. The hydrogen is supplied to a tube in the rear of the handle from which it is led into the two current carrying tubes by means of a manifold. One of the two electrode holders is movable, and the gap between this and the other holder is adjusted by means of a trigger on the handle (fig. 5-26).
(3) Carbon-arc electrode holder. This holder is manufactured in three specific types. One type holds two electrodes and is similar in design to the atomic hydrogen torch, but has no gas tubes; a second equipped with a heat shield; the third type is watercooled.
(1) Chipping hammer and wire brush. A chipping hammer is required to loosen scale, oxides and slag. A wire brush is used to clean each weld bead before further welding. Figure 5-27 shows a chipping hammer with an attachable wire brush.
(2) Welding table. A welding table should be of all-steel construction. A container for electrodes with an insulated hook to hold the electrode holder when not in use should be provided. A typical design for a welding table is shown in figure 5-28.
(3) Clamps and backup bars. Workpieces for welding should be clamped in position with C-clamps or other clamp brackets. Blocks, strips, or bars of copper or cast iron should be available for use as backup bars in welding light sheet aluminum and in making certain types of joints. Carbon blocks, fire clay, or other fire-resistant material should also be available. These materials are used to form molds which hold molten metal within given limits when building up sections. A mixture of water, glass, and fire clay or carbon powder can be used for making molds.
d. Goggles. Goggles with green lenses shaped to cover the eye orbit should be available to provide glare protection for personnel in and around the vicinity of welding and cutting operations (other than the welder).
These goggles should not be used in actual welding operations.
5-25. ELECTRODES AND THEIR USE
a. General. When molten metal is exposed to air, it absorbs oxygen and nitrogen, and becomes brittle or is otherwise adversely affected. A slag cover is needed to protect molten or solidifying weld metal from the atmosphere. This cover can be obtained from the electrode coating, which protects the metal from damage, stabilizes the arc, and improves the weld in the ways described below.
b. Types of Electrodes. The metal-arc electrodes may be grouped and classified as bare electrodes, light coated electrodes, and shielding arc or heavy coated electrodes. The type used depends on the specific properties required in the weld deposited. These include corrosion resistance, ductility, high tensile strength, the type of base metal to be welded; the position of the weld (i. e., flat, horizontal, vertical, or overhead); and the type of current and polarity required.
c. Classification of Electrodes. The American Welding Society’s classification number series has been adopted by the welding industry. The electrode identification system for steel arc welding is set up as follows:
(1) E indicates electrode for arc welding.
(2) The first two (or three) digits indicate tensile strength (the resistance of the material to forces trying to pull it apart) in thousands of pounds per square inch of the deposited metal.
(3) The third (or fourth) digit indicates the position of the weld. 0 indicates the classification is not used; 1 is for all positions; 2 is for flat and horizontal positions only; 3 is for flat position only.
(4) The fourth (or fifth) digit indicates the type of electrode coating and the type of power supply used; alternating or direct current, straight or reverse polarity.
(5) The types of coating, welding current, and polarity position designated by the fourth (or fifth) identifying digit of the electrode classification are as listed in table 5-4.
(6) The number E6010 indicates an arc welding electrode with a minimum stress relieved tensile strength of 60,000 psi; is used in all positions; and reverse polarity direct current is required.
(3) The electrode identification system for stainless steel arc welding is set up as follows:
(a) E indicates electrode for arc welding.
(b) The first three digits indicated the American Iron and Steel type of stainless steel.
(c) The last two digits indicate the current and position used.
(d) The number E-308-16 by this system indicates stainless steel Institute type 308; used in all positions; with alternating or reverse polarity direct current.
d. Bare Electrodes. Bare electrodes are made of wire compositions required for specific applications. These electrodes have no coatings other than those required in wire drawing. These wire drawing coatings have some slight stabilizing effect on the arc but are otherwise of no consequence. Bare electrodes are used for welding manganese steel and other purposes where a coated electrode is not required or is undesirable. A diagram of the transfer of metal across the arc of a bare electrode is shown in figure 5-29.
e. Light Coated Electrodes.
(1) Light coated electrodes have a definite composition. A light coating has been applied on the surface by washing, dipping, brushing, spraying, tumbling, or wiping to improve the stability and characteristics of the arc stream. They are listed under the E45 series in the electrode identification system.
(2) The coating generally serves the following functions:
(a) It dissolves or reduces impurities such as oxides, sulfur, and phosphorus.
(b) It changes the surface tension of the molten metal so that the globules of metal leaving the end of the electrode are smaller and more frequent, making the flow of molten metal more uniform.
(c) It increases the arc stability by introducing materials readily ionized (i. e., changed into small particles with an electric charge) into the arc stream.
(3) Some of the light coatings may produce a slag, but it is quite thin and does not act in the same manner as the shielded arc electrode type slag. The arc action obtained with light coated electrodes is shown in figure 5-30.
f. Shielded Arc or Heavy Coated Electrodes. Shielded arc or heavy coated electrodes have a definite composition on which a coating has been applied by dipping or extrusion. The electrodes are manufactured in three general types: those with cellulose coatings; those with mineral coatings; and those with coatings of combinations of mineral and cellulose. The cellulose coatings are composed of soluble cotton or other forms of cellulose with small amounts of potassium, sodium, or titanium, and in some cases added minerals. The mineral coatings consist of sodium silicate, metallic oxides, clay, and other inorganic substances or combinations thereof. Cellulose coated electrodes protect the molten metal with a gaseous zone around the arc as well as slag deposit over the weld zone. The mineral coated electrode forms a slag deposit only. The shielded arc or heavy coated electrodes are used for welding steels, cast iron, and hard surfacing. The arc action obtained with the shielded arc or heavy coated electrode is shown in figure 5-31.
g. Functions of Shielded Arc or Heavy Coated Electrodes.
(1) These electrodes produce a reducing gas shield around the arc which prevents atmospheric oxygen or nitrogen from contaminating the weld metal. The oxygen would readily combine with the molten metal, removing alloying elements and causing porosity. The nitrogen would cause brittleness, low ductility, and in some cases, low strength and poor resistance to corrosion.
(2) The electrodes reduce impurities such as oxides, sulfur, and phosphorus so that these impurities will not impair the weld deposit.
(3) They provide substances to the arc which increase its stability and eliminate wide fluctuations in the voltage so that the arc can be maintained without excessive spattering.
(4) By reducing the attractive force between the molten metal and the end of the electrode, or by reducing the surface tension of the molten metal, the vaporized and melted coating causes the molten metal at the end of the electrode to break up into fine, small particles.
(5) The coatings contain silicates which will form a slag over the molten weld and base metal. Since the slag solidifies at a relatively slow rate, it holds the heat and allows the underlying metal to cool and slowly solidify. This slow solidification of the metal eliminates the entrapment of gases within the weld and permits solid impurities to float to the surface. Slow cooling also has an annealing effect on the weld deposit.
(6) The physical characteristics of the weld deposit are modified by incorporating alloying materials in the electrode coating. The fluxing action of the slag will also produce weld metal of better quality and permit welding at higher speeds.
(7) The coating insulates the sides of the electrode so that the arc is concentrated into a confined area. This facilitates welding in a deep U or V groove.
(8) The coating produces a cup, cone, or sheath (fig. 5-31) at the tip of the electrode which acts as a shield, concentrates and directs the arc, reduces heat losses and increases the temperature at the end of the electrode.
h. Storing Electrodes. Electrodes must be kept dry. Moisture destroys the desirable characteristics of the coating and may cause excessive spattering and lead to the formation of cracks in the welded area. Electrodes exposed to damp air for more than two or three hours should be dried by heating in a suitable oven (fig. 5-32) for two hours at 500°F (260°C). After they have dried, they should be stored in a moisture proof container. Bending the electrode can cause the coating to break loose from the core wire. Electrodes should not be used if the core wire is exposed.
i. Tungsten Electrodes.
(1) Nonconsumable electrodes for gas tungsten-arc (TIG) welding are of three types: pure tungsten, tungsten containing 1 or 2 percent thorium, and tungsten containing 0.3 to 0.5 percent zirconium.
(2) Tungsten electrodes can be identified as to type by painted end marks as follows.
(a) Green — pure tungsten.
(b) Yellow — 1 percent thorium.
(c) Red — 2 percent thorium.
(d) Brown — 0.3 to 0.5 percent zirconium.
(3) Pure tungsten (99. 5 percent tungsten) electrodes are generally used on less critical welding operations than the tungstens which are alloyed. This type of electrode has a relatively low current-carrying capacity and a low resistance to contamination.
(4) Thoriated tungsten electrodes (1 or 2 percent thorium) are superior to pure tungsten electrodes because of their higher electron output, better arc-starting and arc stability, high current-carrying capacity, longer life, and greater resistance to contamination.
(5) Tungsten electrodes containing 0.3 to 0.5 percent zirconium generally fall between pure tungsten electrodes and thoriated tungsten electrodes in terms of performance. There is, however, some indication of better performance in certain types of welding using ac power.
(6) Finer arc control can be obtained if the tungsten alloyed electrode is ground to a point (fig. 5-33). When electrodes are not grounded, they must be operated at maximum current density to obtain reasonable arc stability. Tungsten electrode points are difficult to maintain if standard direct current equipment is used as a power source and touch-starting of the arc is standard practice. Maintenance of electrode shape and the reduction of tungsten inclusions in the weld can best be accomplished by superimposing a high-frequency current on the regular welding current. Tungsten electrodes alloyed with thorium and zirconium retain their shape longer when touch-starting is used.
(7) The electrode extension beyond the gas cup is determined by the type of joint being welded. For example, an extension beyond the gas cup of 1/8 in. (3.2 mm) might be used for butt joints in light gage material, while an extension of approximately 1/4 to 1/2 in. (6.4 to 12.7 mm) might be necessary on some fillet welds. The tungsten electrode of torch should be inclined slightly and the filler metal added carefully to avoid contact with the tungsten. This will prevent contamination of the electrode. If contamination does occur, the electrode must be removed, reground, and replaced in the torch.
j. Direct Current Welding. In direct current welding, the welding current circuit may be hooked up as either straight polarity (dcsp) or reverse polarity (dcrp). The polarity recommended for use with a specific type of electrode is established by the manufacturer.
(1) For dcsp, the welding machine connections are electrode negative and workpiece positive (fig. 5-34); electron flow is from electrode to workpiece. For dcrp, the welding machine connections are electrode positive and workpiece negative; electron flow is from workpiece to electrode.
(2) For both current polarities, the greatest part of the heating effect occurs at the positive side of the arc. The workpiece is dcsp and the electrode is dcrp. Thus, for any given welding current, dcrp requires a larger diameter electrode than does dcsp. For example, a 1/16-in. (1.6-mm) diameter pure tungsten electrode can handle 125 amperes of welding current under straight polarity conditions. If the polarity were reversed, however, this amount of current would melt off the electrode and contaminate the weld metal. Hence, a 1/4-in. (6.4-mm) diameter pure tungsten electrode is required to handle 125 amperes dcrp satisfactorily and safely. However, when heavy coated electrodes are used, the composition of the coating and the gases it produces may alter the heat conditions. This will produce greater heat on the negative side of the arc. One type of coating may provide the most desirable heat balance with straight polarity, while another type of coating on the same electrode may provide a more desirable heat balance with reverse polarity.
(3) The different heating effects influence not only the welding action, but also the shape of the weld obtained. DCSP welding will produce a wide, relatively shallow weld (fig. 5-35). DCRP welding, because of the larger electrode diameter and lower currents generally employed, gives a narrow, deep weld.
(4) One other effect of dcrp welding is the so-called plate cleaning effect. This surface cleaning action is caused either by the electrons leaving the plate or by the impact of the gas ions striking the plate, which tends to break up the surface oxides, and dirt usually present.
(5) In general, straight polarity is used with all mild steel, bare, or light coated electrodes. Reverse polarity is used in the welding of non-ferrous metals such as aluminum, bronze, monel, and nickel. Reverse polarity is also used with sane types of electrodes for making vertical and overhead welds.
(6) The proper polarity for a given electrode can be recognized by the sharp, cracking sound of the arc. The wrong polarity will cause the arc to emit a hissing sound, and the welding bead will be difficult to control.
k. Alternating Current Welding.
(1) Alternating current welding, theoretically, is a combination of dcsp and dcrp welding. This can be best explained by showing the three current waves visually. As shown in figure 5-36, half of each complete alternating current (ac) cycle is dcsp, the other half is dcrp.
(2) Moisture, oxides, scale, etc., on the surface of the plate tend, partially or completely, to prevent the flow of current in the reverse polarity direction. This is called rectification. For example, in no current at all flowed in the reverse polarity direction, the current wave would be similar to figure 5-37.
(3) To prevent rectification from occurring, it is common practice to introduce into the welding current an additional high-voltage, high-frequency, low-power current. This high-frequency current jumps the gap between the electrode and the workpiece and pierces the oxide film, thereby forming a path for the welding current to follow. Superimposing this high-voltage, high-frequency current on the welding current gives the following advantages:
(a) The arc may be started without touching the electrode to the workpiece.
(b) Better arc stability is obtained.
(c) A longer arc is possible. This is particularly useful in surfacing and hardfacing operations.
(d) Welding electrodes have longer life.
(e) The use of wider current range for a specific diameter electrode is possible.
(4) A typical weld contour produced with high-frequency stabilized ac is shown in figure 5-38, together with both dcsp and dcrp welds for comparison.
l. Direct Current Arc Welding Electrodes.
(1) The manufacturer’s recommendations should be followed when a specific type of electrode is being used. In general, direct current shielded arc electrodes are designed either for reverse polarity (electrode positive) or for straight polarity (electrode negative), or both. Many, but not all, of the direct current electrodes can be used with alternating current. Direct current is preferred for many types of covered, nonferrous, bare and alloy steel electrodes. Recommendations from the manufacturer also include the type of base metal for which given electrodes are suited, corrections for poor fit-ups, and other specific conditions.
(2) In most cases, straight polarity electrodes will provide less penetration than reverse polarity electrodes, and for this reason will permit greater welding speed. Good penetration can be obtained from either type with proper welding conditions and arc manipulation.
m. Alternating Current Arc Welding Electrodes.
(1) Coated electrodes which can be used with either direct or alternating current are available. Alternating current is more desirable while welding in restricted areas or when using the high currents required for thick sections because it reduces arc blow. Arc blow causes blowholes, slag inclusions, and lack of fusion in the weld.
(2) Alternating current is used in atomic hydrogen welding and in those carbon arc processes that require the use of two carbon electrodes. It permits a uniform rate of welding and electrode consumption. In carbon-arc processes where one carbon electrode is used, direct current straight polarity is recommended, because the electrode will be consumed at a lower rate.
n. Electrode Defects and Their Effects.
(1) If certain elements or oxides are present in electrode coatings, the arc stability will be affected. In bare electrodes, the composition and uniformity of the wire is an important factor in the control of arc stability. Thin or heavy coatings on the electrodes will riot completely remove the effects of defective wire.
(2) Aluminum or aluminum oxide (even when present in 0.01 percent), silicon, silicon dioxide, and iron sulphate unstable. Iron oxide, manganese oxide, calcium oxide, and stabilize the arc.
(3) When phosphorus or sulfur are present in the electrode in excess of 0.04 percent, they will impair the weld metal because they are transferred from the electrode to the molten metal with very little loss. Phosphorus causes grain growth, brittleness, and “cold shortness” (i. e., brittle when below red heat) in the weld. These defects increase in magnitude as the carbon content of the steel increases. Sulfur acts as a slag, breaks up the soundness of the weld metal, and causes “hot shortness” (i. e., brittle when above red heat). Sulfur is particularly harmful to bare, low-carbon steel electrodes with a low manganese content. Manganese promotes the formation of sound welds.
(4) If the heat treatment, given the wire core of an electrode, is not uniform, the electrode will produce welds inferior to those produced with an electrode of the same composition that has been properly heat treatedNo tags for this post.