Gtaw and Gmaw Equipment and Supplies: 7.2 Arc Welding Power Sources For GTAW
Gtaw and Gmaw Equipment and Supplies: 7.2 Arc Welding Power Sources For GTAW
Gtaw and Gmaw Equipment and Supplies: 7.2 Arc Welding Power Sources For GTAW
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Welding rods. Optional accessories. o A water cooling system with hoses for heavy duty welding operations. o Foot or finger operated rheostat. o Arc timers. Figure 7-1 shows a schematic drawing of a gas tungsten arc welding outfit. The booth and exhaust system are not shown in this illustration. Refer also to Figure 4-2.
Learning Objectives
After studying this chapter, you will be able to: 1 Identify and describe the function of each component of a GTAW, GMAW, and FCAW station. 1 Name the various types of shielding gases used in GTAW, describe their characteristics, and evaluate their effectiveness. 1 Name the various types of shielding gases used in GMAW, describe their characteristics, and evaluate their effectiveness. 1 Identify and specify the type of electrode used for GTAW, referring to the tables provided in the book and using the AWS electrode classification system. 1 Identify and specify the various electrode wires used for GMAW and FCAW, using the tables provided in the book and the AWS electrode classification system.
able electrode is the filler metal. The weld area is protected by a shielding gas. This process is used in production, in welding shops, and in automobile body repair shops. It is capable of making excellent welds almost continuously. The welding skills required for this process are not as great as those required for some manual welding processes. In shop terms, this process is also known as metal inert gas or MIG welding. Refer to Heading 4.3 for a basic description of the GMAW process. Flux cored arc welding (FCAW) is similar in most respects to the gas metal arc welding (GMAW) process. The difference is in the electrode wire used. FCAW uses a hollow-core electrode that contains flux and alloying materials. The flux core provides a gaseous shield around the arc. FCAW may also use a shielding gas provided through the gun, similar to the GMAW process. Refer to Heading 4.4 for a basic description of the FCAW process. The equipment and supplies used for each process will be explained in this chapter. Chapter 8 will cover gas tungsten arc welding techniques and principles, while Chapter 9 will cover gas metal arc welding and flux cored arc welding principles and techniques. Studying these chapters, coupled with actual welding practice, will build the skills and techniques you need to master these welding processes.
amperage will change slightly as the voltage changes. This allows a welder to vary the amperage by changing the arc length. Newer power supply designs have nearly a vertical line on the volt-ampere curve. This is a true constant current power source, because the amperage is constant even though the voltage changes. This is made possible by using electronics to control the power supply. These electronically controlled power sources are more efficient, more responsive, and more repeatable. They also produce an excellent pulsed welding current. Figure 7-2 shows volt-ampere curves for drooping and constant current welding power sources. Two drooping curves are shown in this illustration. Curve 1 is for an 80V open circuit voltage set for a 150A maximum welding current. A 5V change in the closed circuit (welding) voltage from 20V to 25V, represents a 25% change in voltage with only a 5% change in the current. Curve 2 represents a curve for an 80V open circuit voltage set for 50A maximum welding current. A 25% change in voltage (from 20V to 25V) will only change the amperage about 2A, or 4%. The amperage is therefore considered to be relatively constant with large changes in voltage. Figure 7-2 also shows two curves (3 and 4), for an electronically controlled constant current power source. The same voltage change from 20V to 25V produces almost zero change in amperage.
Gas tungsten arc welding (GTAW) is a welding process in which an arc is struck between a nonconsumable tungsten electrode and the metal workpiece. The weld area is shielded by inert (chemically inactive) gas to prevent contamination. Filler metal may or may not be added to the weld. It is possible to join most weldable metals with GTAW. This process is excellent for welding root passes in heavy metal sections. GTAW is also known, in shop terms, as tungsten inert gas or TIG welding. Heading 4.2 describes the basics of GTAW. Gas metal arc welding (GMAW) is a welding process in which an arc is struck between a consumable metal electrode and the metal workpiece. The consumable electrode wire is fed to the welding gun from a large spool that may hold several hundred feet (meters) of wire. The consum-
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the current flowing, there would be no arc during the electrode positive part of the cycle. When no current flows, the arc is said to be rectified. This is not good for the welding machine or the weld. To reduce or eliminate such rectification, the following methods are suggested: Use a welding machine with a higher open circuit voltage of about 100V (rms). Welding machines for manual GTAW have a maximum open circuit voltage of 80V. Discharge capacitors at the start of the electrode positive half of each cycle. Put a high-frequency voltage supply, which produces several thousand volts, in series with the main transformer. Use a power source that produces a square-wave output. Almost all GTAW machines currently produced use high-frequency or have a square-wave output. The construction, operation, and control of ac, dc and inverter power sources are covered in Headings 5.1, 5.2, and 5.2.2. Review this basic information for a more complete understanding of welding power sources.
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Figure 7-3. An ac/dc, single-phase power source for GTAW (Hobart Brothers Co.)
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Figure 7-2. The volt-ampere curves for a drooper type and constant current power source. Curves 1 and 2 are drooper curves which show a 2A and an 8A change with a change of 5V. Curves 3 and 4 are true constant current curves. They show a nearly constant amperage with voltage changes. The power source for GTAW may be an ac or a dc welding machine. Most often, the machine will produce both ac and dc constant currents. Figures 7-3 and 7-4 show GTAW power sources. A direct current power source may be equipped with a device that produces a high-frequency voltage and feeds it into the welding circuit. High-frequency voltage, when used in a dc circuit, is used only for starting the arc. Once the dc welding arc is stabilized, the superimposed highfrequency starting voltage is automatically stopped. Highfrequency voltage is produced by an oscillator or a similar device. Several thousand volts are produced at a frequency of several million cycles per second or megahertz. The current in this high-frequency circuit is only a fraction of an ampere. When ac welding, the current and voltage reverse directions many times each second. The electrode is positive, then the voltage switches so the electrode is negative. This repeats continuously when ac welding. An ac power source must have a way to keep the current flowing while the current switches from electrodenegative to electrode-positive. Without a method to keep
Figure 7-4. The face of a CC/CV and AC/DC welding power source. (Miller Electric Mfg. Co.)
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Current
One cycle
Time Electrode positive O Electrode negative Time Electrode positive O Electrode negative Time Square wave: Voltage and current go from positive to negative instantly
The welding power source can be adjusted to obtain a balanced wave and obtain the advantages of the balanced current wave listed above. Power source can be adjusted for greater penetration and still have the electrode-positive half cycle be present (not rectified). Power source can be adjusted for greater cleaning than can be obtained with a balanced wave.
Figure 7-6. Comparing voltage and current in a sinusoidal ac wave and a square wave. The square wave goes from a positive value to a negative value instantly. The voltage and current allow the arc to reinitiate (restart). The traditional ac wave has voltage and current near zero for some period of time.
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restart easily. The arc will reinitiate or restart even without the use of high-frequency or another arc stabilizing method. Because the output current and voltage are controlled electronically, the amount of direct current electrode positive and direct current electrode negative can be adjusted. This allows the welder to adjust the amount of cleaning and the amount of penetration. See Figure 8-15, which shows how the amount of current in the electrode-positive and electrode-negative portion of the weld cycle can be adjusted to get a balanced wave, one with more penetration or one with more cleaning. Balanced current is not necessary for most manual welding. It becomes more necessary for automatic welding as the duty cycles increase, often to near 100%. The advantages of a balanced current output wave are: Better oxide cleaning is provided because more current flows during the electrode-positive half cycle. A more stable arc. Reliable restarting of the arc during the electrode-positive half cycle. The output rating of the welding transformer does not need to be reduced because of output imbalance. The advantages of the unbalanced wave, Figure 7-5B (or a square wave with greater electrode-negative current flow), are: Higher currents can be used with a given electrode, since the electrode-positive cycle has less current flowing than with a balanced wave. The power source is less expensive than for a balanced wave machine. Better penetration is possible with an unbalanced wave because penetration takes place during the electrode-negative half cycle. The advantages of a square-wave power source that allows the ac balance to be adjusted are: The square wave provides high voltage and currents while the electrode switches polarities. This eliminates rectification.
Figure 7-5. Alternating current plotted against time. ACompletely rectified ac wave with no current flow during electrode positive. BUnbalanced wave. CBalanced wave. To overcome this lack of flow during one half of the alternating current cycle, a high-frequency voltage is generated and fed into the welding circuit. Several thousand volts, with only a fraction of an ampere, are created. This high voltage has a frequency of several megahertz (million cycles per second). A high-frequency unit is normally built into the gas tungsten arc welding machine. It consists of a step-up transformer, capacitors, a control rheostat, a set of spark gap points, and a coil to couple the high-frequency unit to the welding circuit. High-frequency voltage flows continually in the welding circuit. This high-frequency voltage keeps the shielding gas in the arc area in an ionized state. When the gas is ionized, the arc is maintained during the half of the cycle when the electrode is positive. While the arc is maintained, some current will flow across to the electrode. However, because the work surface does not emit (give off) electrons easily, less current flows during this half of the ac cycle, or wave, as shown in, in Figure 7-5B. The wave shown is an unbalanced wave. A completely balanced wave is shown in Figure 7-5C. In older machines, a balanced current output wave was achieved by using a large number of capacitors in series or a battery in the welding circuit. Newer GTAW power sources use electronics to create and maintain a balanced wave. Most new GTAW power sources produce a squarewave current output. A square-wave power supply can change the current from electrode-positive to electrodenegative very quickly. See Figure 7-6. This provides a high voltage as the current crosses zero and allows the arc to
two to three times more helium than argon to shield a given weld area. The chief advantage of helium over argon is that helium can be used with greater arc voltages. Helium also yields a much higher available heat on the metal than is possible with argon. Helium is therefore used to weld thick sections of metal or metals with a high heat conductivity, such as copper or aluminum. Helium also produces greater weld penetration. To produce the same available heat at the metal, higher currents must be used with argon than with helium. Studies have shown that undercutting will occur at the same current levels with either gas. Helium, therefore, will produce better weld results at higher speeds without undercutting than will argon. Both helium and argon provide good cleaning action with direct current electrode positive (DCEP). However, since DCEN produces greater heat and penetration, DCEN is the polarity normally used for GTAW. With ac, which is used on aluminum and magnesium, argon provides a better cleaning action. Argon also provides better arc stability with ac than helium. Argon and helium can be mixed to obtain desired results. Mixtures provide a combination of the results from each gas. Good arc starting and a stable arc can be combined with increased penetration and increased welding speeds. Argon-hydrogen mixtures are used to produce higher welding speeds. The welding speed possible is in direct proportion to the amount of hydrogen added to argon gas. The addition of hydrogen to argon permits the mixture to carry higher arc voltages. Too much hydrogen will cause porosity (gas pockets) in the weld metal. Porosity weakens a weld. Mixtures of 65% argon and 35% hydrogen have been used on stainless steel with a 0.010" to 0.020" (approximately 0.250mm to 0.500mm) root opening. The most common argon-hydrogen ratio is 85% argon and 15% hydrogen. Argon-hydrogen mixtures are used with stainless steels, nickel-copper, and nickel-based alloys. Hydrogen produces negative welding effects with most other metals. Argon, helium, or argon-helium mixtures can be used for most GTAW welding jobs. Manual welding on thin metals is done best with argon because of the low arc voltages and welding currents required. Whenever there is a cross draft, it is advisable to construct a wind break. This will reduce the possibility of the shielding gas being blown away from the weld area.
Current
Current
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The high pressure gauge or volume scale, if stored as a gas. The weight, if stored as a liquid. Shielding gases, with the exception of hydrogen, are not flammable. Inert gases will not burn, nor will they support combustion. The gases in cylinders are stored under high pressure. The high pressures in the full cylinders make it necessary to handle the cylinders with care. Refer to Chapter 1 and to Heading 13.5.2 for more information on handling cylinders. The cap should be securely threaded over the cylinder valve whenever the cylinders are being moved, or when they are in storage. The cylinder should be fastened to a wall or very stable object when in use. Cylinders should be placed where it is virtually impossible to accidentally damage them with an arc or cutting torch. Gas cylinders should always be stored and used in an upright position.
Tube and ball assembly Highpressure gauge Flowmeter reads in ft3/hr. (cfh) Flowmeter dome Cylinder pressure gauge Argon, helium, CO2, or oxygen
Flowmeter
Float ball
Wing nut
Figure 7-8. A gas regulator and flowmeter. The low pressure to the flowmeter is preset. The flowmeter has two calibrations. One is for argon and the other for CO2. (CONCOA)
Figure 7-9. A two-stage regulator with a flowmeter. The gas is fed to the flowmeter at 50 psig (345 kPa). This flowmeter has four separate calibrated gauges to accurately show the flow of argon, helium, carbon dioxide, or nitrogen. (ESAB Welding and Cutting Products).
Gasket
Gas flow
Flow gauge
scales are accurate only if the gas entering them is at approximately 50 psig (345 kPa). If higher inlet pressures are used, the gas flow rate will be higher than the actual reading; the reverse is true if the inlet pressure is lower than 50 psig (345 kPa). It is therefore important to use accurately adjusted regulators. Figure 7-9 illustrates a twostage regulator for argon gas. The gauge is a high-pressure gauge and is used to indicate the pressure in the cylinder. A regulator is attached to an inert shielding gas cylinder. Start the threads by hand. Be careful not to cross the threads. Once the threads become snug, use a proper fitting wrench to finish tightening the nut on the regulator. Do not overtighten, just firmly secure the nut.
Figure 7-7. A gas regulator with a flowmeter gauge instead of a low-pressure gauge. The flowmeter gauge may be used with CO2 or argon. It is calibrated in cubic feet per hour. (CONCOA)
Figure 7-10 shows a cross section of a tapered-tube flowmeter. For an accurate reading, it is important that this type instrument be mounted in a vertical position. Any slant will cause an inaccurate reading. The tube and return gas housing are either clear plastic or glass. Some have a metal protecting cover. The joints between plastic tubes and the flowmeter body must be gas-tight. The scale on the inner tube is usually calibrated from 0 ft3/hr. (or cfh) to 60 ft3/hr. (0 L/min to 28.3 L/min). The flowmeter scale is usually read by aligning the top of the ball with the ft3/hr. or L/min reading desired. A different flowmeter or a different scale must be used for each gas. The reason for a different flowmeter or scale is that each shielding gas has a different density or weight. Because the weight of the gas is different, the height to which the ball will be lifted is different for helium, argon, and carbon dioxide, even if all have the same flow rate. Universal-type flowmeters that have a square outer tube are available. Each side of the square is calibrated for a different shielding gas. See Figure 7-9. Gas volume is related to pressure if the orifices and gas passages remain constant in size and shape. Therefore, a gas pressure gauge can be used as a volume gauge when the scale is calibrated as shown in Figure 7-7. Most flowmeters have a needle valve to turn the gas flow on and off and to control the volume of the gas flow. Because this valve controls the volume, and because the pressure to the valve is constant, the valve orifice and the needle must be accurate and in good condition at all times.
Body
Hose nipple
Any abuse of the needle and/or seat will result in erratic volume feeds. Also, if the needle threads or packing around the needle have been damaged, the needle will not stay in adjustment. It is necessary that this valve be handled carefully. It should not be forced. Fingertip handling only is recommended. It is sometimes necessary to use mixtures of inert gases in GTAW or GMAW. Most common mixtures are available from local gas distributors in cylinders. For some jobs and in some companies, individual gases are mixed. Such mixing must be done with great accuracy. Gases are mixed by percentages of volume. Each gas to be mixed is fed to a separate flowmeter, where the desired volume of each gas is set. These gases are then mixed by volume in a
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special mixing chamber and fed to the welding torch through a final flowmeter.
shielding gas flows through a rubber or plastic tube which surrounds the electrode lead. Figure 7-14 shows an extremely light-duty GTAW torch which is gas-cooled. Figure 7-15 and 7-16 illustrate light-duty gas-cooled GTAW torches.
Cap Collet
Gas fitting
Figure 7-13. A water-cooled GTAW torch. The inlet water hose also carries the welding current to the electrode.
Figure 7-15. A gas-cooled, gas tungsten arc welding torch. A short cap and electrode permit use of this torch in difficult-to-reach places.
Gas fittings
Figure 7-11. A water circulating and cooling unit used with closed-circuit water-cooled systems. The coolant container is available in larger sizes, as well. This unit can be used with GTAW, GMAW, PAW, resistance, and electroslag welding. (Tweco-Arcair, a Thermadyne Company)
Water fittings
Torch body connection parts 3/16 circle clamps Water-in and gas glands Water-in and gas gland nuts only Body hex, adaptors Outer handle 5/16 circle clamp Inner handle Flexible torch body Collets .040 1/16 3/32 1/8 Ceramic cups 1/4 dia. 5/16 dia. Water-out and power gland 12 1/2 hose assembly includes: power, water, gas supply hoses and hose gland parts
Water nut
Welding machine connections Power lug and water-out coupling Water nut
Figure 7-14. A pen-sized gas tungsten arc welding torch. This torch is used when welding small parts. Note that it is water cooled.
Figure 7-16. A gas-cooled gas tungsten torch. Note that the shielding gas and the electrode lead are separate.
3/16 Water nut Water-in coupling hose glands Water nut Water Shielding Gas Electricity
Figure 7-12. Exploded view of a gas tungsten arc welding torch and hose assembly. This torch has a flexible body that can be bent to suit the job. (Falstrom Co.)
Gas tungsten arc welding torches come in many different designs, sizes, amperages, and shapes. Torches designed for lower amperages and light-duty use are cooled by the shielding gas flowing through the torch. Figures 7-14, 7-15, and 7-16 show examples of gas-cooled torches. Current capacity for the largest shielding gascooled torches is about 200A. Smaller gas-cooled torches carry significantly less current. Torches used for higher amperages and those that are used to do a lot of welding at lower amperages are water-cooled. Most torches used in production and most torches used for continuous automatic welding are watercooled. Figures 7-13 and 7-17 show water-cooled torches. Figure 7-18 shows a cross section of a water-cooled torch. Figure 7-19 shows how the different parts of a watercooled GTAW torch are assembled. These parts are also
Figure 7-17. Three GTAW torches. The two torches to the left are water-cooled; the torch at the right is gas-cooled. Note the difference in length from the nozzle to the cap. (CK Worldwide, Inc.)
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Large dia. gas lens adapter Gas lens insulator Cup gasket Stubby gasket
Nozzles
Collet Nozzle Tungsten electrode End caps Collet bodies Nozzles Stubby end caps
Figure 7-19. The various parts in a typical water-cooled GTAW torch, and how they fit together. (American Torch Tip Co.)
Figure 7-18. A cross section of a water-cooled gas tungsten arc welding torch designed for automatic welding. This torch is designed with a current capacity of up to 500A. (Weldma Co.)
Collets
shown in Figure 7-20. Figure 7-15 shows some of the torch parts as they would be assembled. There are two basic torch designs. These are: Torches designed for general access. These torches usually use a long electrode and have a long cap. They also use standard collets, collet bodies, and nozzles. See Figure 7-17. Torches designed for work in areas where space is limited. These torches are called stubby or stubs. They use a very short electrode and have a very short cap. These also use stubby collets, stubby collet bodies, and stubby nozzles. See Figures 7-16 and 7-17.
There are variations of the general access design. These variations allow a welder to select different parts of the torch for a specific job.
Figure 7-20. Note the different sizes of nozzles, collets, and collet bodies available for a typical water-cooled GTAW torch. (American Torch Tip Co.)
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7.6.2 Nozzles
Torch
Ceramic nozzle
Collet
Figure 7-22. A copper electrode collet being installed in a GTAW torch. The electrode and electrode cap are installed next.
Gas nozzles are used to direct the shielding gas over the tungsten electrode and to cover the weld area with shielding gas. Nozzles must be able to withstand very high temperatures because they are very close to the arc. Different designs are available to meet the requirements of different welding jobs. Nozzles are made from different materials. Most commonly, nozzles are made from a ceramic material. Other materials used include metal-jacketed ceramics, metal, and fused quartz. One end of a gas nozzle must attach to the end of the torch. Different designs are necessary to properly attach to different manufacturers torches. The nozzle may be threaded onto the torch or held by friction. The exit end of the GTAW nozzle is more standard. The exit diameter is identified with a number. The number is the exit diameter measured in 1/16 (1.6mm) increments. A number 6 nozzle thus has a diameter of 3/8 (9.6mm): 6 x 1/16 = 6/16 or 3/8 (6 x 1.6mm = 9.6mm) A number 8 nozzle diameter is 1/2 (12.8mm): 8 x 1/16 = 8/16 or 1/2 (8 x 1.6mm = 12.8mm) Nozzle exit diameters must be the correct size for the job. If they are too small, they will not allow the shielding gas to cover the weld area. They cannot be too large, or the velocity of the gas coming out will be too slow and will easily be blown away. A high velocity and a small diameter is also not good, because air may be caught up in the turbulence and contaminate the weld area. Selecting the correct size nozzle is important. Factors, such as accessibility of the weld area, may affect selection of the nozzle. Different nozzle styles can be seen in Figures 7-19 and 7-20 and other figures in this chapter. See Figure 7-24 for suggested nozzle size for a given electrode diameter.
A collet is installed into the top of a torch assembly, as shown in Figure 7-22. To make a collet tighten around the electrode, the cap is tightened. Tightening the cap forces the collet against the collet body, squeezing the collet. The inside diameter of the collet reduces in size, firmly gripping the electrode. The collet body locates the collet and transfers the electrical current to the collet. The collet in turn transfers the electrical current to the electrode. The collet body is threaded into the end of the torch. Examples of collet bodies are shown in Figures 7-19 and 7-20. Collet bodies are usually made for a single size of collet. When the electrode diameter is changed, both the collet and the collet body need to be changed. A collet body with a gas lens can be very useful to a welder. The purpose of a gas lens is to make the shielding gas exit the nozzle more as a column than as a turbulent stream of gas that begins to spread out after exiting. See Figures 7-20 and 7-23. The column of gas allows the electrode to stick out farther for visibility and for better access to the weld area. A gas lens is a series of stainless steel wire mesh screens. The collet body with gas lens is installed in the torch in place of a standard collet body. See Figures 7-19
Electrode diameter in. mm .010 .020 .040 1/6 .25 .50 1.00 1.6 2.4 3.2 4.0 4.8 6.4
Suggested nozzle size (in.) 1/4 1/4 3/8 3/8 1/2 1/2 1/2 5/8 5/8
Figure 7-23. The torch at the right has a gas lens installed. The gas flow from the conventional GTAW nozzle at the left is turbulent and dissipates more rapidly. (CK Worldwide, Inc.)
and 7-20. A different gas lens is required for each different electrode diameter. Different nozzles are also required because the gas lens has a larger diameter than a standard collet body.
Figure 7-24. A table of suggested nozzle sizes for use with various electrode diameters.
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Percent AWS classification EWP EWCe-2 EWLa-1 EWTh-1 EWTh-2 EWZr-1 EWG Tungsten min. percent (by difference) 99.5 97.3 98.3 98.3 97.3 99.1 94.5 Ceria (CeO) 1.8 - 2.2 Lanthana (LaO) 0.9 - 1.2 Thoria (ThO) 0.8 - 1.2 1.7 - 2.2 Not Specified Zirconia (ZrO) 0.15 - 0.40 Total other elements or oxides, max. 0.5 0.5 0.5 0.5 0.5 0.5
An electrode wire feed mechanism. Shielding gas cylinders. See Heading 7.4.1. A gas regulator. See Heading 7.4.2. A shielding gas flowmeter. See Heading 7.4.3. Shielding gas, coolant hoses and fittings, and the electrode and workpiece leads. A GMAW welding gun.
Electrode wire. Optional equipment. o A coolant system. See Figure 7-11. o Remote controls.
General Content of Booklet Copper Alloys Stainless Steel Aluminum Alloys Tungsten Electrodes Nickel Alloys Cast Iron (FCAW) Titanium Alloys Carbon Steel Magnesium Alloys Carbon Steel (FCAW) Surfacing Alloys Stainless Steel (FCAW) Zirconium Alloys Low-Alloy Steel Low-Alloy Steel Electrodes (FCAW) Consumable Inserts Shielding Gases
Figure 7-25. The chemical composition of the AWS classifications for tungsten electrodes. (AWS A5.12) Tungsten electrode diameters are available in diameters of 0.010, 0.020, 0.040, 1/16, 3/32, 1/8, 5/32, 3/16, and 1/4 (0.25, 0.51, 1.02, 1.59, 2.38, 3.18, 3.97, 4.76, and 6.35mm). Electrodes come in lengths of 3, 6, 7, 12, 18, or 24 (76, 152, 178, 305, 457, or 610mm). The surface of a tungsten electrode, when purchased is either ground or chemically cleaned. It is extremely important that shielding gas always protect the electrode and the weld area. Shielding gas hose connections must be tight to prevent air or moisture from mixing with the shielding gas, then coming in contact with the electrode. Such contamination of the shielding gas would be harmful to the weld and to the electrode. Preparing the electrode end for welding is very important. Refer to Heading 8.3.5 on selecting and preparing a tungsten electrode for welding.
Figure 7-28. AWS filler metal, electrode, and gas specifications. The material covered in the listed A5 specification numbers are for GTAW, GMAW, and PAW unless otherwise indicated.
AWS classification
EWP EWCe-2 EWLa-1 EWTh-1 EWTh-2 EWZr-1 EWG
Defined
Pure tungsten 2% Ceria 1% Lanthana 1% Thoria 2% Thoria 0.15-0.40% Zirconia Other
Color
Green Orange Black Yellow Red Brown Gray
C
Figure 7-27. Three steps are required to prepare a tungsten electrode for welding. ANotching the electrode prior to breaking. BBreaking the electrode in special fixture. CGrinding the electrode in a special grinder. (Intercon Enterprises, Inc.) Figure 7-29. Two models of GTAW power sources with an automatic wire drive to provide filler metal to the weld pool during the welding process. Note the wire guide tube. (CK Worldwide, Inc.)
Figure 7-26. The AWS color code for various types of tungsten electrodes. (AWS A5.12)
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Figure 7-30 is a schematic drawing of a GMAW outfit. A complete station would include the arc welding booth, a ventilation system, and a welding bench. Refer also to Figure 4-13.
Engine
Figure 7-32. An inverter type power source for GMAW. (Hobart Brothers Co.)
Figure 7-33. An engine-driven generator type power source. (Miller Electric Mfg. Co.)
Figure 7-31. A transformer-rectifier type constant voltage (cv) power source for GMAW. (Hobart Brothers Co.)
means it will maintain a constant arc length. To change the arc length, the set voltage must be changed. The output of a constant voltage power source has a very flat voltampere curve, as shown in Figure 7-34. Constant voltage power sources have the ability to self adjust to maintain a constant arc length. When the welding gun is moved closer to the work, the arc length
should decrease. However, the machine will deliver more current to burn off the electrode faster and maintain the arc length determined by the set voltage. Also if the welding gun is moved further from the work, the machine will deliver less current, so the electrode will burn off more slowly. By burning off the wire at a slower rate, the arc length and voltage remain constant. The arc welding
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30 Wire feed drive motor Wire feeder 25 Voltage Manually held gun Auto torch Control system
A B C
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150 Amperage
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Figure 7-30. A diagram of a combination manual and automatic gas metal arc welding outfit.
Figure 7-34. Characteristic volt-ampere curve for a constant voltage arc welding machine. A 100-ampere change (from 200A to 300A) results from a voltage change of 3V from 22V to 19V.
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machine amperage is changed proportionately when the wire feed speed is changed. Figure 7-34 shows how large a current change will occur when the voltage changes only slightly. A welder sets a welding machine and wire feeder to weld at 22V and 200A (Point B on Figure 7-34). If the gun is moved closer to the work, the voltage will attempt to decrease to 19V. To prevent this change in voltage, the machine will increase the current automatically to 300A . See Point C in Figure 7-34. This increase in current will quickly burn off the electrode wire and actually maintain the voltage at 22V. If the gun is moved away from the work and the voltage attempts to increase to 25V, the current will decrease. The electrode will burn off at a slower rate. This will allow the arc length and the voltage to remain very close to their set value of 22V. The slope of the volt-ampere curve is important. Some welding machines allow the slope to be changed; others machines have only one preset slope. Electronically controlled machines have different slopes preset for common types of metal and methods of transfer. The slope of a machine is very important when using short circuiting transfer. Short circuiting transfer and other transfer methods are discussed in Headings 9.1.1 through 9.1.4 Figure 7-35 shows various slopes for GMAW power sources. Nonferrous metals and large-diameter flux cored electrode wires use a slope of 1.5V to 2V per 100A . A medium slope of 2V to 3V per 100A is used for GMAW with carbon dioxide (CO2) gas and small-diameter flux cored electrode wires, as shown in Figure 7-35B. A steeper slope, Figure 7-35C, has a slope of 3V to 4V per 100A. This steeper slope is recommended for short circuiting arc transfer. A slope of 1.5V to 2V per 100A is used with largediameter electrode wires to allow a large current change to burn off the large-diameter electrode wire. A steeper slope of 3V to 4V per 100A is used when using short circuiting
transfer. This prevents excessive current when the short circuit occurs. If too-steep a slope is used with short circuiting transfer, the molten metal drop will not separate from the electrode and the arc will not restart.
Pure argon (Ar) and helium (He) are excellent gases for protecting the arc, metal electrode, and weld metal from contamination. They are not, however, as suitable for some GMAW processes as mixtures of gases. Gas mixtures seem to create arc stability, reduce spatter, and improve the bead contour. Reactive gases like carbon dioxide, oxygen, and nitrogen are not practical to use alone as shielding gases. Carbon dioxide is the exception. It is inexpensive and works well on carbon and low-alloy steels. Carbon dioxide generally costs about one-tenth as much as pure argon gas.
Figure 7-37. A constant voltage (cv) power source with a separate wire feeder are being used to GMAW a small steel wheel. (Lincoln Electric Co.)
the wire drive unit. To reinstall the electrode wire through the cable liner and contact tube, the wire must be moved slowly ahead. The wire is moved slowly so that it does not bend or kink within the electrode cable. The switch on the wire drive control panel is called an inch switch or a jog switch. See the switch in Figure 7-36. The shielding gas hose may have air in it before its first use after a long shutdown period. To clear or purge the hose and gun, the shielding gas is turned on for a short period prior to welding. A purge switch may be provided for this purpose on the wire feed control box.
Set voltage -1 Voltage A -2 B -3 -4 C Gas purge control Wire jog or inch control Wire feed speed control
10
20
30
40
50 60 70 Amperage
80
90 100
Wire feeder
Figure 7-35. Volt-ampere curves for various welding machine slope settings. Curve A1.5V to 2V per 100A for GMAW of nonferrous metals. Curve B2V to 3V per 100A for GMAW with CO2. Curve C3V to 4V per 100A for GMAW short circuiting arc transfer.
Figure 7-36. A wire feeder and controller. Note the controls and the fact that two wire feeders may be used individually or together. (Miller Electric Mfg. Co.)
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Shielding gas or mixture Argon Helium Ar + He (20-80% to 50-50%) Nitrogen Ar + 25-30% N2 Ar + 1-2% O2 Ar + 3-5% O2 CO2 Ar + 20-50% CO2 Ar + 10% CO2 + 5% O2 CO2 + 20% O2 90% He + 7.5% Ar + 2.5% CO2 60-70% He + 25-35% Ar + 4-5% CO2
Metals and applications Virtually all metals except steels. Aluminum, magnesium, and copper alloys for greater heat input and to minimize porosity. Aluminum, magnesium, and copper alloys for greater heat input and to minimize porosity (better arc action than 100% helium). Greater heat input on copper (Europe). Greater heat input on copper (Europe); better arc action than 100% nitrogen. Stainless and alloy steels; some deoxidized copper alloys. Carbon and some low-alloy steels. Carbon and some low-alloy steels. Various steels, chiefly short circuiting arc. Various steels (Europe). Various steels (Japan). Stainless steels for good corrosion resistance, short circuiting arc. Low-alloy steels for toughness, short circuiting arc.
Slightly oxidizing Oxidizing Oxidizing Oxidizing Oxidizing Oxidizing Slightly oxidizing Oxidizing
Figure 7-38. Shielding gases used with various metals. (American Welding Society)
maintained. Carbon dioxide is furnished in liquid form in 50 lb. (23kg) cylinders. These cylinders are approximately 9 (229mm) in diameter and 51 (1.30m) high and weigh 105 lb. (48kg), when empty. Each pound (0.45kg) of liquid will furnish 8.7 ft3 (0.25m3) of gas, which is equal to 435 ft3 (12.3m3) per cylinder. It must change from a liquid to a gas as it is being used. This change from liquid to gaseous CO2 is dependent on the room temperature. When the delivery line from the liquid tank is opened, carbon dioxide gas boils or bubbles out of the liquid. The expansion of the gas as it leaves the liquid and passes through the regulator causes the CO2 gas to cool. If moisture is present, it may condense and freeze in the regulator, blocking the gas passage. Excessive moisture may also be indicated by erratic flowmeter operation. It is recommended that CO2 with a 20F (29C) or lower dew point be used. One cylinder can furnish only about 35 ft3/hr.(16.5 L/min) when the cylinder is in a 70F (21C) room. Sometimes, two or more cylinders must be connected in parallel to furnish enough gas for welding. The pressure in the cylinder when liquid is present is about 835 psig at 70F (5760 kPa at 21C). Carbon dioxide is also available in the gaseous form in cylinders. These cylinders are similar to the oxygen cylinders described in Chapter 12.
Hanging hook
Figure 7-40. A gas-cooled GMAW gun. Note how the electrode wire travels through the center of the combination cable. (Bernard Welding Equipment Co.)
current and wire feeder and starts the shielding gas flowing. Figure 7-41 shows the main parts of a gas-cooled GMAW gun. Gas metal arc welding guns are usually gas-cooled. Gas flowing through the gun and air on the outside of the welding gun keep the gun from overheating. Carbon dioxide GMAW guns may be used for intermittent duty up to 600A. Figure 7-42 shows a gas-cooled GMAW gun. Gas metal arc welding using a gas-cooled gun with argon or helium must be done at much lower currents than are possible with carbon dioxide. GMAW guns are water-cooled when used with argon or helium above 300A or for continuous duty. Torches used with CO2 above 600A, or for continuous duty, are also water-cooled. Figure 7-43 shows a water-cooled GMAW
Shielding gas
Combination cable
Argon
Argon-Helium
Helium
CO2
Figure 7-39. The shape and depth of penetration of beads obtained with various shielding gases used when GMAW.
Electrode wire On-off switch Trigger Torch handle Control switch wire
Adding oxygen or carbon dioxide to argon or helium causes the shielding gas to become slightly oxidizing. This may cause porosity in some ferrous metals. To offset this oxidizing tendency, a deoxidizer is added to the electrode wire.
Carbon dioxide is not an inert gas. It will react with the base metal and oxidize the base metal. When using CO2, an electrode with alloys to eliminate this oxidation must be chosen. Carbon dioxide is 50% heavier than air and its ability to shield the arc is quite good. Moisture-free carbon dioxide must be used, or the hydrogen generated while welding will cause weld porosity and brittleness. Carbon dioxide has a rather high electrical resistance and it, therefore, has a rather critical arc length. Even small changes in the arc length will produce spattering and a wild arc. A very short and constant arc length must be
Nozzle
Figure 7-41. A cross-sectional drawing of an air-cooled GMAW gun. The shielding gas, electrode wire, and control switch wire are carried in a combination cable. The electrode wire runs through the cable liner and contact tube.
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Wire spool and drive wheels Gun oscillation controls Gun speed controls Remote control connector Flexible tube Nozzle
Nozzle
Gas/power cable
Figure 7-42. A gas-cooled GMAW gun. The flow of shielding gas acts to cool this type of gun.
Figure 7-43. A water-cooled GMAW gun. The gas and power cable are combined. The gun has a flexible tube between the handle and nozzle. It can be bent to any convenient angle by the welder.
Figure 7-46. A push-pull wire drive system. The welding gun pulls the wire while the wire drive at the power source pushes the wire. This system is generally used with soft wire like aluminum. (Miller Electric Mfg. Co.)
gun. In addition to an electrical lead and a shielding gas hose, a water-cooled gun must have a hose to carry water to the gun. The water returns to the water cooler through the combination power cable and water tube. Gas metal arc welding guns that have self-contained wire drive units are available. See Figures 7-44 and 7-45. These guns have an electric motor to drive the electrode wire and a small coil of wire enclosed in the gun. Another type of gun that is used is called a pull-type welding gun. When welding long distances from the wire feeder or when welding using aluminum wire (especially
smaller diameter aluminum wire), a pull-type welding gun is used. See Figure 7-45. A pull-type gun has a motor in it, which pulls the electrode wire. Attempting to feed aluminum wire by pushing it through the cable to the gun will cause it to kink or bend. Aluminum electrode wire is not rigid enough to be pushed. Kinking can also happen when feeding steel or stainless steel electrode wire a long distance, because there is a lot of friction on the wire. To prevent kinking the wire, a pull gun is used. The pull-type gun pulls the wire while the regular wire drive unit pushes the wire. This is a pushpull set-up. Figure 7-46 shows a push-pull system that includes a power source with a wire feeder and a pull-type welding gun. Welding can be done 50 (16m) from the wire feeder using this push-pull method. Welding guns used for automatic GMAW often have a straight body and nozzle. They do not have a handle. They are firmly attached to a carriage, robot, or other mechanism for welding, as shown in Figure 7-47. Since automatic welding is done almost continuously, the gun is
almost always water-cooled. Gas-cooled guns are used when welding with lower amperages that will not cause them to overheat. Electrode wire and shielding gas is fed through the gun body to the arc area in the same manner as shown in Figure 7-41. A new GMAW system has been developed for use in emergency or quick repairs. The system is highly portable and relatively light in weight. The power source is two 12 volt long-life batteries to provide 24 volt power. The welding gun contains the wire drive rolls, wire speed controls, and an on-board computer. See Figure 7-48.
Figure 7-47. A GMAW gun mounted on a rigid beam in order to make linear (straight line) welds on a structural I beam. This GMAW system has an oscillator which moves the gun from side to side along the weld line. (Miller Electric Mfg. Co.)
gun using two different methods. One is by threading the nozzle onto the gun. The second way is to use an adaptor and slide-on nozzle. Both types are shown in Figure 7-49. The exit end of the nozzle also varies. It needs to be large enough to allow a continuous flow of shielding gas to cover the weld area. It should not have spatter inside,
Figure 7-44. A self-contained wire feeder and GMAW gun. A small spool of wire, drive motor, and drive wheels are contained within this type of gun to smoothly feed the wire. (Miller Electric Mfg. Co.)
Figure 7-45. A pull type GMAW gun. The drive motor is in the handle, and the drive rolls are just above the motor. (Thermadyne Industries, Inc.)
Figure 7-48. A battery powered GMAW unit. Two 12 volt batteries provide a 24 volt power supply. The gun contains the wire drive and speed controls. (Broco, Inc.)
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since this will affect the flow of the shielding gas. The exit diameter is selected based on the application and joint access. Nozzle exit diameters are measured in 1/16 (1.6mm) increments. An example is a number 6 nozzle which has a 3/8 (6 1/16) or 9.5mm (6 1.6mm) exit diameter. A liner is installed in the cable that carries the electrode from the wire feeder to the welding gun. The electrode wire is constantly moving through this cable and would wear out the cable. To protect the cable, a liner is installed. For most applications, a coiled steel wire liner is used. This coiled steel liner is sometimes called a conduit. When using aluminum electrode wire, a nylon liner is used. Figure 7-50 shows a coiled steel liner.
Deoxidizers are added to most electrode wires. This is done to reduce the porosity in the finished weld. During the welding process, the deoxidizer added to the wire will react with any oxygen, nitrogen, or hydrogen. The deoxidizers reduce the possibility of these gases producing porosity, which would lower the mechanical strength of the weld. The American Welding Society chemical composition specifications for carbon steel filler metal used with GMAW are shown in Figure 7-51. Figure 7-52 lists the AWS filler wire specifications for GMAW low-alloy steels. The numbers and letters used in the AWS classifications in Figures 7-51 and 7-52 have the following meanings: Example: ER70S-2 E electrode R rod 70 tensile strength in ksi (1000 psi) S solid rod 2 variations in chemical compositions Examples of dash numbers: B2 chrome-molybdenum steel B3L chrome-molybdenum steel with a lower carbon content Ni (1-3) Nickel steel D2 manganese-molybdenum steel
A
Liner
Another method to remove fumes is to use a large air removal system, as shown in Figure 7-55. This pickup hood can be located near the welding area to remove the contaminated air. This type of exhaust system can be used with any welding process or other process in the welding shop, such as grinding. An inexpensive option to move fumes away from the welder is to use a portable fan. This does not eliminate the fumes, only moves them. The fan cannot blow too much air or it will blow the shielding gas away from the weld and cause contamination of the weld. Sometimes, welding must be done in a closed or confined area, or on metals that are hazardous. In such cases, it is necessary for the welder to wear a purified air breathing apparatus to supply fresh, clean air. Refer to Heading 1.3.2. Figure 7-56 shows a welder wearing a purified air breathing apparatus. Whenever it is necessary to use a purified air breathing apparatus, always make sure the system is operating properly before entering a closed, confined, or contaminated area and before doing any welding.
B
Figure 7-50. A cable liner is used to carry the electrode through the combination cable to the welding gun. AThe wire guide used in the wire drive. BThe liner within the welding gun.
Figure 7-49. Parts for a gas-cooled GMAW gun. (American Torch Tip Co.)
The GMAW process can generate some smoke. Flux cored arc welding (FCAW) often produces quite a bit of smoke. FCAW is discussed in the next few headings. When welding with carbon dioxide (CO2), there is some carbon monoxide (CO) created. Some ozone is also created. Both of these gases are toxic. It is especially important to prevent breathing these gases or any metal or metal oxide fumes. Removal of these gases from the weld area can be done by means of a GMAW torch equipped with a fume extractor. Fume extractors may be built into the gun, as shown in Figure 7-53. Gas fumes, contaminated air, and air near the weld are drawn away by the torch, filtered, and released to the atmosphere. Figure 7-54 shows a fume extractor in use.
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Chemical Composition, Weight Percent AWS classification ER70S-2 C 0.07 0.06 to 0.15 0.07 to 0.15 0.07 to 0.19 0.07 to 0.15 0.07 to 0.15 Mn 0.90 to 1.40 0.90 to 1.40 1.00 to 1.50 0.90 to 1.40 1.40 to 1.85 1.50 to 2.00 Si 0.40 to 0.70 0.45 to 0.70 0.65 to 0.85 0.30 to 0.60 0.80 to 1.15 0.50 to 0.80 0.50 to 0.90 P 0.025 S 0.035 Nia Cra Moa Va Cub 0.50 Ti 0.05 to 0.15 Zr 0.02 to 0.12 Al 0.05 to 0.15
The self-shielded FCAW process also is preferred for welding in hard-to-reach or hard-to-see places. Since there is no shielding gas, there is no nozzle so visibility is improved. The electrode extension may be greater. Moving the arc out and away from the electrode contact tube permits the welder to see the joint more easily.
A shielding gas flowmeter. See Heading 7.4.3. Shielding gas and coolant hoses and fittings. See Heading 7.5. Shielding gas is used only for gasshielded FCAW. Electrode lead and workpiece lead.
ER70S-3
ER70S-4
ER70S-5
ER70S-6
ER70S-7 ER70S-G
No chemical requirementsc a. b. c. These elements may be present but are not intentionally added. The maximum weight percent of copper in the rod or electrode due to any coating plus the residual copper content in the steel shall be 0.50. For this classification, there are no chemical requirements for the elements listed, with the exception that there shall be no intentional addition of Ni, Cr, Mo, or V.
Figure 7-51. Chemical composition specifications for carbon steel filler metal used with GMAW. For exact limitations and more information, see AWS specification A5.18.
Figure 7-54. A smoke pickup and filter set up on top of a GMAW power source. The pickup hose is attached to the GMAW gun. (Kemper Purification Systems, Inc.)
Chemical Composition
AWS classification ER80S-B2 ER80S-B2L ER90S-B3 ER90S-B3L Carbon Manganese Silicon Phosphorus 0.025 0.025 0.025 0.025 Sulfur 0.025 0.025 0.025 0.025 Nickel 0.20 0.20 0.20 0.20 Chromium Molybdenum Vana- Tita- Zirco- Alumidium nium nium num Coppera 0.35 0.35 0.35 0.35 Total other elements 0.50 0.50 0.50 0.50
A
Smoke extractor and filter
Chromium-molybdenum steel electrodes and rods 0.07-0.12 0.40-0.70 0.40-0.70 0.05 0.05 0.40-0.70 0.40-0.70 0.40-0.70 0.40-0.70 0.07-0.12 0.40-0.70 0.40-0.70 1.20-1.50 0.40-0.65 1.20-1.50 0.40-0.65 2.30-2.70 0.90-1.20 2.30-2.70 0.90-1.20
Nickel steel electrodes and rods ER80S-Ni1 ER80S-Ni2 ER80S-Ni3 ER80S-D2 ER100S-1 ER100S-2 ER110S-1 ER120S-1 ERXXS-G 0.12 0.12 0.12 1.25 1.25 1.25 0.40-0.80 0.40-0.80 0.40-0.80 0.025 0.025 0.025 0.025 0.010 0.010 0.010 0.010 0.025 0.025 0.025 0.025 0.010 0.010 0.010 0.010 0.80-1.10 2.00-2.75 3.00-3.75 0.15 1.40-2.10 0.80-1.25 1.90-2.60 2.00-2.80 0.15 0.30 0.30 0.50 0.60 0.35 0.40-0.60 0.25-0.55 0.20-0.55 0.25-0.55 0.30-0.65 0.05 0.05 0.05 0.04 0.03 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.35 0.35 0.35 0.50 0.25 0.35-0.65 0.25 0.25 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Pickup hood
Manganese-molybdenum steel electrodes and rods 0.07-0.12 1.60-2.10 0.50-0.80 0.08 0.12 0.09 0.10 1.25-1.80 0.20-0.50 1.25-1.80 0.20-0.60 1.40-1.80 0.20-0.55 1.40-1.80 0.25-0.60 Other low-alloy steel electrodes and rods
a. The maximum weight percent of copper in the rod or electrode due to any coating plus the residual copper content in the steel shall comply with the stated value. b. In order to meet the requirements of the G classification, the electrode must have as a minimum either 0.50 percent nickel, 0.30 percent chromium, or 0.20 percent molybdenum.
B
Figure 7-53. Smoke extractors. AThis GMAW gun has a built-in smoke extractor. BA portable smoke collector and filter. (Thermadyne Industries, Inc.)
Figure 7-52. The chemical composition specifications for low-alloy steel filler metals used with GMAW. For exact limitations and more information, see AWS specification A5.28.
Figure 7-55. A smoke extractor is attached to a pickup hood by a flexible tube and positioned over the weld area. The smoke is removed from the weld area and filtered. (Kemper Purification Systems, Inc.)
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Bead
A
Fume filter and cleaned air pump to helmet End cap (not a nozzle)
Figure 7-59. A GMAW or FCAW wire feeder. A FCAW gun is attached. Note that no nozzle is required when a shielding gas is not used. (Miller Electric Mfg. Co.)
Flux cored arc welding electrodes are tubular and easily flattened. The wire drive wheels used for FCAW are usually knurled to firmly grip, but not crush, the tubular electrodes. The adjustment of these drive wheels must be carefully done to permit the wire to be driven, but not flattened.
Figure 7-56. This welder is wearing a powered air purifier around the waist. The purifier pumps cleaned air to the hood and helmet combination. (Racal Health and Safety, Inc.)
B
Figure 7-57. Flux cored arc welding. AThis sketch shows a self-shielded flux cored arc weld in progress. BAn FCAW torch. Note that no nozzle is required when using the self-shielded process. (Thermadyne Industries, Inc.)
Current-carrying contact tube
1. Flat strip steel is formed by rolls into a U shape. 2. The U-shaped area is then filled with a carefully prepared granular flux and/or alloying material. 3. After filling, the metal is closed and rolled into a round shape. This closing and rolling compresses the flux material. 4. The tubular wire is then passed through drawing (forming) dies. This further compresses the granular flux and forms a perfectly round form of an exact diameter. 5. In a continuous operation, the completed flux cored wire is then wrapped on spools or into coil drums. Flux cored wire has the advantage of varying the core ingredients to match any weld requirements. Such welding requirements may include: Adding deoxidizers, such as silicon, manganese, or aluminum, to reduce weld porosity. Adding denitrifiers, such as aluminum, to reduce nitrogen by forming stable nitrides. Providing mechanical, metallurgical, and corrosion-resistant properties to the weld metal by adding alloying elements. Forming gases to shield the weld area from the oxygen and nitrogen in the atmosphere. Creating a slag covering over the weld bead to shield it while it cools. Stabilizing the arc by providing for better ionization of the arc. Trapping the impurities in the molten weld metal and floating them to the top of the weld to form slag. Figure 7-60 gives the chemical composition for flux cored arc welding electrode wire used on carbon steels. These electrodes may be used for self-shielded or gasshielded FCAW. Dash numbers are used at the end of the carbon steel electrode classifications. They are used to indicate electrode polarity, the shielding gas used, or the
Nozzle
A FCAW welding gun. See GMAW gun, Heading 7.13. Flux cored metal electrode wire. Optional equipment: o A coolant system. See Figure 7-11. o Remote controls. A booth, table, and a ventilation system. The differences occur in the wire drive and shielding gas equipment. Shielding gas is used with the gas-shielded FCAW process. Therefore, shielding gas cylinders, regulators, flowmeters, gauges, and hoses are required. When selfshielded FCAW wire electrodes are used, none of the shielding gas equipment listed above is necessary. Also when self-shielded FCAW is done, there is no need for a gas nozzle. See Figures 7-57 and 7-59. The gun for FCAW may be gas-cooled or watercooled. Gas-cooled guns are generally used up to 200A. Water-cooled guns are used when using currents of more than 200A. Water cooling is also used on guns which operate on a 100% duty cycle.
0.04
0.03
0.08
0.09
0.50
0.20
0.30
1.75
1.8
No chemical requirements
Weld pool
Values are maximums. Composition limits are intended to insure a plain carbon steel deposit. For self-shielded electrodes only.
Figure 7-58. A gas-shielded flux cored arc weld in progress is shown in this sketch.
Figure 7-60. The chemical composition of various carbon steel electrodes for flux cored arc welding. Note that aluminum is used as a denitrifier only for self-shielded electrodes. See Figure 7-61 for the meanings of the dash numbers. (American Welding Society, A5.20.)
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number of weld passes recommended. See Figure 7-61 for the meaning of these dash numbers. The number and letter classifications for mild steel FCAW electrodes are explained as follows: Example: EXXT-1 E electrode XX The first numbers in a two-digit number specify minimum tensile strength of the deposited weld metal in thousands of pounds per square inch (ksi). The second digit represents the position in which the electrode is to be used. A 0 is for flat and horizontal fillet welds. A 1 is for all-position welding. T indicates a flux cored electrode 1 to 11 a grouping of chemical composition, method of shielding, or its suitability to make single or multiple pass welds. Example: E70T-3 This is a tubular electrode with 70 ksi (483MPa) tensile strength in the deposited weld metal to be used for flat or horizontal fillet welds. The dash three (-3) in this case means it is self-shielded and intended for single-pass welds. For a complete description of these FCAW electrodes, refer to the AWS 5.15, 5.20, 5.22, 5.29 electrode specification manuals. Chemical compositions and AWS classification numbers for low-alloy FCAW electrodes are shown in Figure 7-62. The meaning of electrode numbers and letters used in Figure 7-62 are shown below: Example: E80T1-B2H E electrode 80 80 ksi (80,000 psi or 552MPa) tensile strength
T tubular flux cored electrode 1 intended usage B major alloying ingredients as follows: A carbon-molybdenum B chromium-molybdenum Ni nickel D manganese-molybdenum K all other low-alloy electrodes 2 Chemical composition group. See brackets on Figure 7-60. H comparative carbon content H higher carbon L lower carbon Figure 7-63 lists the chemical composition of the deposited weld metal when using chromium and chromium-nickel flux cored electrodes. The electrode classification numbers are explained below: Example: E308T-X and E308LT-X E electrode 308 the stainless steel classification T tubular flux cored electrode LT low carbon, tubular electrode X numbers from 1 to 3 plus the letter G 1 used with carbon dioxide (CO2) 2 used with argon plus 2% oxygen 3 self-shielded. No external shielding gas used. G not specified. Used only for special compositions. Example: E316T-3 This describes a tubular (T) stainless steel (316) electrode (E) used without any external shielding gas (-3). For complete specifications and information on flux cored arc welding electrodes, refer to one of the following AWS electrode specifications: A5.20, Specifications for Carbon Steel Electrodes for Flux Cored Arc Welding. A5.22, Specifications for Flux Cored Corrosion-Resisting Chromium and Chromium-Nickel Steel Electrodes. A5.29, Specifications for Low-Alloy Steel Flux Cored Arc Welding Electrodes.
Mn
Si
Ni
Cr
Mo
Ala
Cu
Carbon-molybdenum steel electrodes 0.12 0.12 0.05 0.12 0.10/0.15 0.05 0.12 1.25 1.25 1.25 1.25 1.25 1.25 1.25 0.03 0.03 0.80 0.40/0.65 0.40/0.65 0.40/0.65 0.40/0.65 0.40/0.65 0.90/1.20 0.90/1.20
Chromium-molybdenum steel electrodes 0.03 0.03 0.80 0.40/0.65 0.03 0.03 0.80 1.00/1.50 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.80 0.80 0.80 0.80 1.00/1.50 1.00/1.50 2.00/2.50 2.00/2.50
0.10/0.15
1.25
0.03
0.03
2.00/2.50
0.90/1.20
0.12
1.50
0.03
0.03
0.15
0.35
0.05
1.8
0.12
1.50
0.03
0.03
0.80
1.75/2.75
1.8
0.03 0.03 0.80 2.75/3.75 Manganese-molybdenum steel electrodes 0.03 0.03 0.80 0.03 0.03 0.03 0.80 0.03 0.80 All other low-alloy steel electrodes 0.03 0.03 0.80 0.80/1.10 0.15
0.05
0.15
0.50/1.75
0.03
0.03
0.80
1.00/2.00
0.15
0.35
0.05
1.8
AWS classification
EXXT-1 (Multiple-pass) EXXT-2 (Single-pass) EXXT-3 (Single-pass) EXXT-4 (Multiple-pass) EXXT-5 (Multiple-pass) EXXT-6 (Multiple-pass) EXXT-7 (Multiple-pass) EXXT-8 (Multiple-pass) EXXT-10 (Single-pass) EXXT-11 (Multiple-pass) EXXT-G (Multiple-pass) EXXT-GS (Single-pass) a.
0.15
0.75/2.25
0.03
0.03
0.80
1.25/2.60
0.15
0.25/0.65
0.05
1.8 1.8
0.30/0.75
a. For self-shielded electrodes only. * All values are maximum except where min (minimum) is indicated.
Figure 7-62. The chemical composition and AWS classification numbers for low-alloy FCAW electrodes. Numbers and letters are explained in Heading 7.18.
Figure 7-61. The meanings and uses of the dash numbers at the end of carbon steel FCAW electrode designations. The letter G is used when a manufacturer and purchaser develop a special electrode.
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Modern Welding
AWS classification
E307T-X E308T-X E308LT-X E308MoT-X E308MoLT-X E309T-X E309CbLT-X E309LT-X E310T-X E312T-X E316T-X E316LT-X E317LT-X E347T-X E409T-X E410T-X E410NiMoT-X E410NiTiT-X E430T-X E502T-X E505T-X E307T-3 E308T-3 E308LT-3 E308MoT-3 E308MoLT-3 E309T-3 E309LT-3 E309CbLT-3 E310T-3 E312T-3 E316T-3 E316LT-3 E317LT-3 E347T-3 E409T-3 E410T-3 E410NiMoT-3 E410NiTiT-3 E430T-3 EXXXT-G
C
0.13 0.08 a 0.08 a 0.10 a a 0.20 0.15 0.08 a a 0.08 0.10 0.12 0.06 a 0.10 0.10 0.10 0.13 0.08 0.03 0.08 0.03 0.10 0.03 0.03 0.20 0.15 0.08 0.03 0.03 0.08 0.10 0.12 0.06 0.04 0.10
Cr
18.0-20.5 18.0-21.0 18.0-21.0 18.0-21.0 18.0-21.0 22.0-25.0 22.0-25.0 22.0-25.0 25.0-28.0 28.0-32.0
Ni
9.0-10.5 9.0-11.0 9.0-11.0 9.0-12.0 9.0-12.0 12.0-14.0 12.0-14.0 12.0-14.0 20.0-22.5 8.0-10.5
Mo
0.5-1.5 0.5 0.5 2.0-3.0 2.0-3.0 0.5
Cb + Ta
Mn
3.3-4.75 0.5-2.5
Si
1.0
P
0.04
Fe
Cu
0.5
0.03 Rem
1.0-2.5 0.5-2.5
0.03 0.04
water flow decreases below a set limit, a switch opens and shuts off the electrical power source. Figure 7-64 shows a flow safety device. Some systems use a (thermal) heat fuse to protect the water-cooled welding gun. If the water flow stops or decreases to a dangerous minimum, the fuse link will overheat and open to interrupt the electrical current flow. See Figure 7-65. Most arc welding machines are equipped with a remote control circuit switch and external plug. The remote control is used to vary the voltage or amperage within a rough setting range. The welder at the job site can adjust the arc welding machine at a distance with this control. Figure 7-66 shows one type of remote control device.
17.0-20.0 11.0-14.0 17.0-20.0 11.0-14.0 18.0-21.0 12.0-14.0 18.0-21.0 10.5-13.0 11.0-13.5 11.0-12.5 11.0-12.0 15.0-18.0 4.0-6.0 8.0-10.5 19.5-22.0 19.5-22.0 19.5-22.0 18.0-21.0 18.0-21.0 9.0-11.0 0.60 0.60 4.0-5.0 3.6-4.5 0.60 0.40 0.40 9.0-10.5 9.0-11.0 9.0-11.0 9.0-12.0 9.0-12.0
2.0-3.0 2.0-3.0 3.0-4.0 0.5 0.5 0.5 0.40-0.70 0.05 0.5 0.45-0.65 0.85-1.20 0.5-1.5 0.5 0.5 2.0-3.0 2.0-3.0 0.5
Several types of remote controls and their uses are described in Heading 5.9.
0.60 1.0 0.03 0.04
23.0-25.5 12.0-14.0 23.0-25.5 12.0-14.0 23.0-25.5 12.0-14.0 25.0-28.0 20.0-22.5 28.0-32.0 8.0-10.5 18.0-20.5 11.0-14.0 18.0-20.5 11.0-14.0 18.5-21.0 13.0-15.0 19.0-21.5 10.5-13.0 11.0-13.5 11.0-12.5 11.0-12.0 15.0-18.0 9.0-11.0 0.60 0.60 4.0-5.0 2.6-4.5 0.60
Figure 7-64. Water flow safety control. If the water flow decreases below a set limit, the current is shut off until the flow of water resumes. (Hayes Fluid Controls)
1.0-2.5 0.5-2.5 0.03 0.04
Insulator cover
Thermal fuse
a. The carbon content shall be 0.04% maximum when the suffix X is 1; it shall be 0.03% when the suffix X is 2.
Figure 7-65. Fuse and hose assembly with insulator cover removed. If the thermal fuse overheats, it will shut off the current flow to the torch. (ESAB Welding and Cutting Products)
Fire-resistant clothing and accessories of leather are recommended. Wool also is satisfactory. Cotton does not provide sufficient protection and deteriorates rapidly under infrared and ultraviolet rays. However, flameresistant cotton clothing is made and is often used while arc welding. Always wear dark clothing to reduce reflection of light behind the helmet. The clothing should be without cuffs or open pockets, since these can collect sparks. Leather or leather-palm gloves should be worn. See Chapters 1 and 11 for additional information on clothing recommended for arc welding.
Figure 7-63. The chemical composition of the deposited weld metal for flux cored chromium and chromium-nickel steel electrodes. (American Welding Society, A5.22)
Chapter 7
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200
Modern Welding
Guide for Shade Numbers Electrode size 1/32 in. (mm) Less than 3 (2.5) 3-5 (2.5-4) 5-8 (4-6.4) More than 8 (6.4) Minimum protective shade 7 8 10 11 7 10 10 10 8 8 10 10 11 6 8 10 11 8 9 10 Suggested* shade no. (comfort) 10 12 14 11 12 14 10 12 14 12 14 6 to 8 10 12 14 9 12 14 3 or 4 2 14
Arc current (A) Less than 60 60-160 160-250 250-550 Less than 60 60-160 160-250 250-500 Less than 50 50-150
Gas metal arc welding and flux cored arc welding Gas tungsten arc welding Air carbon Arc cutting (Light) (Heavy)
150-500 Less than 500 500-1000 Less than 20 20-100 100-400 400-800
Figure 7-67. An electronic quick change lens is installed in this arc welding helmet. The welder can see the weld joint clearly until the arc is struck. When the arc is struck, the lens darkens in about 0.00004 second. (Jackson Products, Inc.) Figure 7-68. The action of an electronic quick-change lens is shown in this pair of photos. AView before arc is struck. BView after arc is struck. (Hornell Speedglas, Inc.)
in. Gas welding Light Medium Heavy Oxygen cutting Light Medium Heavy Under 1/8 1/8 to 1/2 Over 1/2 Under 1 1 to 6 Over 6
mm Under 3.2 3.2 to 12.7 Over 12.7 Under 25 25 to 150 Over 150 4 or 5 5 or 6 6 or 8 3 or 4 4 or 5 5 or 6
*As a rule of thumb, start with a shade that is too dark to see the weld zone. Then go to a lighter shade which gives sufficient view of the weld zone without going below the minimum. In oxyfuel gas welding or cutting where the torch produces a high yellow light, it is desirable to use a filter lens that absorbs the yellow or sodium line in the visible light of the (spectrum) operation. **These values apply where the actual arc is clearly seen. Experience has shown that lighter filters may be used when the arc is hidden by the workpiece.
Figure 7-69. A guide to the correct welding lens shade for various welding processes and applications. (ANSI/AWS Z49.1)
Chapter 7
201
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Modern Welding
14. The arc welding power source used for GMAW and FCAW produces a constant _____ . 15. GMAW power sources have a sloping voltage curve. How much slope should the curve have for use with carbon dioxide (CO2) gas? 16. Increasing the rate of speed on the wire feed in GMAW or FCAW increases the _____ in the circuit. 17. The shielding gas mixture suggested in Figure 13-37 for welding low-alloy steel with the short circuit GMAW process is _____% _____% He,_____% _____% Ar, and_____% _____% CO2. 18. Which gas produces the deepest penetration when used in GMAW? 19. To produce better metal transfer through the arc, reduce spatter, and stabilize the arc, _____ and _____ are added to argon or helium. 20. Describe the following GMAW electrode completely: ER90S-B3L 21. What two methods are used in FCAW to shield the weld from atmospheric contamination? 22. When feeding a FCAW electrode through the wire feed mechanism, care must be taken not to _____ the flux cored wire. 23. List five ways that FCAW wires are altered to meet the requirements of a weld. 24. Describe the following FCAW electrode completely: E80T-2. 25. What number filter lens is recommended when GTAW steel with 80A? When GMAW steel, using 220A?
Manual GTAW requires greater skill than other manual welding processes, but is capable of producing very high quality welds. Careful addition of filler metal at the proper time is critical. (Miller Electric Mfg. Co.)