Industrial Machinery Repair Episode 2 Part 11 pps

484 Maintenance Welding Argon is used more extensively than helium for GTAW because: 1 It produces a smoother, quieter arc action; 2 It operates at a lower arc voltage for any given current and arc length; 3 There is greater cleaning action in the welding of materials such as aluminum and magnesium in the AC mode; 4 Argon is more available and lower in cost than helium; 5 Good shielding can be obtained with lower flow rates; 6 Argon is more resistant to arc zone contamination by cross drafts; 7 The arc is easier to start in argon. The density of argon is approximately 1.3 times that of air and 10 times that of helium. For this reason, argon will blanket a weld area and be more resistant than helium to cross drafts. Helium, being much lighter than air, tends to rise rapidly and cause turbulence, which will bring air into the arc atmosphere. Since helium costs about three times as much as argon, and its required flow rate is two to three times that for argon, the cost of helium used as a shielding gas can be as much as nine times that of argon. Although either helium or argon can be used successfully for most GTAW applications, argon is selected most frequently because of the smoother arc operation and lower overall cost. Argon is preferred for welding thin sheet to prevent melt-through. Helium is preferred for welding thick materials and materials of high thermal conductivity such as copper and aluminum. Electrode Material for GTAW In selecting electrodes for GTAW, five factors must be considered: material, size, tip shape, electrode holder, and nozzle. Electrodes for GTAW are classified as pure tungsten, tungsten containing 1 or 2% thoria, tungsten containing 0.15 to 0.4% zirconia, and tungsten that contains an internal lateral segment of thoriated tungsten. The internal segment runs the full length of the electrode and contains 1 or 2% thoria. Overall, these elec- trodes contain 0.35 to 0.55% thoria. All tungsten electrodes are normally available in diameters from 0.010 to 0.250" and lengths from 3" to 24". Chemical composition requirements for these electrodes are given in AWS A5.12, “Specification for Tungsten Arc Welding Electrodes.” Pure tungsten electrodes, which are 99.5% pure, are the least expensive but also have the lowest current-carrying capacity on AC power and a low Maintenance Welding 485 resistance to contamination. Tungsten electrodes containing 1 or 2% thoria have greater electron emissivity than pure tungsten and, therefore, greater current-carrying capacity and longer life. Arc starting is easier, and the arc is more stable, which helps make the electrodes more resistant to contamina- tion from the base metal. These electrodes maintain a well sharpened point for welding steel. Tungsten electrodes containing zirconia have properties in between those of pure tungsten and thoriated tungsten electrodes with regard to arc starting and current-carrying capacity. These electrodes are recommended for AC welding of aluminum over pure tungsten or thoriated tungsten electrodes because they retain a balled end during welding and have a high resistance to contamination. Another advantage of the tungsten-zirconia electrodes is their freedom from the radioactive element thorium, which, although not harmful in the levels used in electrodes, is of concern to some welders. GTAW Electrode Size and Tip Shape The electrode material, size, and tip shape (Figure 24.21) will depend on the welding application, material, thickness, type of joint, and quantity. Electrodes used for AC or electrode-positive polarity will be of larger diameter than those used for electrode-negative polarity. The total length of an electrode will be limited by the length that can be accommodated by the GTAW torch. Longer lengths allow for more redressing of the tip than short lengths and are therefore more econom- ical. The extension of the electrode from the collet or holder determines the heating and voltage drop in the electrode. Since this heat is of no value to the weld, the electrode extension should be kept as short as necessary to provide access to the joint. It is recommended that electrodes to be used for DC negative-polarity weld- ing be of the 2% thoria type and be ground to a truncated conical tip. Excessive current will cause the electrode to overheat and melt. Too low a current will permit cathode bombardment and erosion caused by the low operating temperature and resulting arc instability. Although a sharp point on the tip promotes easy arc starting, it is not recommended because it will melt and form a small ball on the end. For AC and DC electrode-positive welding, the desirable electrode tip shape is a hemisphere of the same diameter as the electrode. This tip shape on the larger electrodes required for AC and DC electrode-positive welding provides a stable surface within the operating current range. Zirconia-type 486 Maintenance Welding Electrode for use with DC electrode negative Electrode tip angle Tip radius Tip radius Electrode diameter Electrode diameter Electrode diameter Tip diameter Tip diameter Electrode for use with AC or DC electrode positive Taper angle Figure 24.21 electrodes are preferred for AC and DC electrode-positive operation because they have a higher current-carrying capacity than the pure tungsten elec- trodes, yet they will readily form a molten ball under standard operating conditions. Thoriated electrodes do not ball readily and, therefore, are not recommended for AC or DC electrode-positive welding. The degree of taper on the electrode tip affects weld penetration, where the smaller taper angles tend to reduce the width of the weld bead and thus increase penetration. When preparing the tip angle on an electrode, grinding should be done parallel to the length of the electrode. Special machines are available for grinding electrodes. These canbe set to accurately grind any angle required. GTAW Electrode Holders and Gas Nozzles Electrode holders usually consist of a two-piece collet made to fit each standard-sized tungsten electrode. These holders and the part of the GTAW torch into which they fit must be capable of handling the required weld- ing current without overheating. These holders are made of a hardenable copper alloy. The function of the gas nozzle is to direct the flow of inert gas around the holder and electrode and then to the weld area. The nozzles are made of a hard, heat-resistant material such as ceramic and are available in various Maintenance Welding 487 sizes and shapes. Large sizes give a more complete inert gas coverage of the weld area but may be too big to fit into restricted areas. Small nozzles can provide adequate gas coverage in restricted areas where features of the component help keep the inert gas at the joint. Most nozzles have internal threads that screw over threads on the electrode holder. Some nozzles are fitted with a washer-like device that consists of several layers of fine-wire screen or porous powder metal. These units provide a nonturbulent or lamellar gas flow from the torch, which results in improved inert gas cover- age at a greater distance from the nozzle. In machine or automatic welding, more complete gas coverage may be provided by backup gas shielding from the fixture and a trailer shield attached to the torch. Characteristics of GTAW Power Supplies Power supplies for use with GTAW should be of the constant-current, drooping-voltage type (Figure 24.22). They may have other optional fea- tures such as up slope, down slope, pulsing, and current programming capabilities. Constant-voltage power supplies should not be used for GTAW. Constant current power source Operating poin t Current, A Voltage, V ∆V ∆A Figure 24.22 488 Maintenance Welding The power supply may be a single-phase transformer-rectifier, which also can supply AC for welding aluminum. Engine generator-type power sup- plies are usually driven by a gasoline or diesel engine and will produce DC with either a constant-current, drooping-voltage or constant-voltage char- acteristics. Engine alternator power supplies will produce AC for GTAW. A power supply capable of operating on either constant current or constant voltage should be set for the constant-current mode for GTAW. Power supplies made specifically for GTAW normally will include a high- frequency source for arc starting and valves that control the flow of inert gas and cooling water for the torch. Timers allow the valves to be opened a short time before the arc is initiated and closed a short time after the arc is extinguished. The high frequency is necessary for arc starting instead of torch starting, where tungsten contamination of the weld is likely. It should be possible to set the high frequency for arc starting only, or for continuous operation in the AC mode. Power supplies should include a secondary contactor and a means of con- trolling arc current remotely. For manual welding, a foot pedal would perform these functions of operating the contactor and controlling weld current. A power supply with a single current range is desirable because it allows the welder to vary the arc current between minimum and maximum without changing a range switch. The more advanced power supplies incorporate features that permit pulsing the current in the DC mode with essentially square pulses. Both background and pulse peak current can be adjusted, as well as pulse duration and puls- ing frequency (Figure 24.23). In the AC mode, the basic 60-Hz sine wave can be modified to produce a rectangular wave. Other controls permit the AC wave to be balanced or varied to favor the positive or negative half-cycles. This feature is particularly useful when welding aluminum and magnesium, where the control can be set to favor the positive half-cycle for maximum cleaning. In the DC mode, the pulsing capability allows welds to be made in thin material, root passes, and overhead with less chance of melt-through or droop. GTAW Torches A torch for GTAW must perform the following functions: 1 Hold the tungsten electrode so that it can be manipulated along the weld path. Maintenance Welding 489 Peak current Background current Pulse time Cycle time Arc current Time Ϫ Ϫ Ϫ Ϫ 0 0 0 0 Figure 24.23 2 Provide an electrical connection to the electrode. 3 Provide inert-gas coverage of the electrode tip, arc, and hot weld zone. 4 Insulate the electrode and electrical connections from the operator or mounting bracket. Typical GTAW torches consist of a metallic body, a collet holder, a collet, and a tightening cap to hold the tungsten electrode. The electrical cable is connected to the torch body, which is enclosed in a plastic insulating outer sheath. For manual torches, a handle is connected to the sheath. Power, gas, and water connections pass through the handle or, in the case of automatic operation, through the top of the torch. In the smaller, low-current torches, the electrode, collet, and internal components are cooled by the inert-gas flow. Larger, high-current torches are water-cooled and require connections to tap water and a drain or to a water cooler circulator. A cooler circulator 490 Maintenance Welding with distilled or deionized water is preferred to prevent buildup of mineral deposits from tap water inside the torch. Inert gas flows through the torch body and through holes in the collet holder to the arc end of the torch. A cup or nozzle is fitted over the arc end of the torch to direct inert gas over the electrode and the weld pool. The nozzles normally screw onto the torch and are made of a hard, heat-resistant ceramic. Some are made of high-temperature glass such as Vicor and are pressed on over a compressible plastic taper. Some nozzles can be fitted with an insert washer made up of several layers of fine-wire screen sometimes called a gas lens. This produces a lamellar rather than turbulent flow of inert gas to increase the efficiency of shielding. On most manual GTAW torches, the handle is fixed at an angle of approxi- mately 70 degrees to the torch body. Some makes of torches have a flexible neck between the handle and torch body which allows the angle between the handle and the torch body to be adjusted over a range from about 50 degrees to 90 degrees. Manual GTAW Techniques To become proficient in manual gas tungsten arc welding, the welder must develop skills in manipulating the torch with one hand, while controlling weld current with a foot pedal or thumb control and feeding filler metal with the other hand (see Figure 24.24). Before welding is started on any job, a rough idea of the welding conditions, such as filler material, current, shielding gas, etc., is needed. Establishing Welding Parameters for GTAW The material, thickness, joint design, and service requirements will deter- mine the weld current, inert gas, voltage, and travel speed. This information may be available in a “welding procedure specification” (WPS) or from hand- book data on the material and thickness. If welding parameters are not provided in a WPS, the information given in Tables 24.6, 24.7, and 24.8 can be used as starting-point parameters for carbon and low-alloy steels, stain- less steels, and aluminum. These should be considered starting values; final values should be established by running a number of test parts. Gas Tungsten Arc Starting Methods The gas tungsten arc may be started by touching the work with the elec- trode, by a superimposed high frequency pulse, or by a high-voltage pulse. Maintenance Welding 491 Figure 24.24 The touch method is not recommended for critical work because there is a strong possibility of tungsten contamination with this technique. Most weld power supplies intended for GTAW contain a high-frequency genera- tor (usually a spark-gap oscillator), which superimposes the high-frequency pulse on the main weld power circuit. When welding with DC electrode- negative or -positive, the high-frequency switch should be set in the HT start position. When the welder presses the foot pedal to start welding, a timer is activated, which starts the high-frequency pulse and stops it when the arc initiates. Once started, the arc will continue after the high-frequency pulse stops as long as the power and proper arc gap are maintained. When welding with AC, the switch should be set in the HF continuous position to ensure that the arc restarts after voltage reversal on each half-cycle. High-frequency generators on welders produce frequencies in the radio communications range. Therefore, manufacturers of power supplies must certify that the radio frequency radiation from the power supply does not exceed limitations established by the Federal Communications Commission (FCC). The allowable radiation may be harmful to some computer and microprocessor systems and to communications systems. These possibili- ties for interference should be investigated before high-frequency starting is used. Installation instructions provided with the power supply should be studied and followed carefully. 492 Maintenance Welding Table 24.6 60-Hz 3 wires in conduit or Welder input Ampere 3-conductor cable, Grounding size voltage rating Type R conductor 200 230 44 8 8 460 22 12 10 575 18 12 14 300 230 62 6 8 460 31 10 10 575 25 10 12 400 230 78 6 6 460 39 8 8 575 31 10 10 600 230 124 2 6 460 62 6 8 575 50 8 8 900 230 158 1 3 460 79 6 6 575 63 4 8 Table 24.7 Amp input Wire size (3 in conduit) Wire size (3 in free air) With- With- With- Volts With out With out Ground With out Ground Welder input condsr. condsr. condsr. condsr. conduct. condsr. condsr. conduct. 300 200 84 104 2 1 1 4 4 4 440 42 52 6 6 6 8 8 8 550 38 42 8 6 6 10 8 8 400 220 115 143 0 00 00 3 1 1 440 57.5 71.5 4 3 3 6 6 6 550 46 57.2 8 4 4 8 6 6 500 220 148 180 000 0000 0000 1 0 0 440 74 90 3 2 2 6 4 4 550 61 72 4 3 3 6 6 6 Maintenance Welding 493 Table 24.8 Machine size, Cable sizes for lengths (electrode plus ground) amp Up to 50 ft 50–100 ft 100–250 ft 200 2 2 1/0 300 1/0 1/0 3/0 400 2/0 2/0 4/0 ∗ 600 3/0 3/0 4/0 ∗ 900 Automatic application only ∗ Recommended longest length of 4/0 cable for 400-amp welder, 150 ft; for 600-amp welder, 100 ft. For greater distances, cable size should be increased; this may be a question of cost- consider ease of handling versus moving of welder closer to work. Oxyacetylene Cutting Steel can be cut with great accuracy using an oxyacetylene torch (see Figure 24.25). However, not all metals cut as readily as steel. Cast iron, stainless steel, manganese steels, and nonferrous materials cannot be cut and shaped satisfactorily with the oxyacetylene process because of their reluctance to oxidize. In these cases, plasma arc cutting is recommended. The cutting of steel is a chemical action. The oxygen combines readily with the iron to form iron oxide. In cast iron, this action is hindered by the presence of carbon in graphite form, so cast iron cannot be cut as read- ily as steel. Higher temperatures are necessary, and cutting is slower. In steel, the action starts at bright-red heat, whereas in cast iron, the tem- perature must be nearer the melting point in order to obtain a sufficient reaction. Because of the very high temperature, the speed of cutting is usually fairly high. However, since the process is essentially one of melting without any great action, tending to force the molten metal out of the cut, some provision must be made for permitting the metal to flow readily away from the cut. This is usually done by starting at a point from which the molten metal can [...]... 0.15 Se (min) , – – – 308 309 309S 310 310S 0.08 0 .20 0.08 0 .25 0.08 19.0 21 .0 22 .0 24 .0 22 .0 24 .0 24 .0 26 .0 24 .0 26 .0 10.0– 12. 0 12. 0–15.0 12. 0–15.0 19.0 22 .0 19.0 22 .0 – – – 1.5 Si 1.5 Si 314 316 316L 317 321 0 .25 0.06 0.04 0.06 0.06 23 .0 26 .0 16.0–18.0 16.0–18.0 18.0 20 .0 17.0–19.0 19.0 22 .0 10.0–14.0 10.0–14.0 11. 0–15.0 9.0– 12. 0 1.5–3.0 Si 2. 0–3.0 Mo 2. 0–3.0 Mo 3.0–4.0 Mo Ti (5 × %C min) 347 348 0.08... costs of making the joint 508 Maintenance Welding Table 24 .15 Composition∗ (%) AISI type Carbon Chromium Manganese Other† 405 430 430F 430FSe 4 42 446 0.08 0. 12 0. 12 0. 12 0 .20 0 .20 11. 5–14.5 14.0–18.0 14.0–18.0 14.0–18.0 18.0 23 .0 23 .0 27 .0 1.0 1.0 1 .25 1 .25 1.0 1.5 0.1-0.3 Al – 0.060 P 0.15 S(min), 0.60 Mo (opt) , 0.060 P 0.060 S, 0.15 Se (min) , – 0 .25 N ∗ Single values denote maximum percentage unless... Welding 507 Table 24 .14 Composition* (%) AISI type Carbon Chromium Nickel 20 1 20 2 301 3 02 302B 0.15 0.15 0.15 0.15 0.15 16.0–18.0 17.0–19.0 16.0–18.0 17.0–19.0 17.0–19.0 3.5–5.5 4.0–6.0 6.0–8.0 8.0–10.0 8.0–10.0 303 303Se 304 304L 305 0.15 0.15 0.08 0.03 0. 12 17.0–19.0 17.0–19.0 18.0 20 .0 18.0 20 .0 17.0–19.0 8.0–10.0 8.0–10.0 8.0– 12. 0 8.0– 12. 0 10.0–13.0 0 .20 P 0.15 S (min), 0.60 Mo (opt) , 0 .20 P 0.06 S,... diameter Should be used only with AC electrodes AC-DC Table 24 . 12 Maximum and minimum current (amp) Type of electrode and power Electrode size (in.) 5/ 32 DC electrodes, DECP power AC electrodes, AC power AC electrodes, DCEN power 3/16 1/4 5/16 3/8 1 /2 90–150 150 20 0 20 0–400 25 0–450 350–600 600–1000 — 150 20 0 20 0–300 — 300–500 400–600 — 150–180 20 0 25 0 — 300–400 400–500 Plasma Arc Cutting Plasma arc cutting... is not as good as other processes for through-cutting Maintenance Welding 495 Figure 24 .26 Electrode type and air supply specifications for arc gouging are outlined in Tables 24 .9 and 24 .10 Applications The CAC-A process may be used to provide a suitable bevel or groove when preparing plates for welding (see Fig 24 .27 ) It also may be used to back-gouge a seam prior to welding the side CAC-A provides... recommended Figure 24 .27 FCAW Gas Self shielded GTAW SAW (TIG) (Sub arc) 498 Maintenance Welding (b) Weldment: Quality Category: Drawing No Critical Functional Process Electrode Size/Type Polarity Volts WFS/AMP (either) ESO SMAW GMAW SAW TIG FCAW Sketch of the Joint Special instructions: Welder: Proced Apprvd by: Welder: Date: Figure 24 .27 continued Shield Gas/Flux Maintenance Welding 499 Table 24 .11 Type of... 0.08 0.08 17.0–19.0 17.0–19.0 9.0–13.0 9.0–13.0 Other† 0 .25 N, 5.5–7.5 Mn, 0.060 P 0 .25 N, 7.5–10.0 Mn, 0.060 P – – 2. 0–3.0 Si Cb + Ta (10 × %C min) Cb + Ta (10 × %C min but 0.10 Ta max), 0 .20 Co ∗ Single values denote maximum percentage unless otherwise noted † Unless otherwise noted, other elements of all alloys listed include maximum contents of 2. 0% Mn, 1.0% Si, 0.045% P and 0.030% S Balance is Fe... either rectifier or motor generator, will give the best results With DC, it is operated with DCEP (electrode-positive) Arc voltages normally range from about 35 to 56 volts Table 24 .11 lists recommended power sources, and Table 24 . 12 lists suggested current ranges It is recommended that the power source have overload protection in the output circuit High current surges of short duration occur with arc gouging,... resistance of the material These steels are Table 24 .13 Low, % Carbon Manganese Silicon Sulfur Phosphorus Preferred, % High, % 0.06 0.30 0.10 to 0 .25 0.35 to 0.80 0.10 or under 0.035 or under 0.03 or under 0.35 1.40 0.30 max 0.05 max 0.04 max 504 Maintenance Welding selectively used by manufacturers of railroad equipment, farm machinery, construction machinery, material-handling equipment, and other... Shielding cup Plasma cutting gas Work piece (DC+) Figure 24 .25 flow readily This method is followed until the desired amount of metal has been melted away Air-Carbon Arc Cutting and Gouging Air-carbon arc cutting (CAC-A) is a physical means of removing base metal or weld metal using a carbon electrode, an electric arc, and compressed air (see Figure 24 .26 ) In the air-carbon arc process, the intense heat . 10 575 18 12 14 300 23 0 62 6 8 460 31 10 10 575 25 10 12 400 23 0 78 6 6 460 39 8 8 575 31 10 10 600 23 0 124 2 6 460 62 6 8 575 50 8 8 900 23 0 158 1 3 460 79 6 6 575 63 4 8 Table 24 .7 Amp input Wire. condsr. conduct. 300 20 0 84 104 2 1 1 4 4 4 440 42 52 6 6 6 8 8 8 550 38 42 8 6 6 10 8 8 400 22 0 115 143 0 00 00 3 1 1 440 57.5 71.5 4 3 3 6 6 6 550 46 57 .2 8 4 4 8 6 6 500 22 0 148 180 000 0000. carefully. 4 92 Maintenance Welding Table 24 .6 60-Hz 3 wires in conduit or Welder input Ampere 3-conductor cable, Grounding size voltage rating Type R conductor 20 0 23 0 44 8 8 460 22 12 10 575 18 12 14 300