GMAW

Introduction GAS-METAL ARC WELDING (GMAW) is an arc welding process that joins metals together by heating them with an electric arc that is established between a consumable electrode (wire) and the workpiece. An externally supplied gas or gas mixture acts to shield the arc and molten weld pool. Although the basic GMAW concept was introduced in the 1920s, it was not commercially available until 1948. At first, it was considered to be fundamentally a high-current-density, small-diameter, bare-metal electrode process using an inert gas for arc shielding. Its primary application was aluminum welding. As a result, it became known as metal-inert gas (MIG) welding, which is still common nomenclature. Subsequent process developments included operation at low current densities and pulsed direct current, application to a broader range of materials, and the use of reactive gases (particularly carbon dioxide) and gas mixtures. The latter development, in which both inert and reactive gases are used, led to the formal acceptance of the term gas-metal arc welding. The GMAW process can be operated in semi-automatic and automatic modes. All commercially important metals, such as carbon steel, high-strength low-alloy steel, stainless steel, aluminum, copper, and nickel alloys can be welded in all positions by this process if appropriate shielding gases, electrodes, and welding parameters are chosen. Advantages. The applications of the process are dictated by its advantages, the most important of which are: · ELECTRODE LENGTH DOES NOT FACE THE RESTRICTIONS ENCOUNTERED WITH SHIELDED-METAL ARC WELDING (SMAW). · WELDING CAN BE ACCOMPLISHED IN ALL POSITIONS, WHEN THE PROPER PARAMETERS ARE USED, A FEATURE NOT FOUND IN SUBMERGED ARC WELDING. · WELDING SPEEDS ARE HIGHER THAN THOSE OF THE SMAW PROCESS. · DEPOSITION RATES ARE SIGNIFICANTLY HIGHER THAN THOSE OBTAINED BY THE SMAW PROCESS. · CONTINUOUS WIRE FEED ENABLES LONG WELDS TO BE DEPOSITED WITHOUT STOPS AND STARTS. · PENETRATION THAT IS DEEPER THAN THAT OF THE SMAW PROCESS IS POSSIBLE, WHICH MAY PERMIT THE USE OF SMALLER-SIZED FILLET WELDS FOR EQUIVALENT STRENGTHS. · LESS OPERATOR SKILL IS REQUIRED THAN FOR OTHER CONVENTIONAL PROCESSES, BECAUSE THE ARC LENGTH IS MAINTAINED CONSTANT WITH REASONABLE VARIATIONS IN THE DISTANCE BETWEEN THE CONTACT TIP AND THE WORKPIECE. · MINIMAL POSTWELD CLEANING IS REQUIRED BECAUSE OF THE ABSENCE OF A HEAVY SLAG. These advantages make the process particularly well suited to high-production and automated welding applications. With the advent of robotics, gas-metal arc welding has become the predominant process choice. Limitations. The GMAW process, like any welding process, has certain limitations that restrict its use: · THE WELDING EQUIPMENT IS MORE COMPLEX, USUALLY MORE COSTLY, AND LESS PORTABLE THAN SMAW EQUIPMENT. · THE PROCESS IS MORE DIFFICULT TO APPLY IN HARD-TO-REACH PLACES BECAUSE THE WELDING GUN IS LARGER THAN A SMAW HOLDER AND MUST BE HELD CLOSE TO THE JOINT (WITHIN 10 TO 19 MM, OR 3 8 TO 3 4 IN.) TO ENSURE THAT THE WELD METAL IS PROPERLY SHIELDED. · THE WELDING ARC MUST BE PROTECTED AGAINST AIR DRAFTS THAT CAN DISPERSE THE SHIELDING GAS, WHICH LIMITS OUTDOOR APPLICATIONS UNLESS PROTECTIVE SHIELDS ARE PLACED AROUND THE WELDING AREA. · RELATIVELY HIGH LEVELS OF RADIATED HEAT AND ARC INTENSITY CAN HINDER OPERATOR ACCEPTANCE OF THE PROCESS. Gas-Metal Arc Welding D.B. Holliday, Westinghouse Electric Corporation Process Fundamentals Principles of Operation. In the GMAW process (Fig. 1), an arc is established between a continuously fed electrode of filler metal and the workpiece. After proper settings are made by the operator, the arc length is maintained at the set value, despite the reasonable changes that would be expected in the gun-to-work distance during normal operation. This automatic arc regulation is achieved in one of two ways. The most common method is to utilize a constant-speed (but adjustable) electrode feed unit with a variable-current (constant-voltage) power source. As the gun-to-work relationship changes, which instantaneously alters the arc length, the power source delivers either more current (if the arc length is decreased) or less current (if the arc length is increased). This change in current will cause a corresponding change in the electrode melt-off rate, thus maintaining the desired arc length. The second method of arc regulation utilizes a constant-current power source and a variable-speed, voltage-sensing electrode feeder. In this case, as the arc length changes, there is a corresponding change in the voltage across the arc. As this voltage change is detected, the speed of the electrode feed unit will change to provide either more or less electrode per unit of time. This method of regulation is usually limited to larger electrodes with lower feed speeds. Metal Transfer Mechanisms. The characteristics of the GMAW process are best described by reviewing the three basic means by which metal is transferred from the electrode to the work: short-circuiting transfer, globular transfer, or spray transfer. The type of transfer is determined by a number of factors, the most influential of which are: · MAGNITUDE AND TYPE OF WELDING CURRENT · ELECTRODE DIAMETER · ELECTRODE COMPOSITION · ELECTRODE EXTENSION BEYOND THE CONTACT TIP OR TUBE · SHIELDING GAS · POWER SUPPLY OUTPUT Short-circuiting transfer encompasses the lowest range of welding currents and electrode diameters associated with the GMAW process. This type of transfer produces a small, fast-freezing weld pool that is generally suited for joining thin sections, for out-of-position welding, and for bridging of large root openings. Metal is transferred from the electrode to the workpiece only during a period when the electrode is in contact with the weld pool, and there is no metal transfer across the arc gap (Fig. 2). FIG. 2 TRANSFER MODES IN GMAW PROCESS The electrode contacts the molten weld pool at a steady rate that can range from 20 to over 200 times per second. As the wire touches the weld metal, the current increases and the liquid metal at the wire tip is pinched off, initiating an arc. The rate of current increase must be high enough to heat the electrode and promote metal transfer, yet low enough to minimize spatter caused by violent separation of the molten drop. The rate of current increase is controlled by adjusting the power source inductance. The optimum setting depends on the electrical resistance of the welding circuit and the melting temperature of the electrode. When the arc is initiated, the wire melts at the tip as it is fed forward toward the next short circuit. The open-circuit voltage of the power source must be low enough so that the drop of molten metal cannot transfer until it contacts the weld metal. Because metal transfer only occurs during short circuiting, the shielding gas has very little effect on the transfer itself. However, the gas does influence the operating characteristics of the arc and the base-metal penetration. The use of carbon dioxide generally produces high spatter levels, when compared with inert gases, but it allows deeper penetration when welding steels. To achieve a good compromise between spatter and penetration, mixtures of carbon dioxide and argon are often used. With nonferrous metals, argon-helium mixtures are used to achieve this compromise. Globular Transfer. With a positive electrode, globular transfer takes place when the current density is relatively low, regardless of the type of shielding gas. However, the use of carbon dioxide or helium results in this type of transfer at all usable welding currents. Globular transfer is characterized by a drop size with a diameter that is greater than that of the electrode. This large drop is easily acted upon by gravity, which limits successful transfer to the flat position. At average currents that are slightly higher than those used in short-circuiting transfer, axially directed globular transfer can be achieved in a substantially inert gas shield. However, if the arc length is too short, then the enlarging drop can short to the workpiece, become superheated, and disintegrate, producing considerable spatter. Therefore, the arc length must be long enough to ensure that the drop detaches before it contacts the weld pool. However, when higher voltage values are used, the weld is likely to be unacceptable, because of a lack of fusion, insufficient penetration, and excessive reinforcement. This limits the use of this transfer mode to very few production applications. Carbon dioxide shielding produces a randomly directed globular transfer when the welding current and voltage values are significantly higher than the range used for short-circuiting transfer. Although severe spatter conditions result when conventional techniques are used, carbon dioxide is still the most commonly used shielding gas for welding mild steel when the quality requirements are not too rigorous. The spatter problem is controlled by "burying" the arc below the weld/base-metal surface. The resulting arc forces are adequate enough to produce a depression that contains the spatter. This technique requires relatively high currents and results in very deep penetration. Good operator setup skills are required. However, poor wetting action can result in an excessive weld reinforcement. Spray Transfer. A very stable, spatter-free "spray" transfer mode can be produced when argon-rich shielding is used. This type of transfer requires the use of direct current with the electrode positive and a current level that is above a critical value called the "transition current." Below this current level, transfer occurs in the globular mode at the rate of a few drops per second. At values above the transition current, transfer occurs in the form of very small drops that are formed and detached at the rate of hundreds per second and are accelerated axially across the arc gap. The transition current is proportional to the electrode diameter, and, to a lesser extent, to the electrode extension. It also has a direct relationship to the filler metal melting temperature. Transition currents for various materials and electrode diameters are shown in Table 1. The spray transfer mode results in a highly directed stream of discrete drops that are accelerated by arc forces to velocities that overcome the effects of gravity. This enables the process to be used in any position, under certain conditions. Because the drops are separated, short circuits do not occur, and the spatter level is negligible, if not totally eliminated. Another characteristic of spray transfer is the "finger" penetration pattern that it produces directly below the electrode tip. Although the penetration can be deep, it can be affected by magnetic fields that must be controlled to ensure that it is always located at the center of the weld penetration profile. Otherwise, a lack of fusion and an irregular bead surface profile can result. The spray transfer mode can be used to weld almost any metal or alloy, because of the inert characteristics of the argon shield. Sometimes, thickness can be a factor, because of the relatively high current levels required. The resultant arc forces can cut through, rather than weld, thin sheets. In addition, high deposition rates can result in a weld pool size that cannot be supported by surface tension in the vertical and overhead positions. However, the thickness and position limitations of spray transfer have been largely overcome by specially designed power supplies. These machines produce carefully controlled current outputs that "pulse" the welding current from levels below the transition current to levels above it. Figure 3 shows the two levels of current provided by these machines. One is a constant, low-background current that sustains the arc without providing enough energy to cause the formation of drops on the wire tip. The other is a superimposed pulsing current with an amplitude that is greater than the transition current necessary for spray transfer. During this pulse, one or more drops are formed and transferred. The frequency and amplitude of the pulses control the energy level of the arc and, therefore, the rate at which the wire melts. By reducing the arc energy and the wire melting rate, it is possible to retain many of the desirable features of spray transfer while joining sheet metals and welding thick metals in all positions Many variations of such machines are available. The simplest provide a single frequency of pulsing (60 or 120 pulses/s) and independent control of the background and pulsing current levels. Synergic machines, which are sophisticated, automatically provide the optimum combination of background and pulsing current levels for any given setting of wire feed speed. Normally, these settings are specific to an electrode/shielding gas combination and must be changed or reprogrammed when the combination is changed. Process Variables. The important variables of the GMAW process that affect weld penetration, bead geometry, and overall weld quality are: · WELDING CURRENT (ELECTRODE FEED SPEED) · POLARITY · ARC VOLTAGE (ARC LENGTH) · TRAVEL SPEED · ELECTRODE EXTENSION · ELECTRODE ORIENTATION (GUN ANGLE) · ELECTRODE DIAMETER Knowledge and control of these variables are essential to consistently produce welds of satisfactory quality. Because they are not completely independent of one another, changing one variable generally requires changing one or more of the others to produce the desired results. The effects of these variables on deposit attributes are shown in Table 2. Considerable skill and experience are necessary to select the optimal combination for each application. This selection is further complicated by the fact that the optimal settings for the variables are also affected by the type of base metal, the electrode composition, the welding position, quality requirements, and the number of completed weldments required. Thus, no single set of parameters provides optimal results in every case. Welding Current. As the electrode feed speed is varied, the welding current varies in a like manner when a constantvoltage power source is used. This occurs because the current output of the power source varies dramatically with the slight changes in the arc voltage (arc length) that result when changes are made in the electrode feed speed. When all other variables are held constant, an increase in welding current results in an increase in the depth and width of penetration, deposition rate, and weld bead size. Polarity is the term used to described the electrical connection of the welding gun in relation to the terminals of a directcurrent (dc) power source. When the gun power lead is connected to the positive terminal, the polarity is designated as direct current, electrode positive (DCEP). Alternatively, a connection to the negative terminal is designated as direct current, electrode negative (DCEN). The vast majority of GMAW applications utilize DCEP, because it provides for a stable arc, low spatter, a good weld bead profile, and the greatest depth of penetration. Arc voltage and arc length are related terms that are often used interchangeably. However, they are different. Arc voltage is an approximate means of stating the physical arc length in electrical terms. The same physical arc length, however, could yield different arc voltage readings, depending on factors such as shielding gas, current, and electrode extension. When all variables are held constant, a reliable relationship exists between the two: an increase in voltage setting will result in longer arc length. Although the arc length is the variable of interest and the one that should be controlled, arc voltage is more easily monitored. Because of this fact, and because the arc voltage is normally required to be specified in welding procedures, it is the term that is more commonly used. From any specific value of arc voltage, an increase tends to flatten the weld bead and increase the width of the fusion zone. Excessively high voltage can cause porosity, spatter, and undercut. A reduction in voltage results in a narrower weld bead with a higher crown. Travel speed is the linear rate at which the arc is moved along the weld joint. When all other conditions are held constant, weld penetration is a maximum at an intermediate travel speed. When travel speed is decreased, the filler metal deposition per unit length increases. At very slow speeds, the welding arc impinges on the molten weld pool, rather than the base metal, thereby reducing the effective penetration. As the travel speed is increased, the thermal energy transmitted to the base metal from the arc increases, because the arc acts more directly on the base metal. However, further increases in travel speed impart less thermal energy to the base metal. Thus, melting of the base metal first increases and then decreases with increasing travel speed. As travel speed is increased further, there is a tendency toward undercutting along the edges of the weld bead, because there is insufficient deposition of filler metal to fill the path melted by the arc. Electrode orientation is described in two ways: by the relationship of the electrode axis with respect to the direction of travel (the travel angle) and by the angle between the electrode axis and the adjacent work surface (work angle). When the electrode points in a direction opposite to the travel direction, it results in a trail angle and is known as the backhand welding technique. When the electrode points in the direction of travel, it results in a lead angle and is called the forehand welding technique. For all positions, a trailing travel angle that ranges from 5 to 15° (from perpendicular) provides a weld with maximum penetration and a narrow, convex surface configuration. It also provides for maximum shielding of the molten weld pool. However, the common technique utilizes a leading travel angle, which provides better visibility for the operator and a weld with a flatter surface profile. For some materials, such as aluminum, a leading angle is preferred, because it provides a "cleaning action" ahead of the molten weld metal, which promotes wetting and reduces base-material oxidation. When producing fillet welds in the horizontal position, the work angle should be about 45° to the vertical member. The electrode extension is the distance between the last point of electrical contact (usually the gun contact tip or tube) and the end of the electrode. An increase in the amount of this extension causes an increase in electrical resistance. This, in turn, generates additional heat in the electrode, which contributes to greater electrode melting rates. Without an increase in arc voltage, the additional metal will be deposited as a narrow, high-crowned weld bead. The optimum electrode extension generally ranges from 6.4 to 13 mm ( 1 4 to 1 2 in.) for short-circuiting transfer and from 13 to 25 mm ( 1 2 to 1 in.) for spray and globular transfers. The electrode diameter influences the weld bead configuration. A larger electrode requires a higher minimum current than a smaller electrode does to achieve the same metal transfer characteristics. Higher currents, in turn, produce additional electrode melting and larger, more-fluid weld deposits. Higher currents also result in higher deposition rates and greater penetration, but may prevent the use of some electrodes in the vertical and overhead positions. Gas-Metal Arc Welding D.B. Holliday, Westinghouse Electric Corporation Equipment The basic equipment for a typical GMAW installation is shown in Fig. 1. The major components are discussed below. A welding gun provides electrical current to the electrode, directs it to the workpiece, and provides a vehicle for directing shielding gas to the weld area. Different types of guns have been designed for many varied applications, ranging from heavy-duty guns for high-current, high-volume production to lightweight guns for low-current or out-of-position welding. The most commonly used guns are designed to be cooled by the surrounding air (Fig. 4). However, as amperage requirements increase, a water-cooled gun may be required. Guns are rated based on their current-carrying capacity, generally with a CO2 shielding gas. If inert gases are used, these gun ratings must be reduced significantly. Guns can also be equipped with their own integral electrode feed units. The contact tube, usually made of copper or a copper alloy, is used to transmit welding current to the electrode, as well as to direct the electrode toward the work. The contact tube is connected electrically to the welding power supply by the power cable. The inner surface of the contact tube is very important, because the electrode must feed easily through this tube while making a good electrical contact. Generally, the hole in the contact tube would be from 0.13 to 0.25 mm (0.005 to 0.010 in.) larger than the wire being used, although larger sizes may be required for materials such as aluminum. The hole should be checked periodically and replaced if it has become elongated because of excessive wear. If a tip in this condition is used, it can result in poor electrical contact and erratic arc characteristics. The nozzle directs an even-flowing column of shielding gas into the welding zone. It is extremely important to maintain an even flow in order to adequately protect the molten weld metal from atmospheric contamination. Different-sized nozzles are available and should be chosen according to the application, that is, larger nozzles for high-current work where the puddle is large and smaller nozzles for low-current and short-circuiting welding. The electrode conduit and liner are connected to a bracket adjacent to the feed rolls on the electrode feed motor. The conduit and liner support both protect and direct the electrode from the feed rolls to the gun and contact tube. Uninterrupted electrode feeding is necessary to ensure good arc stability. A steel liner is recommended when using hard electrode materials, such as steel and copper, whereas nylon liners should be used for soft electrode materials, such as aluminum and magnesium. The electrode feed unit, or wire feeder, consists of an electric motor, output shaft, drive rolls, and accessories for maintaining electrode alignment and pressure (Fig. 5). These units can be separate or integrated with the speed control or located remotely from it. The electrode feed motor is usually a direct-current type and provides the mechanical energy for pushing the electrode through the gun and to the work. It has a control circuit that varies the motor speed over a broad range. The feed unit can be an integral component of the gun (Fig. 6) or dual-feed units, one in the gun and one in a separate feeder, can be electrically coupled together to provide a "push-pull" system. The spool-gun and push-pull systems are often used on aluminum, where difficulty can be encountered in trying to push the wire through a conduit to the gun. The feed motor is connected through a gear reducer to a set of wire feed rolls that transmit mechanical energy to the electrode, pulling it from the source and pushing it through the welding gun. Various types of rolls are available, including knurled, "U" groove, "V" groove, and flat. The knurled design is used for harder wires, such as steel, and allows maximum frictional force to be transmitted to the wire with a minimum of drive roll pressure. These types of rolls are not recommended for soft wires, such as aluminum, because they tend to cause the wire to flake, which can eventually clog the gun or liner. For these softer wires, the "U" groove or "V" groove type will allow the application of uniform pressure around the wire without deforming it. The flat rolls can be used with the smaller-diameter wires and in combination with a "U" or "V" groove. The welding control mechanism and the electrode feed motor for semiautomatic operation are usually provided in one integrated package. The main function of the welding control mechanism is to regulate the speed of the electrode feed motor, usually through an electronic governor. The control also regulates the starting and stopping of the electrode feed through a signal received from the gun switch. Normally, shielding gas, water (when used), and welding power are also delivered to the gun through the control mechanism, which requires direct connection to these facilities and the power supply. Gas and water flow are regulated to coincide with the weld start and stop by using solenoid valves. The control mechanism can also sequence gas flow starts and stops and energize the power source contractor. The control mechanism may allow some gas to flow before welding starts (preflow) and after welding stops (postflow) to protect the molten weld puddle. The control mechanism is usually independently powered by 115 V ac. The welding power source provides suitable electrical power (generally 20 to 80 V) that is delivered to the electrode and workpiece to produce the arc. Because the vast majority of GMAW applications utilize DCEP, the positive lead is connected to the gun and the negative lead, to the workpiece. The power source can be the "static" type in which incoming utility power (120 to 480 V) is reduced to welding voltage by a transformer or solid-state inverter. It could also be the "rotating" type in which the welding power is provided by a rotating generator driven by a motor or internal combustion engine. The static type is normally used in shops where there is an available source of power. It has advantages over the rotating type in that it can respond more rapidly to varying arc conditions. The rotating type is generally used at field sites where external power in unavailable. Both types of power sources can be designed and built to provide either a constant current or constant potential (cp) output, the latter of which is the most common by far. On newer power sources, this cp output can be pulsed at either a constant or variable frequency. With the advent of solidstate electronic power sources, such as inverters, even further control over the pulsing variables (for example, frequency, pulse width, and so on) can be obtained. When used in conjunction with a constant-speed wire feeder, the constant-voltage power source compensates for the variations in the contact-tip-work-distance that can occur during normal welding operations. It does this by instantaneously increasing or decreasing welding current to increase or decrease the electrode burnoff rate. The initial arc length is established by adjusting the voltage at the power source. Once this is set, no other changes are required during welding. The wire feed speed, which is also the current control, is then set by the operator and adjusted as necessary. In addition to this self-regulating feature of the CP power source, control over slope and inductance is included on those machines intended for short-circuiting transfer. Additional controls are also provided when using power sources that have pulsing capabilities. Additional information is available in the article "Power Sources" in this Volume. Electrode Source. The GMAW process uses a continuously fed electrode that is consumed at relatively high speeds. Therefore, the electrode source must provide a large volume of material that can readily be fed to the gun to ensure maximum process efficiency. This source is usually in the form of a spool or coil that can hold from 7 to 27 kg (15 to 60 lb) of wire that has been wound to allow free feeding without kinks or tangles. Larger spools of up to 115 kg (250 lb) are also available, and material can be provided in drums of 340 to 455 kg (750 to 1000 lb). Small spools of 0.45 to 0.9 kg (1 to 2 lb) are used for spool-on-gun equipment. Regulated Shielding Gas Supply. A system is required to provide constant shielding gas pressure and flow rate during welding. This system consists of a regulator connected to a supply of "welding grade" shielding gas, as well as the necessary hoses or piping. The regulator is a device that reduces the source gas pressure to a constant working pressure, regardless of variations at the source. It can be a single-stage or dual-stage type and may have a built-in flowmeter. The shielding gas source can be a high-pressure cylinder, a liquid-filled cylinder, or a bulk-liquid tank. Gas mixtures are available in a single cylinder. Mixing devices can also be used to obtain the correct proportions when two or more gases or liquids are used. The type and size of the gas storage source depend on economic considerations that are based on the volume of shielding gas consumed per unit of time. Gas-Metal Arc Welding D.B. Holliday, Westinghouse Electric Corporation Consumables The two consumable, but essential, elements of the GMAW process are the electrode and the shielding gas, each of which is described below. The chemical composition of the electrode must be selected to achieve the desired properties in the weld metal. The composition is designed with extra deoxidizers or other scavenging agents to compensate for reactions with the atmosphere and the base metal. The deoxidizers most commonly used in steel electrodes are silicon and manganese. Silicon can also be added in all transfer modes to increase weld metal fluidity or it can be added when a 300 series stainless steel electrode is used. The physical characteristics (finish, straightness, and others) of electrodes used in the GMAW process are important to successful welding. The material specifications for these electrodes establish manufacturing requirements to ensure that users receive a uniform product that feeds smoothly through the equipment and has these characteristics, as well: · UNIFORM WINDING ON THE SPOOL OR COIL WITH NO KINKS OR BENDS · SMOOTH SURFACE FINISH FREE OF SLIVERS, SCRATCHES, OR SCALE · PRESCRIBED CAST AND HELIX · UNIFORM DIAMETER Cast and helix refer to dimensions of a single coil of wire removed from a spool or coil and layed (that is, cast) on a flat surface. If this coil is too small in diameter (cast) or shows an excessive lift from the flat surface (helix) wire, feeding problems during welding can be anticipated. Shielding Gas The primary function of the shielding gas in most of the welding processes is to protect the surrounding atmosphere from contact with molten metal. In the GMAW process, this gas plays an additional role in that it has a pronounced effect on arc characteristics, mode of metal transfer, depth of fusion, weld bead profile, welding speed, and cleaning action. Inert gases, such as argon and helium, are commonly used, as is the active gas, CO2. It is also common to use mixtures of these gases and to employ small additions of oxygen. Information about shielding gas compositions and about which gases to use for specific joining applications is provided in the article "Shielding Gases" in this Volume. Gas-Metal Arc Welding D.B. Holliday, Westinghouse Electric Corporation Safety The major hazards of concern during GMAW are: the fumes and gases, which can harm health; the high-voltage electricity, which can injure and kill; the arc rays, which can injure eyes and burn skin; and the noise which may be present that can damage hearing. The type and amount of fumes and gas present during welding depend on the electrode being used, the alloy being welded, and the presence of any coatings on the base metal. To guard against potential hazards, a welder should keep his head out of the fume plume and avoid breathing the fumes and gases caused by the arc. Ventilation is always required. Electrode shock can result from exposure to the high open-circuit voltages associated with welding power supplies. All electrical equipment and the workpiece must be connected to an approved electrical ground. Cables should be of sufficient size to carry the maximum current required. Insulation should be protected from cuts and abrasion, and the cable should not come into contact with oils, paints, or other fluids which may cause deterioration. Work areas, equipment, and clothing must be kept dry at all times. The welder should be well insulated, wearing dry gloves, rubbersoled shoes, and standing on a dry board or platform while welding. Radiant energy, especially in the ultraviolet range, is intense during GMAW. To protect the eyes from injury, the proper filter shade for the welding-current level selected should be used. These greater intensities of ultraviolet radiation can cause rapid disintegration of cotton clothing. Leather, wool, and aluminum-coated cloth will better withstand exposure to arc radiation and better protect exposed skin surfaces. When noise has been determined to be excessive in the work area, ear protection should be used. This can also be used to prevent spatter from entering the ear. Conventional fire prevention requirements, such as removal of combustibles from the work area, should be followed. Sparks, slag, and spatter can travel long distances, so care must be taken to minimize the start of a fire at locations removed from the welding operation. For further information, see the guidelines set forth in the National Fire Protection Association Standard NFPA No. 51B, "Fire Protection in Use of Cutting and Welding Processes." Care should be exercised in the handling, storage, and use of cylinders containing high-pressure and liquefied gases. Cylinders should be secured by chains or straps during handling or use. Approved pressure-reducing regulators should be used to provide a constant, controllable working pressure for the equipment in use. Lubricants or pipe fitting compounds should not be used for making any connections, as they can interfere with the regulating equipment, and in the case of oxygen service, they can contribute to a catastrophic fire.