CHAPTER 7 SHIELDED METAL-ARC WELDING AND WEARFACING
The shielded metal-arc welding process, referred to as metallic-arc welding, arc welding, or stick welding, is extensively used in welding ferrous and nonferrous metals. It has many applications for producing a vast assortment of metal products. Shielded metal-arc welding is found in the ship building industry and in the construction industry for fabricating girders, beams, and columns. Because it is easy to use and portable, shielded metal-arc welding is universally used in the repair and servicing of equipment, machinery, and a host of other items.
MANUAL SHIELDED METAL-ARC WELDING
Arc welding provides you the ability to join two metals by melting them with an arc generated between a coated-metal electrode and the base metal. The temperatures developed by the arc can reach as high as 10000°F. The arc energy is provided by a power source that generates either direct or alternating current. The electrodes that carry the current produce a gas that shields the arc from the atmosphere and supplies filler metal to develop the weld shape.
A wide variety of welding equipment is available, and there are many differences between the makes and models of the equipment produced by the manufacturers. However, all types of arc-welding equipment are similar in their basic function of producing the high-amperage, low-voltage electric power required for the welding arc. In this discussion, we are primarily concerned with the typical items of arc-welding equipment, rather than the specific types. For specific information about the equipment your battalion or duty station has available, consult the manufacturer's instruction manual. For additional operational information and safety instruction, have your leading welding petty officer explain the operation to you.
The basic parts of a typical shielded metal-arc welding outfit include a welding machine, cables, electrode holder (stinger), and electrodes. The Steelworker also requires a number of accessories that include a combination chipping hammer and wire brush, welding table (for shopwork), C-clamps, and protective apparel.
Before we discuss the different types of welding machines, you must first have a basic knowledge of the electrical terms used with welding.
Many terms are associated with arc welding. The following basic terms are especially important.
ALTERNATING CURRENT.- Alternating current is an electrical current that has alternating negative and positive values. In the first half-cycle, the current flows in one direction and then reverses itself for the next half-cycle. In one complete cycle, the current spends 50 percent of the time flowing one way and the other 50 percent flowing the other way. The rate of change in direction is called frequency, and it is indicated by cycles per second. In the United States, the alternating current is set at 60 cycles per second.
AMPERE.- Amperes, sometimes called "amps," refers to the amount of current that flows through a circuit. It is measured by an "amp" meter.
CONDUCTOR.- Conductor means any material that allows the passage of an electrical current.
CURRENT.- Current is the movement or flow of an electrical charge through a conductor.
DIRECT CURRENT.- Direct current is an electrical current that flows in one direction only.
ELECTRICAL CIRCUIT.- Electrical circuit is the path taken by an electrical current flowing through a conductor from one terminal of the source to the load and returning to the other terminal of the source.
POLARITY.- Polarity is the direction of the flow of current in a circuit. Since current flows in one direction only in a do welder, the polarity becomes an important factor in welding operations.
Figure 7-1.-A 300 amp ac/dc portable welding machine.
VOLT.- A volt is the force required to make the current flow in an electrical circuit. It can be compared to pressure in a hydraulic system. Volts are measured with a volt meter.
The power source used in arc welding is called a welding machine or a welder. Three basic types of welding machines are presently in use: motorgenerators, transformers, and rectifiers.
MOTOR-GENERATOR WELDING MACHINES.- These types of welding machines are powered by electrical, gasoline, or diesel motors. The diesel and gasoline motors are ideal for use in areas where electricity is not available. Portable gas/diesel welding machines are part of the equipment allowance for Naval Mobile Construction Battalions. These machines usually have the capability of generating alternating or direct current. On the newer machines, when you are welding in the directcurrent mode, the polarity can be changed by turning a switch. Some of the older machines require reversing the cable connections. One of the advantages of a direct-current (dc) welding generator is that you have the choice of welding with either straight or reverse polarity. The welding machine, as shown in figure 7-1, consists of a heavy-duty, ac/dc 300 amp generator powered by a diesel engine. The generator is also capable of producing 3 kilowatts of 60 cycle ac power.
Welding machines are made in six standardized ratings for general purposes and are listed as follows:
Figure 7-2.-An ac arc-welding transformer.
1. Machines rated 150 and 200 amperes-30 volts are for light-shielded metal-arc welding and for inertgas arc welding. They are also for general-purpose jobs or shopwork.
2. Machines rated 200,300, and 400 amperes-40 volts are for general welding purposes by machine or manual application.
3. Machines rated 600 amperes-40 volts are for submerged-arc welding or carbon-arc cutting.
ALTERNATING-CURRENT TRANSFORMER WELDING MACHINES.- Practically all the alternating current (at) arc-welding machines in use are the static-transformer type, as shown in figure 72. These types of machines are the smallest, least expensive, and the lightest type of welders made. Industrial applications for manual operation use machines having 200, 300, and 400 ampere ratings. Machines with a 150-ampere rating are used in light industrial, garage, and job/shop welding.
The transformers are usually equipped with arcstabilizing capacitors. Current control is provided in several ways by the welding transformer manufacturers. One such method is an adjustable reactor that is set by turning a crank until the appropriate setting is found. Another method is by plugging the electrode cable into different sockets located on the front of the machine.
One major advantage of ac transformers is the freedom from arc blow, which often occurs when welding with direct-current (dc) machines. Arc blow causes the arc to wander while you are welding in corners on heavy metal or using large coated electrodes.
Table 7-1.-Cable Size Selection Guide
Figure 7-3.-Combination ac, do transformer-rectifier arc welder.
RECTIFIER WELDING MACHINES.- Rectifier welders are single-phase or three-phase transformers that have selenium or silicon rectifiers added to rectify (change) the output current from alternating to direct current. Most of these machines have the capability of producing either ac or do straight or reverse polarity current. By flicking a switch, the welder can select the current that best suits the job. Figure 7-3 shows an example of a combination ac/dc rectifier.
Welding cables carry the current to and from the workpiece. One of the cables runs from the welding machine to the electrode holder and the other cable connects the workpiece to the welding machine. The cable that connects the workpiece to the welding machine is called the ground. When the machine is turned on and the operator touches the electrode to the workpiece, the circuit is completed, current begins to flow, and the welding process commences.
The welding cables must be flexible, durable, well insulated, and large enough to carry the required current. Only cable that is specifically designed for welding should be used. A highly flexible cable must be used for the electrode holder connection. This is necessary so the operator can easily maneuver the electrode holder during the welding process. The ground cable need not be so flexible because once it is connected, it does not move.
Two factors determine the size of welding cable to use: the amperage rating of the machine and the distance between the work and the machine. If either amperage or distance increases, the cable size also must increase. (See table 7-1.) A cable that is too small for the amperage or the distance between the machine and the work will overheat. On the other hand, larger size cables are more difficult to handle, especially if you are working on a structure that requires a lot of moving around. The best size cable is one that meets the amperage demand but is small enough to manipulate with ease.
As a rule, the cable between the machine and the work should be as short as possible. Use one continuous length of cable if the distance is less than 35 feet. If you must use more than one length of cable, join the sections with insulated lock-type cable connectors. Joints in the cable should be at least 10 feet away from the operator.
An electrode holder, commonly called a stinger, is a clamping device for holding the electrode securely in any position. The welding cable attaches to the holder through the hollow insulated handle. The design of the electrode holder permits quick and easy electrode exchange. Two general types of electrode holders are in use: insulated and noninsulated. The noninsulated holders are not recommended because they are subject to accidental short circuiting if bumped against the workpiece during welding. For safety reasons, try to ensure the use of only insulated stingers on the jobsite.
Electrode holders are made in different sizes, and manufacturers have their own system of designation. Each holder is designed for use within a specified range of electrode diameters and welding current. You require a larger holder when welding with a machine having a 300-ampere rating than when welding with a 100-ampere machine. If the holder is too small, it will overheat.
The use of a good ground clamp is essential to producing quality welds. Without proper grounding, the circuit voltage fails to produce enough heat for proper welding, and there is the possibility of damage to the welding machine and cables. Three basic methods are used to ground a welding machine. You can fasten the ground cable to the workbench with a C-clamp (fig. 74), attach a spring-loaded clamp (fig. 7-5) directly onto the workpiece, or bolt or tack-weld the end of the ground cable to the welding bench (fig. 7-6). The third way creates a permanent common ground.
Strong welds require good preparation and procedure. The surface area of the workpiece must be free of all foreign material, such as rust, paint, and oil. A steel brush is an excellent cleaning tool and is an essential
Figure 7-4.---C-clamped ground cable.
Figure 7-5.-A spring-loaded ground clamp for the ground lead.
Figure 7-6.-Bolted and tack-welded ground clamps.
part of the welder's equipment. After initial cleaning and a weld bead has been deposited, the slag cover must be removed before additional beads are added. The chipping hammer was specifically designed for this task. The chipping operation is then followed by more brushing, and this cycle is repeated until the slag has been removed. When the slag is not removed, the result is porosity in the weld that weakens the weld joint.
Cleaning can also be accomplished by the use of power tools or chemical agents. If these items are used, it is essential that all safety precautions are followed.
Arc welding not only produces a brilliant light, but it also emits ultraviolet and infrared rays that are very dangerous to your eyes and skin. In chapter 3, personal safety items, such as helmets, lenses, and gloves, were covered. An important item that needs to be covered here is welding screens. The welder not only has to protect himself but he also must take precautions to protect other people who may be working close by. When you are welding in the field, you must install a welding screen around your work area. It can be an elaborate factory-manufactured screen or as simple as one constructed on site from heavy fire-resistant canvas. WARNING
Never look at thes welding arc without proper eye protection. Looking at the arc with the naked eye could result in permanent eye damage. If you receive flash burns, they should be treated by medical personnel.
Another area often overlooked is ventilation. Welding produces a lot of smoke and fumes that can be injurious to the welder if they are allowed to accumulate. This is especially true if you are welding in a tank or other inclosed area. Permanent welding booths should be equipped with a exhaust hood and fan system for removal of smoke and fumes.