Chapter 3 Shielded Metal Arc Equipment, Setup, and Operation
OBJECTIVES • After completing this chapter, the student should be able to • describe the process of shielded metal arc welding (SMAW). • list and define the three units used to measure a welding current. • tell how adding chemicals to the coverings of the electrodes affects the arc. • discuss the three different types of current used for welding. • explain the types of welding power supplies and which type the shielded metal arc welding process requires. • define open circuit voltage and operating voltage. • explain arc blow, what causes it, and how to control it. • tell what the purpose of a welding transformer is and what kind of change occurs to the voltage and amperage with a step-down transformer.
OBJECTIVES (cont.) • compare generators and alternators. • tell the purpose of a rectifier. • read a welding machine duty cycle chart and explain its significance. • demonstrate how to determine the proper welding cable size. • demonstrate how to service and repair electrode holders. • discuss the problems that can occur as a result of poor work lead clamping. • describe the factors that should be considered when placing an arc welding machine in a welding area.
KEY TERMS • amperage • anode • cathode • duty cycle • electrons • inverter • magnetic flux lines • open circuit voltage • operating voltage • output • rectifier • step-down transformer • voltage • wattage • welding cables • welding leads
INTRODUCTION • Shielded metal arc welding (SMAW) is a welding process that uses a flux-covered metal electrode to carry an electrical current, Figure 3-1.
INTRODUCTION • The current forms an arc across the gap between the end of the electrode and the work. • The electric arc creates sufficient heat to melt both the electrode and the work. • Molten metal from the electrode travels across the arc to the molten pool on the base metal, where they mix together.
INTRODUCTION (cont.) • The end of the electrode and molten pool of metal is surrounded, purified, and protected by a gaseous cloud and a covering of molten flux produced as the flux coating of the electrode burns or vaporizes. • As the arc moves away, the mixture of molten electrode and base metal solidifies and becomes one piece. • At the same time, the molten flux solidifies, forming a solid slag. Some electrode types produce heavier slag coverings than others.
INTRODUCTION cont. • SMAW is a widely used welding process because of its low cost, flexibility, portability, and versatility. • The SMAW process is very versatile because the same SMA welding machine can be used to make a wide variety of weld joint designs in a wide variety of metal types and thicknesses, and in all positions: • Joint designs—In addition to the standard butt, lap, tee, and outside corner joints, at some time SMAW has been certified to be used to weld every possible joint design.
INTRODUCTION cont. • Metal types—Although mild steel is the most commonly metal welded a wide variety of electrode types allow SMA to be used to weld and hardface almost any metal or alloy, including cast iron, aluminum, stainless steel, and nickel. • Metal thickness—Metal as thin as 16 gauge, approximately 1/16 in. (2 mm) thick up to several feet thick, can be SMA welded.
INTRODUCTION cont. • All position—The flat welding position is the easiest and most productive because large welds can be made fast using SMA welding, but the process can be used to make welds in any position.
Welding Current • The source of heat for arc welding is an electric current. An electric current is the flow of electrons. • Electrons flow through a conductor from negative (−) to positive (+), Figure 3-2.
Welding Current • Resistance to the flow of electrons (electricity) produces heat. • The greater the resistance, the greater the heat. Air has a high resistance to current flow. • As the electrons jump the air gap between the end of the electrode and the work, a great deal of heat is produced. • Electrons flowing across an air gap produce an arc.
Electrical Measurement • Three units are used to describe any electrical current. The three units are voltage (V), amperage (A), and wattage (W). • Voltage, or Volts (V), is the measurement of electrical pressure in the same way that pounds per square inch is a measurement of water pressure. Voltage controls the maximum gap the electrons can jump to form the arc. A higher voltage can jump a larger gap. Welding voltage is associated with the welding arc’s temperature.
Electrical Measurement (cont.) • Amperage, or Amps (A), is the measurement of the total number of electrons flowing, in the same way that gallons is a measurement of the amount of water flowing. Amperage controls the size of the arc. Amperage is associated with the welding arc’s heat. • Wattage, or Watts (W), is a measurement of the amount of electrical energy or power in the arc. Watts are calculated by multiplying voltage (V) times amperes (A), Figure 3-3. Watts are the welding arc’s power or how much energy the arc is producing, Figure 3-4.
Temperature and Heat • The term temperature refers to the degree or level of thermal energy in a material and can be measured in degrees with a thermometer. • The term heat refers to the quantity of thermal energy in a material and cannot be easily measured. • Temperature and heat are to some degree related to each other, but can change independently.
Temperature and Heat • For example, the small, red-hot spark from a grinder and a red-hot weld are both at the same temperature but the weld has more heat. So the quantity of heat in a material is a function of both its temperature and its weight (mass). • Another example is a match and a bonfire, both are burning at the same temperature but the burning match does not produce the same quantity of heat as the bonfire. Likewise, the temperature of an arc from both a small diameter electrode and a large diameter electrode is the same, but the larger arc has more heat.
SMA Welding Arc Temperature • An arc’s temperature is dependent on the voltage, arc length, and the atmosphere (gas or vapor) it is passing through. • The arc temperature can range from around 5500°F (3000°C) to above 36,000°F (20,000°C), but most SMA welding arcs have effective temperatures around 11,000°F (6100°C). • The voltage and arc length are closely related because as the arc length increases the arc resistance increases so it takes a higher voltage (pressure) to keep the electrons flowing (jumping) across the gap. • The shorter the arc, the lower the arc’s resistance and the lower the arc’s voltage and the lower the temperature produced, and as the arc lengthens, the resistance increases, thus causing a rise in the arc voltage and temperature.
SMA Welding Arc Temperature • Most shielded metal arc welding electrodes have chemicals added to their coverings to stabilize the arc. • These arc stabilizers form conductive ions (gas or vapors) that make the arc more stable and reduce the arc resistance. This makes it easier to hold an arc. • By lowering the resistance, the arc stabilizers also lower the arc temperature. Changing the chemicals in the electrode flux will cause changes within the gaseous cloud around the arc. • These changes may raise or lower the resistance thus raising or lowering the welds temperature and heat.
SMA Welding Arc Heat • The amount of heat produced by the arc is determined by the amperage. • The higher the amperage setting the higher the heat produced by the welding arc, and the lower the amperage setting the lower the heat produced. • Each diameter of electrode has a recommended minimum and maximum amperage range and therefore a recommended heat range. • Not all of the heat produced by an arc reaches the weld. Some of the heat is radiated away in the form of light and heat waves, Figure 3-5.
SMA Welding Arc Heat • Some additional heat is carried away with the hot gases formed by the electrode covering. • Heat is also lost through conduction in the work. In total, about 50% of all heat produced by an arc is missing from the weld. • The 50% of the remaining heat the arc produces is not distributed evenly between both ends of the arc. • This distribution depends on the composition of the electrode’s coating and type of welding current.
Types of Welding Currents • The three different types of current used for welding are alternating current (AC), direct-current electrode negative (DCEN), and direct-current electrode positive (DCEP). • The terms DCEN and DCEP have replaced the former terms direct-current straight polarity (DCSP) and direct-current reverse polarity (DCRP). • DCEN and DCSP are the same currents, and DCEP and DCRP are the same currents. • Some electrodes can be used with only one type of current. Others can be used with two or more types of current. • Each welding current has a different effect on the weld.
DCEN • In direct-current electrode negative, the electrode is negative, and the work is positive, Figure 3-6.
DCEN (cont.) • The electrons are leaving the electrode and traveling across the arc to the surface of the metal being welded. • DCEN welding current produces a high electrode melting rate.
DCEP • In direct-current electrode positive, the electrode is positive, and the work is negative, Figure 3-7.
DCEP (cont.) • The electrons are leaving the surface of the metal being welded and traveling across the arc to the electrode.
AC • In alternating current, the electrons change direction every 1/120 of a second so that the electrode and work alternate from anode to cathode, Figure 3-8.
AC (cont.) • The positive side of an electrode arc is called the anode, and the negative side is called the cathode. • The rapid reversal of the current flow causes the welding heat to be evenly distributed on both the work and the electrode • Figure 3-9.
Types of Welding Power Supplies (Machines) • Welding power can be supplied as • Constant voltage (CV) • Rising arc voltage (RAV) • Constant current (CC) • This type of power is also called drooping arc voltage (DAV) because the arc voltage decreases as the amperage increases. • The shielded metal arc welding (SMAW) process requires a constant current arc voltage characteristic, illustrated by the constant current line in Figure 3-10.
Types of Welding Power Supplies (Machines) (cont.) • The shielded metal arc welding machine’s voltage output decreases as current increases. • This output power source provides a reasonably high open circuit voltage before the arc is struck. • The high open circuit voltage quickly stabilizes the arc. The arc voltage rapidly drops to the lower closed circuit level after the arc is struck.
Open Circuit Voltage • Open circuit voltage is the voltage at the electrode before striking an arc (with no current being drawn). • The open circuit voltage is much like the higher surge of pressure you might observe when a water hose nozzle is first opened, Figure 3-11A and B.
Open Circuit Voltage (cont.) • The higher the open circuit voltage, the easier it is to strike an arc because of the initial higher voltage pressure.
Operating Voltage • Operating, welding, or closed circuit voltage is the voltage at the arc during welding. • Operating voltage is much like the water pressure observed as the water hose is being used, Figure 3-11C.
Operating Voltage (cont.) • The operating voltage will vary with arc length, type of electrode being used, and type of current, and polarity. • The welding voltage will be between 17 V and 40 V.
Arc Blow • When electrons flow, they create lines of magnetic force that circle around the path of flow, Figure 3-12.
Arc Blow (cont.) • These lines of magnetic force are referred to as magnetic flux lines. • They space themselves evenly along a current-carrying wire. • If the wire is bent, the flux lines on one side are compressed together, and those on the other side are stretched out, Figure 3-13.
Arc Blow • The welding current flowing through a plate or any residual magnetic fields in the plate will result in unevenly spaced magnetic flux lines. • The term arc blow refers to this movement of the arc. • Arc blow makes the arc drift like a string would drift in the wind. • Figure 3-14. • The more complex a weldment becomes, the more likely arc blow will become a problem.
Controlling Arc Blow • Arc blow can be controlled or reduced by connecting the work lead to the end of the weld joint, and then welding away from the work lead, Figure 3-15.
Controlling Arc Blow (cont.) • Another way of controlling arc blow is to use two work leads, one on each side of the weld. • The best way to eliminate arc blow is to use alternating current. • Arc blow may not be a problem as you are learning to weld in the shop because most welding tables are all steel. • Try changing where you have your practice plates clamped to change the path the welding current takes through the plate, which can change the affects of arc blow.
Types of Power Sources • Two types of electrical devices can be used to produce the low-voltage, high-amperage current combination that arc welding requires. • One type uses electric motors or internal combustion engines to drive alternators or generators. • The other type uses step-down transformers. • Because transformer-type welding machines are quieter, are more energy efficient, require less maintenance, and are less expensive, they are now the industry standards. • However, engine-powered generators are still widely used for portable welding.
Transformer-Type Welding Machines • A welding transformer uses the alternating current (AC) supplied to the welding shop at a high voltage to produce the low-voltage welding power. • The heart of these welders is the step-down transformer. All transformers have the following three major components: • Primary coil—the winding attached to the incoming electrical power • Secondary coil—the winding that has the electrical current induced and is connected to the welding lead and work leads • Core—made of laminated sheets of steel and used to concentrate the magnetic field produced in the primary winding into the secondary winding, Figure 3-16
Transformer-Type Welding Machines (cont.) • As electrons flow through a wire, they produce a magnetic field around the wire. • Figure 3-17.
Transformer-Type Welding Machines (cont.) • A transformer with more turns of wire in the primary winding than in the secondary winding is known as a step-down transformer. • A step-down transformer takes a high-voltage, low-amperage current and changes it into a low-voltage, high-amperage current. • A transformer welder is a step-down transformer. It takes the high line voltage (110 V, 220 V, 440 V, etc.) and low-amperage current (30 A, 50 A, 60 A, etc.) and changes it into 17 V to 45 V at 190 A to 590 A.
Transformer-Type Welding Machines (cont.) • Welding machines can be classified by the method by which they control or adjust the welding current. The major classifications are multiple coil (called tap type), movable coil, movable core, (see Figure 3-18), and inverter type.
Multiple-Coil • The multiple-coil machine, or tap-type machine, allows the selection of different current settings by tapping into the secondary coil at a different turn value.
Multiple-Coil • The greater the number of turns, the higher the amperage is induced in the turns. • These machines may have a large number of fixed amperes, Figure 3-19, or they may have two or more amperages that can be adjusted further with a fine adjusting knob.
EXPERIMENT 3-1 • Estimating Amperages • Using a pencil and paper, you will prepare a rough estimate of the amperage setting of a welding machine listed in Table 3-1.
EXPERIMENT 3-1 (cont.) • Figure 3-20 shows a welding machine with low, medium, and high tap amperage ranges.