Shielded Metal Arc Equipment, Setup, and Operation
INTRODUCTION
Shielded metal arc welding (SMAW) is a welding process that uses a flux-covered metal electrode to carry an electrical current. The current forms an arc across the gap between the end of the electrode and the work.
The electric arc creates sufficient heat and temperature to melt both the end of the electrode and the base metal being welded. Molten metal from the end of the electrode travels across the arc to the molten pool on the metal being welded. The metal from the electrode and the molten base metal are mixed together to form the weld. The high temperature at the electrode end causes the flux covering around the electrode to burn or vaporize into a gaseous cloud. This gaseous cloud surrounds, purifies, and protects at the end of the electrode and molten pool of base metal. Some of the electrode flux forms a molten protective slag on top of the molten weld pool. As the arc moves away, the weld metal cools, forming one solid piece of metal. SMAW is the most widely used welding process for metal fabrication because of its low cost, flexibility, portability, and versatility. The welding machine itself can be as simple as a 110-volt, step-down transformer that can be plugged into a normal electrical outlet. The electrodes are available from a large number of manufacturers in packages ranging from 1 lb (0.5 kg) to 50 lb (22 kg).
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 SMAW has been certified to be used to weld every possible joint design.
Metal types—Although mild steel is the most common SMA-welded metal; stainless steel, aluminum, and cast iron are easily SMA welded.
Metal thickness—Metal as thin as 16 gauge and approximately 1/16 in. (2 mm) thick up to several feet thick can be SMA welded.
All positions—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.
SMAW is a very portable process because it is easy to move the equipment from the shop to the job site. Engine-driven generator-type SMA welders are available that can be used almost anywhere. The limited amount of equipment required for the process makes moving easy.
WELDING CURRENT
Welding current is the term used to describe the electricity that jumps across the arc gap between the end of the electrode and the metal being welded. An electric current is the flow of electrons. The resistance to the flow of electrons (electricity) produces heat. The greater the electrical resistance, the greater the heat and temperature that the arc will produce. Air has a high resistance to current flow, so there is a lot of heat and temperature produced by the SMA welding arc. Electrons flow from negative (-) to positive (+).
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 that the electrons can jump to form the arc. A higher voltage can jump a larger gap. Welding voltage is associated with the welding temperature.
Amperage, or amps (A), is the measurement of the total number of electrons flowing, in the same way that gallons are a measurement of the amount of water flowing. Amperage controls the size of the arc. Amperage is associated with the welding 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). Watts are associated with welding power or how much heat and temperature an arc produces.
SMA Welding Arc Temperature and Heat
The temperature of a welding arc is dependent on the voltage, arc length, and atmosphere. The arc temperature can range from around 5500°F to above 36,000°F, but most SMA welding arcs have effective temperatures around 11,000°F. The voltage and arc length are closely related. The shorter the arc, the lower the arc 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. Most shielded metal arc welding electrodes have chemicals added to their coverings to stabilize the arc. These arc stabilizers form conductive ions 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. Other chemicals within the gaseous cloud around the arc may raise or lower the resistance. 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 the electrode has a recommended minimum and maximum amperage range and therefore a recommended heat range. If you were to try to put too many amps through a small diameter electrode, it would overheat and could even melt. If the amperage setting is too low for an electrode diameter, the end of the electrode may not melt evenly, if at all. 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. 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 produced by the arc is not distributed evenly between both ends of the arc. This distribution depends on the composition of the electrode’s coating and the type of welding current.
WELDING POWER
The shielded metal arc welding process (SMAW) requires a constant current arc voltage characteristic. Gas tungsten arc welding (GTAW) also uses this same type of welding power, but gas metal arc welding (GMAW) and flux cored arc welding (FCAW) both use a different type of welding power called constant voltage. The SMA welding machines’ voltage output decreases as current increases. This output power supply 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. Following this short starting surge, the power (watts) remains almost constant despite the changes in arc length. With a constant voltage output, small changes in arc length would cause the power (watts) to make large swings. The welder would lose control of the weld.