© COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON II
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Lesson 1
The Basics of Arc Welding
Lesson 2
Common Electric
Arc Welding Processes
Lesson 3
Covered Electrodes for Welding
Mild Steels
Lesson 4
Covered Electrodes for Welding Low Alloy Steels
Lesson 5
Welding Filler Metals for Stainless Steels
Lesson 6
Carbon & Low Alloy
Steel Filler Metals -
GMAW,GTAW,SAW
Lesson 7
Flux Cored Arc Electrodes Carbon Low Alloy Steels
Lesson 8
Hardsurfacing Electrodes
Lesson 9
Estimating & Comparing Weld Metal Costs
Lesson 10
Reliability of Welding Filler Metals
wire, the weld puddle, and the area in the arc zone are protected from the atmosphere by a gaseous shield.  Inert gases, reactive gases, and gas mixtures are used for shielding.  The metal transfer mode is dependent on shielding gas choice and welding current level.  Figure 9 is a sketch of the process showing the basic features. FIGURE 9 WELDING WIRE WELDING CABLE SHIELDING GAS GAS NOZZLE CONTACT TIP WORK PIECE MOLTEN POOL WELD METAL ARC GAS SHIELD SOLID WIRE ELECTRODE TRAVEL DIRECTION GAS METAL ARC WELDING 2.4.0.1 Gas metal arc welding is a versatile process that may be used to weld a wide variety of metals including carbon steels, low alloy steels, stainless steels, aluminum alloys, magnesium, copper and copper alloys, and nickel alloys.  It can be used to weld sheet metal or relatively heavy sections.  Welds may be made in all positions, and the process may be used for semiautomatic welding or automatic welding.  In semiautomatic welding, the wire feed speed, voltage, amperage, and gas flow are all preset on the control equipment.  The operator needs merely to guide the welding gun along the joint at a uniform speed and hold a relatively constant arc length.  In automatic welding, the gun is mounted on a travel carriage that moves along the joint, or the gun may be stationary with the work moving or revolving beneath it. 2.4.0.2 Practically all GMAW is done using DCEP (Electrode positive).  This polarity provides deep penetration, a stable arc and low spatter levels.  A small amount of GMAW welding is done with DCEN and although the melting rate of the electrode is high, the arc is erratic.  Alternating current is not used for gas metal arc welding. 2.4.1 Current Density - To understand why gas metal arc welding can deposit weld metal at a rapid rate, it is necessary that the term "current density" be understood.  Figure 10 shows a 1/4" coated electrode and a 1/16" solid wire drawn to scale.  Both are capable of carrying 400 amperes.  Notice that the area of the 1/16" wire is only 1/16 that of the core wire of the coated electrode.  We can say that the current density of the 1/16" wire is 16 times