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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 -
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
© COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON II arc must be reestablished each time it does so.  The oxide coating on metals, such as aluminum and magnesium, can act much like a rectifier as discussed in Lesson I.  The positive half-cycle will be eliminated if the arc does not reignite, causing an unstable condition. Continuous high frequency maintains an ionized path for the welding arc, and assures arc re- ignition each time the current changes direction.  AC is extensively used for welding aluminum and magnesium. AC/DC Constant Current Power Sources - Designed for gas tungsten arc welding, are available, and can be used for welding practically all metals.  The gas tungsten arc welding process is usually chosen because of the high quality welds it can produce.  The metals that are commonly welded with this process, such as stainless steel, aluminum and some of the more exotic metals, cost many times the price of mild steel; and therefore, the power sources designed for this process have many desirable features to insure high quality welds.  Among these are: 1. Remote current control, which allows the operator to control welding amperage with a hand control on the torch, or a foot control at the welding station. 2. Automatic soft-start, which prevents a high current surge when the arc is initiated. 3. Shielding gas and cooling water solenoid valves, which automatically control flow before, during and for an adjustable length of time after the weld is completed. 4. Spot-weld timers, which automatically control all elements during each spot-weld cycle. Other options and accessories are also available.    Power sources for automatic welding with complete programmable output are also available.  Such units are used extensively for the automatic welding of pipe in position.  The welding current is automatically varied as the torch travels around the pipe.  Some units provide a pulsed welding current where the amperage is automatically varied between a low and high several times per second.  This produces welds with good penetration and improved weld bead shape. 2.3.3 Torches - The torch is actually an electrode holder that supplies welding current to the tungsten electrode, and an inert gas shield to the arc zone.  The electrode is held in a collet-like clamping device that allows adjustment so that the proper length of electrode pro- trudes beyond the shielding gas cup.  Manual torches are designed to accept electrodes of 3