- Pure Tungsten : ?Green
- 2% Ceriated : ?Gray
- 2% Thoriated : Red
- 2% Zirconiated : Brown
- 1.5% Lanthanated : Gold
- 2% Lanthanated : Blue
?Pure tungsten electrodes are generally used on less critical welding operations than the tungsten electrodes that are alloyed. This type of electrode is used for AC welding, has a relatively low current carrying capacity and a low resistance to contamination.
Thoriated tungsten electrodes (1 or 2 percent thorium) are superior to pure tungsten electrodes because of their higher electron output, better arc-starting and arc stability, high current-carrying capacity, longer life, and greater resistance to contamination. 2% thoriated tungsten electrodes can be used for AC welding, but they excel in DC electrode negative (straight polarity) GTAW on carbon and stainless steel, nickel, and titanium. ?Welding operators should note that 2% thoriated tungsten electrodes contain low levels of radioactivity. Therefore, they must always follow manufacturer’s warnings, instructions, and the MSDS?(Material Safety Data Sheet) for their use.
Lanthanated tungsten electrodes?contain a minimum of 97.80 percent tungsten and 1.30 percent to 1.70 percent lanthanum, or lanthana, both 1.5 and 2% lanthanated tungsten electrodes offer excellent arc starting at low amperages, provide long electrode life and stability, and have a low burn-off rate. They also resist tip wear. The?2% lanthanated tungsten electrodes are considered to have superior characteristics. 2% lanthanated tungsten electrodes are often used in critical applications, such as in the aviation industry, and can be used to replace 2% thoriated tungsten electrodes. Both electrodes are well suited to welding carbon steel, stainless steel, nickel alloys, titanium, and aluminum. They work well on AC or DC electrode negative with a pointed end, or they can be balled for use with AC sine wave power sources. ?Unlike thoriated tungsten, lanthanated electrodes are suitable for AC welding and, like ceriated electrodes, allow the arc to be started and maintained at lower voltages. Compared with pure tungsten, the addition of 1.5 percent lanthana increases the maximum current-carrying capacity by approximately 50 percent for a given electrode size.
Ceriated tungsten electrodes?contains a min of 97.3 % tungsten, with 1.8 to 2.2 % cerium, ?is used for low current settings and has a low-amp arc.?These electrodes perform best in DC welding at low current settings but can be used proficiently in AC processes.
Zirconium tungsten electrodes contain 0.7–0.9% zirconium and are known for their ability to ball up easily in AC applications. ?Zirconium electrodes generally fall between pure tungsten electrodes and thoriated tungsten electrodes in terms of performance. There is, however, some indication of better performance in certain types of welding using ac power.
Finer arc control can be obtained if the tungsten alloyed electrode is ground to a point (fig. 5-33). When electrodes are not ground, they must be operated at maximum current density to obtain reasonable arc stability. Tungsten electrode points are difficult to maintain if standard direct current equipment is used as a power source and touch-starting of the arc is standard practice. Maintenance of electrode shape and the reduction of tungsten inclusions in the weld can best be accomplished by superimposing a high-frequency current on the regular welding current. Tungsten electrodes alloyed with thorium and zirconium retain their shape longer when touch-starting is used.
The angle of taper on a tungsten electrode has a direct affect on arc stability and bead profile.?
To properly grind tungsten electrodes, use a grinding wheel specially designated for tungsten grinding (to avoid contamination). Note: if you are grinding thoriated tungsten, make sure you control and collect the dust, have an adequate ventilation system at the grinding station and that you follow manufacture’s warnings, instructions and MSDS sheets.
Grind the tungsten electrode straight on the grinding wheel rather than at a 90-degree angle to ensure that the grind marks run the length of the electrode. Doing so reduces the presence of ridges on the tungsten that could create arc wandering or melt into the weld puddle, causing contamination.
Generally, you will want to grind the taper on the tungsten to a distance of no more than 2.5 times the electrode diameter (for example, with a 1/8-in. electrode you would grind a surface 1/4 to 5/16-in. long). Grinding the tungsten to a taper eases the transition of arc starting and creates a more focused arc for better welding performance.
The tungsten electrode extension beyond the gas cup is determined by the type of joint being welded as well as the amount of shielding gas coverage provided. For example, an extension beyond the gas cup of 1/8 in. (3.2 mm) might be used for butt joints in light gauge material, while an extension of approximately 1/4 to 1/2 in. (6.4 to 12.7 mm) might be necessary on some fillet welds. The tungsten electrode of torch should be inclined slightly and the filler metal added carefully to avoid contact with the tungsten. This will prevent contamination of the electrode. If contamination does occur, the electrode must be removed, reground, and replaced in the torch.
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