July 12, 2001
The author outlines basic components of a welding booth suitable for GTAW, and offers possible solutions to a decline in availability of the thoriated tungsten used in a GTAW electrode. He also offers ideas for providing a clean air supply for the GTAW operator.
Possibly the greatest development of the last decade was increased awareness of environmental and health concerns, which in the welding industry has prompted improvements in air quality, ergonomic workstations, and decreased exposure to materials deemed harmful to health.
It is important to consider the environment in which gas tungsten arc welding (GTAW) will be done—specifically, the tungsten electrode used and the overall operator environment.
One current concern is the declining availability of the most commonly used material in a GTAW electrode—thoriated tungsten. The low levels of radiation present in thoria, the oxide that improves electrode migration on the surface of the electrode, is not a cause for alarm; still, thoriated tungsten is being phased out of some suppliers' inventories. Alternatives must be considered as availability diminishes. While many GTAW tungsten electrode suppliers have different experiences and recommendations regarding which oxide is the best additive for pure tungsten for different applications, each user's application is unique.
Only one reliable rule applies in all cases: Test different materials for your applications, quantifying all critical factors, such as arc-starting characteristics, weld quality, cost, and electrode durability versus the materials currently used.
The term rare earth, which describes some new tungsten oxides, may be unclear. Rare earth is a descriptive term for elements between 51 and 71 on the periodic table of elements, including cerium, lanthanum, and yttrium, whose oxides have been used to emulate the properties of thoria to improve the behavior of pure tungsten.
The two most common oxides recommended as substitutes for thorium are lanthanum and cerium, each of which offers specific advantages and limitations. In most cases, using these electrodes in a particular GTAW process can result in an improved welding experience and a good exercise in awareness of the variables affecting performance.
Testing is a critical part of the selection process because each application is unique. If you change electrode materials based only on sales propaganda, you may miss important factors that affect performance and cost.
Lanthanated tungsten has become a widely used, nonradioactive substitute for thoriated tungsten. It is available in 1, 1.5, and 2 percent concentrations. The 11¼2 percent concentration has shown to behave most like 2 percent thoriated tungsten in many applications, while 2 percent lanthanated tungsten can offer greater durability in high-current applications.
A very small amount of oxide has a measurable impact on performance, because the oxides facilitate electron migration on the surface of the tungsten, thereby aiding arc starting and current flow.
Two percent ceriated tungsten is another nonradioactive choice. It is versatile, but it is best-suited for low-amperage operations. It can be used effectively in AC applications for aluminum welding, DC for stainless and mild steel, and on many exotic metals. High current tends to exhaust the oxide content quickly because it fires the arc, but each project has different activity, and this material may work well for your next job.
Possibly the best advantage of the GTAW process is that the electrode does not come in contact with the weld puddle. This means that different electrode materials may be used with no impact on shield gas or filler metal requirements, because they remain specific to the materials being welded.
Varying oxide percentages can affect current flow. This can be best seen in the width of the resulting weld bead, so it is important to be aware of this and adjust the current accordingly to produce the proper weld bead.
Like any welding process, GTAW generates a variety of noxious fumes in the direct presence of the welder. The direct action of applying high heat to metal consumes oxygen from the local atmosphere and generates other fumes as a result. It is imperative that a good, clean air supply be provided for the operator and others in the affected area.
One thing to keep in mind when setting up ventilation for GTAW is the purity of the weld: It requires an undisturbed shield gas around the weld puddle. Sufficient air currents must provide fresh air to the operator without an intensity that can affect the flow patterns of the inert shield gas.
An effective system for providing fresh-air exchange is a downdraft table with variable fan speeds. When selecting a booth, make certain to specify an adjustable fan speed and flow for levels suitable for your applications.
One way to test the influence of draft on your welding zone is to light a small candle and monitor the disturbance of the flame at different fan speeds. You do not want to turn down the speed so far that it will not move fresh air into the area, so it often helps to experiment with at least a 2-foot-square section of steel over the table's grates to force the airflow to pass farther away from the arc. The best part of this situation is that a properly tuned downdraft will draw fresh air from above and behind the operator while drawing all fumes downward and away.
A simple GTAW booth with a downdraft table can be built with typical materials found in most fabricating shops and a minimum of store-bought parts. A 4-foot-wide by 2-1/2-foot-deep booth can be made of an angle iron or steel shelving framework enclosed with sheet metal.
A replaceable grid or rail system for the work area should be positioned at a comfortable height, preferably built with an overhang at the front to make room for the operator's knees when he sits on a stool.
Beneath the work area, a 1-foot-deep boxed chamber with air vents on the lower sides or bottom will catch debris and act as a plenum through which to draw air. A large squirrel-cage fan mounted below this box is used to suck the air and fumes downward through the booth.
A 6-inch-diameter exhaust pipe mounted to the fan should be filtered and sent outside to keep the air in the building fresh for all employees. The entire system must be made of fireproof materials and be mounted on a cement or protected floor.
It is advisable to reduce reflective glare generated by the GTAW arc by spray-painting the inside of the booth with a nonreflective color. Flat black paint reflects the least, but a flat gray primer color takes the shine off steel walls without darkening the work area.
Adequate lighting from the uppermost corners of the booth properly illuminate the work subject. Be sure to eliminate or protect against all sharp edges and corners to protect fingers and expensive welding hoses.
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