The properties of tungsten change when it is alloyed with thorium. For instance, a thoriated tungsten electrode has a greater current carrying capacity than pure tungsten. This allows it to carry more heat and reduces the potential for tungsten inclusions. It also makes a weld arc easier to start and makes the geometry of that arc more stable. Thoriated tungsten was and still is a mainstay in tungsten inert gas (TIG) arc welding due to the predictability it brings to the TIG welding process and to welds. However, its major drawback is that it is mildly radioactive.
The majority of thorium’s radiation is released in the form of alpha particles. This is a low energy type of radiation that can be blocked by something as thin as a sheet of paper. Clothes and skin form a sufficient barrier to allow thorium to be used safely. However, tungsten electrodes are ground before use, and when ground thorium particulate is ingested or inhaled, it becomes a carcinogen. For this reason, many welding projects choose tungsten electrodes that use a different alloying oxide. Since this is true, you may be wondering what thoriated tungsten is used for. This post will discuss when and why thoriated tungsten is still used in welding projects.
What Is Thoriated Tungsten Used For?
In many TIG welding processes, thoriated tungsten has been replaced by tungsten alloyed with another metal oxide, such as cerium, lanthanum, or zirconium. With a range of non-radioactive tungsten electrode types widely available to choose from, it is natural to wonder why thoriated tungsten electrodes are still used. Part of the reason is that thoriated tungsten has a long history of performing well in nearly every TIG welding role. It isn’t easily replaced.
Very high-specification welding in the aerospace and nuclear industry was engineered, practiced, and qualified using thoriated tungsten electrodes. It is not easy to develop new welding procedure specifications. Thoriated tungsten also has a positive reputation among welders, and is reputed to be superior in maintaining a sharp tungsten grind angle through extended welding without spitting or splitting, unlike other tungsten electrodes.
Thoriated tungsten electrodes remain the only qualified electrodes for certain high-specification welding. Even in welding processes where other electrodes are qualified and available, the welders themselves often prefer thoriated tungsten, choosing it over other GTAW tungsten electrode types even when those types are qualified. Fortunately, there are ways of working with thoriated tungsten safely.
How to Use Thoriated Tungsten Electrodes Safely
Because the radioactive alpha particles in thorium are only a risk to humans if they are inhaled, ingested, or manage to work past the barrier of the skin, the risk of thoriated tungsten is mainly associated with grinding the tungsten. This risk can be mitigated to an extent by taking safety measures during grinding.
Common safety precautions for grinding thoriated tungsten may include:
- Ventilation: The American Welding Society (AWS) recommends local exhaust ventilation near the source of grinding to capture dust. It also recommends that this be supplemented with other measures where deemed necessary.
- Respiratory Equipment: The AWS also suggests the use of respiratory protection, but does not give specifics on the form that respiratory equipment should take. It is largely up to management to decide whether masks or respirators are more appropriate.
- Cleaning and Disposal: The Welding Institute (TWI), a British trade organization, recommends that any dust collected by exhaust ventilation or swept from the grinding area and any spent tips be placed into a sealed container, marked, and taken to a landfill area.
The safety precautions above are fairly simple common-sense measures that are typically already used in welding work environments even around nonradioactive materials. The real issue, however, is that grinding and disposing of thoriated tungsten represents a potential source of liability.
Even when ventilation and cleaning are strictly observed and thoroughly performed, there is no way to be completely certain that all dust from grinding thoriated tungsten has been removed from an area. The solution for most welders using thoriated tungsten electrodes is to avoid grinding it by ordering tungsten electrodes that are pre-ground by the manufacturer in a controlled environment.
The Advantage of Pre-Ground Thoriated Tungsten Electrodes
The obvious advantage of a pre-ground thoriated tungsten electrode is that it completely avoids any risk of contact with radioactive dust. It is also easier to dispose of spent thoriated tungsten electrodes as they can simply be returned to the manufacturer.
A less obvious advantage of pre-ground thoriated tungsten is that the manufacturer is able to put a more precise and more polished grind on the tungsten than what is possible in a shop or manufacturing environment. This reduces the number of burrs on the tungsten tip that occur as part of the machining process and, as a result, significantly lowers the likelihood of tungsten inclusions forming in the weld. In the high-specification welding processes where thoriated tungsten is still used, this very refined machining is a significant benefit.
So, what processes are thoriated tungsten used for? Orbital welding is one example. Orbital TIG welding often takes place in the aerospace and nuclear power generation industries where there is very little tolerance for flaws. In these projects, orbital welding parameters have been developed to meet the exacting needs of demanding applications over several decades and the cost in time and money for qualifying welds performed with a different tungsten electrode type is simply prohibitive. A pre-ground thoriated tungsten electrode meets existing welding parameters without the need to requalify welds, averts the risk of radiation exposure, removes the liability of disposal, and results in a better quality weld in these high-specification applications.
Arc Machines, Inc. recommends and offers pre-ground Wolfram tungsten electrodes for all of its orbital welding machines. For more information about high-quality thoriated tungsten, contact sales@arcmachines.com. For service contact service@arcmachines.com. Contact us to learn more about custom orbital welding solutions.
