For quite some time, tungsten manufacturers have added an oxide to pure tungsten to improve the arc starting characteristics and the longevity of welding electrodes.
While 2 per cent Thoriated tungsten has been the most commonly used tungsten material for many years, this electrode type is increasingly being scrutinised because of concerns for the environment and for the safety of users of this material. The concerns arise from the fact that the element thorium is radioactive.
On this basis, many organisations have evaluated alternative tungsten types to see what changes in performance they would see by transitioning to non-radioactive tungsten.
The radioactivity problem
The Thorium used in 2 per cent Thoriated tungsten is a radioactive element and therefore can be dangerous to the health of those exposed to it and to the environment.
It is an alpha emitter, however when it is inside a tungsten electrode rod, it is enclosed in a tungsten matrix and so there is little radiation emitted externally.
The main risk to the welder occurs in the ingestion of the material. This occurs primarily during the inhalation of dust caused during grinding of tips for welding, but also to a lesser extent during breathing of any fumes released during welding.
The American Welding Society, in their A5.12 "Specification for Tungsten and Tungsten-Alloy Electrodes for Arc Welding and Cutting" states the following on this issue: "Thorium is radioactive and may present hazards by external and internal exposure. If alternatives are technically feasible, they should be used."
They confirm that the primary concern in using this material is in ingesting dust produced while grinding points on electrodes.
They go on to say: "...during the grinding of electrode tips there is generation of radioactive dust, with the risk of internal exposure. Consequentially, it is necessary to use local exhaust ventilation to control the dust at the source, complemented if necessary by respiratory protective equipment."
Europe has also recognised the dangers of 2 per cent Thoriated tungsten. TWI (The Welding Institute), which is the British equivalent of the AWS, reports that "The British Health and Safety Executive (HSE) have issued an information document to provide for the storage and use of Thoriated tungsten electrodes.
"This states that local exhaust ventilation should be provided during the grinding operation, and that the dust from the grinding equipment and the spent tips are disposed of in a sealed container to a landfill disposal site."
Further: "The HSE has recommended to factory inspectors that, where Thoriated tungsten electrodes are not necessary for the quality of the weld, users should be encouraged to look for alternatives."
In a highly detailed study conducted by the DVS (the German Welding Society), they measured the amounts of radioactive alpha particles and gamma energy sprectra in the ambient air during the grinding of tungsten electrodes.
Some of the recommendations of this study included: (i) it is recommended to work, if possible, without the use of welding electrodes containing Thorium; (ii) if this is not possible, precautions should be taken to protect employees against the contamination with or the inhalation of grinding dust; and (iii) the working area shall be cleaned regularly in order to avoid contamination caused by deposited dust.
Another study was conducted in Germany by the State of Bavaria, Department for Development and Environmental issues.
They concluded that in facilities where there is no dust extraction system for Thoriated tungsten grinding, the exposure to harmful particles is four times greater than with extraction. However, even though dust collection provides a significant improvement, levels may still be too high.
In addition, they mentioned that Thoriated tungsten electrodes stored in boxes on a shelf do not seem to pose any potential hazards.
At the moment, a proposal has been placed before the European Community Commission to drastically increase the severity of legal regulations with respect to the use and disposal of Thoriated electrodes.
In the State of California, tungsten manufacturers now include packaging of 2 per cent Thoriated tungsten electrodes that include a warning such as the one Osram Sylvania uses, which is, "WARNING: This product contains or produces a chemical known to the State of California to cause cancer. (California Health and Safety Code 25249.5 et seq.)"
Finally, one of the major US tungsten manufacturers, Teledyne Tungsten Products, list the following cautions/warnings in their MSDS Sheets:
"Thorium is a naturally occurring low level radioactive element. Thorium is primarily an alpha emitter. Daughters in the decay chain emit alpha, beta, and gamma radiation. Radioactive elements are regulated by the Nuclear Regulatory Commission. The NRC publishes regulations regulations regarding radioactive materials."
"Effects of overexposure: ...Chronic inhalation of the dust may cause lung damage in humans."
"Carcinogenic assessment: NTP-1 (Thoria). Note: NTP-1: Substances or groups of substances that are known to be carcinogenic. 'Known carcinogens' are defined by the NTP (National Toxicology Program) report as those substances for which there is sufficient evidence of carninogenicity from studies in humans to indicate a causal relationship between the agent and human cancer."
"Toxicological Information: ...Heavy exposure to the dust or the ingestion of large amounts of the soluble compounds produces changes in body weight, behavior, blood cells, choline esterase activity and sperm in experimental animals."
Non-radioactive tungsten materials
Due to the problem discussed in the section above, two significant alternative, non-radioactive tungsten materials have been developed. The first one, introduced in the 1980's, is most commonly available as 2 per cent ceriated tungsten. This material is commonly used for lower amperage DC welding applications. In fact, it holds a very high market share in sales for the orbital welding process.
More recently, 1½ per cent lanthanated tungsten has emerged as what could be the future standard for tungsten electrodes.
The 1½ per cent by weight content (as opposed to 2 per cent) was chosen by three of the largest manufacturers as the optimum content amount based on scientific studies which showed that this content amount most closely mirrors the conductivity characteristics of 2 per cent Thoriated tungsten.
Therefore, welders can usually easily replace their radioactive 2 per cent Thoriated material with this tungsten and not have to make any welding program changes.
In addition, since the lanthanum oxide material is less dense that thorium oxide, a stick of 1½ per cent (by weight) lanthanated tungsten actually contains 15 per cent more oxides by volume than a stick of 2 per cent (by weight) Thoriated tungsten. This aids in arc starting and stability, as well as longevity, because the additional volume of oxides keep the tip cooler.
Finally, 1½ per cent lanthanated tungsten is suitable for both AC and DC welding applications. Therefore, facilities that stock both 2 per cent Thoriated tungsten for DC welding and another tungsten type (usually pure or zirconiated tungsten) for AC welding, could stock only one tungsten type.
In order to evaluate the feasibility of switching to 1½ per cent lanthanated tungsten for DC welding applications, a number of studies have been performed. The following are summaries of a few of them:
At the 1998 American Welding Society Exhibition in Detroit, Michigan, one major tungsten manufacturer presented an independent study performed on 1½ per cent lanthanated tungsten. In summary, 2 per cent Thoriated, 2 per cent ceriated and two manufacturer's 1½ per cent lanthanated tungsten were compared by observing tip erosion after 300 DC arc strikes at both 70 amps and 150 amps. In both the lower and higher amperage tests, the 1½ per cent lanthanated tungsten showed the least amount of tip erosion and the most consistent arc behavior.
The Air Force conducted a study of 1½ per cent lanthanated tungsten versus 2 per cent Thoriated tungsten to evaluate arc starting characteristics, current capability, and penetration.
They used 5/32" diameter electrodes to weld 6061-T6 Aluminum and 304 stainless steel with a Miller 350 Synchrowave GTAW machine.
Using both high frequency and touch start, the lanthanated electrodes performed better than or equal to the Thoriated electrodes.
Also, the tip geometry was more stable at high current on the lanthanated electrodes. There was no significant difference found in arc stability, penetration, or weld bead shape.
Finally, they concluded that it was not necessary to recertify existing weld procedures or operators when using lanthanated electrodes.
Independent investigations were carried out at the Institut de Soudure in Paris, the Institut fuer Schweisstechnische Fertigungsverfahren of the TH-Aachen and at the Schweisstechnische Zentralanstalt in Vienna.
They found that in every case the service life of lanthanated electrodes was longer than Thoriated electrodes. In addition, the electrode consumption rate was higher for Thoriated electrodes than for lanthanated. No differences could be ascertained regarding the arc starting behavior.
The commonly used 2 per cent Thoriated tungsten type is a potential health hazard. Non-radioactive 1½ per cent lanthanated tungsten is an alternative to this material that provides for an easy transition and often produces better performance.
Welding Automation and its distributors carry stocks of 2% Lathinated Tungsten, which has been shown in Australian trials carried out by the Australian (former Submarine, now) Shipbuilding Corporation to most closely match the performance of 2% Thoriated Tungsten.