Frequently Asked Questions
The processes most widely used for repair and refurbishment in nuclear environments are tungsten inert gas (TIG), metal inert gas or metal active gas (MIG/MAG) and manual metal arc (MMA) welding. There is increasing pressure on the industry to remove personnel, wherever possible, from contaminated work areas; this will result in reduced use of MMA welding.
Although both TIG and MIG/MAG processes are currently used for remote applications, they both have limitations. The TIG process can be used remotely but it does not give deep penetration welds, is relatively slow, and may need a number of welding passes to complete each welded joint. The process is prone to variable quality due to cast-to-cast variation and requires a separate wire feed system for adding filler metal.
The TIG process does, however, have a major advantage in that the size of the processing head is mechanically simple and can designed to be relatively small and with a small stand-off distance (to the workpiece). Although slow, it is noteworthy that speed of processing is rarely a consideration in remote nuclear repair, with the time to set up these repairs greatly outweighing the arc welding times.
Compared with TIG, MIG/MAG welding is a higher productivity welding process but it also requires local wire feed, and its remote use has been limited to carbon-manganese steel applications. Recent developments in power source technology have improved the performance and quality of MIG/MAG welding to the point where they could be considered for high quality remote applications.
New processes which have potential for remote processing include plasma powder arc, high-power fibre-delivered laser, and TIG welding using activated fluxes (A-TIG). All these processes have potential to improve productivity and performance.
Plasma powder arc and TIG welding with activated fluxes, although not widely used in general industry at the present time, are potentially easier to use in remote applications. This is because powder delivery is local and direct to the weld pool with the plasma, and the depth of penetration is large with A-TIG (reducing the number of passes required).
The use of lasers for remote applications has been largely restricted by their lack of portability. However, fibre and disk lasers are now widely available and these lasers combine an optical fibre delivery with high wall-plug efficiencies and high output powers (several kW) from a single source, which could be transported close to a repair site. However, it should also be noted that in using these higher powers and high power densities, higher processing speeds are also required, which may be difficult to control in a remote repair situation where joint tracking is critical. The relatively large size of the processing head (compared with TIG, for instance) and the stand-off distance also needs to be considered when working in restricted areas.
Further information
Please note that the following two items are only available to Industrial Members of TWI -
A system for integrated off-line programming of welding robots
Portable welding robots - an industrial survey