TWI Industrial Member Report Sumamry 842/2006
By D Bertaso and R L Jones
The A-TIG welding process involves a novel method of increasing the penetration capability of the arc in TIG welding. The process is effected through the application of thin coating of an activating flux material onto the joint surface prior to welding. The effect of the flux is to constrict the arc which produces a narrow deep weld compared with a wide shallow weld bead obtained with a conventional TIG arc. Activating fluxes are commercially available from several sources and these fluxes claim to be suitable for the welding of a range of materials, including C-Mn steel, Cr-Mo steels, stainless steels and nickel-based alloys.
The specific advantages claimed for the A-TIG process compared with conventional TIG include:
- Increased productivity due to greater depth of penetration, i.e. up to 12mm in stainless steel compared to 3mm for conventional TIG welding. Increased productivity is derived through a reduction in welding time and/or are duction in the number of weld passes.
- Reduced distortion, i.e. the use of a square edge closed butt joint preparation reduces weld shrinkage compared with a conventional multipass V butt joint. Alternatively, for single pass TIG welding applications, the useof activating fluxes enables the heat input to be decreased and distortion to be reduced.
- Problems of inconsistent weld penetration associated with cast-to-cast material variations can be eliminated. For example, deep penetration welds can be made in low sulphur stainless steel (~0.002%), which would otherwise show ashallow wide weld bead for conventional TIG welding.
The benefits of the A-TIG process can be exploited in many industrial applications. For example, butt-welding of C-Mn and stainless steel plate up to 12mm thickness can be welded in a single pass in the PA position using backing support. Butt-welding of tubes, typically up to 5mm wall thickness, can be welded in a single pass in the PG position when using conventional orbital TIG welding equipment.
Despite the productivity benefits of the A-TIG process, industry to date has been slow to exploit the process. This is because the use of the flux is seen as an additional cost and its application an additional operation. Furthermore, the commercial fluxes tend to produce an inferior surface finish compared to conventional TIG welding and produce a surface slag residue, which is required to be removed. In order to mitigate these disadvantages, TWI has developed a low cost activating flux with the following characteristics:
- It comprises a low cost, readily available flux ingredient, which can be applied as a water based paste using a brush or spray applicator. It can be procured as a standard chemical reagent. Procured in this form, the flux cost is estimated to be less than 10pence per metre of weld length.
- The flux ingredient is non-toxic. It contains no halides or fluorides.
- Flux performance, including depth of weld penetration, is similar to alternative commercial fluxes.
- It is suitable for welding carbon steel and stainless steel.
- It produces a satisfactory weld deposit surface appearance with minimal slag residue.
The objective of the present work was to investigate A-TIG welding using the low cost activating flux for orbital welding of stainless steel tube.
For the welding of stainless steel tube using autogenous orbital TIG welding equipment in combination with a low cost activating flux:
- To develop welding procedures for thick wall tube (> 3mm).
- To develop welding procedures which eliminate arc deflections when joining laser seam welded tube.
- To develop improved welding procedures for small diameter, thin wall (1.6mm) tube.