Activating Flux - Improving TIG Process Performance

(Published in Welding and Metal Fabrication, 2000, Vol. 68, No. 2, February, pp 7-10 by DMG Business Media Ltd - http://www.dmgworldmedia.com )

US shipbuilders have used NJC/EWI activating fluxes for TIG welding stainless steel pipework for surface ships such as this Arleigh Burke Class destroyer

Activating fluxes to improve the performance of the TIG process were first reported by the Paton Welding Institute (PWI), Ukraine, in the 1960's ^[1] . More recently, the Navy Joining Center (NJC)/Edison Welding Institute (EWI), US, has developed an alternative range of fluxes ^[2] . The principle of the technique is that by applying a thin coating of the flux to the surface of the material, the arc is constricted which increases the current density at the anode root and the arc force acting on the weld pool^[3] . The characteristic appearance of the constricted arc compared with the more diffused conventional TIG arc is shown in Fig 1. As demonstrated in welding 6mm thick stainless steel, arc constriction significantly increases weld pool penetration producing a deep narrower weld compared with a wide, shallow weld bead with the conventional TIG process ( Fig 1).

Fig.1. The characteristic appearances of the activated and conventional TIG arcs and the comparative depths of penetration in 6mm thick stainless steel (left: Conventional TIG welding, right: A-TIG welding)

Activating fluxes have led to a dramatic improvement in the operating characteristics of the TIG process notably a greater depth of penetration, higher welding speed and a reduction in the sensitivity to cast to cast material variation. As companies are considering the benefits of activating fluxes for their specific products, it is now timely to review the benefits to be derived from arc constriction and the scope for industrial application.

Availability of fluxes

The PWI activating fluxes are available commercially from WTC and the NJC fluxes from EWI, Miller Electric (ITW) and Liburdi Engineering. The PWI fluxes are produced in the form of either an aerosol spray or as a paste (powdered flux mixed with acetone) which is applied to the surface with a brush. The NJC fluxes are likewise available in powder form and mixed with isopropanol which evaporates when brushed on to the surface. Dispensers have recently become available, similar to a 'high-lighting' pen, for applying the flux directly to the surface. 

There are two PWI fluxes which have been designed for welding the following range of materials:

PATIG - S-A	C Mn steel Low alloy steel Cr Mo steel Stainless steel
PATIG - N-A	Nickel alloys (Nimonic alloys, alloy 600, 690)
The NJC/EWI fluxes are suitable for the following range of materials:
FASTIG TM SS-7 CS-325	Stainless steel C Mn steels Low alloy steels Cr Mo steels
Fi-600	Nickel based alloys

Both the PWI and NJC/EWI are researching new fluxes with the intention of extending the range of materials to copper nickel alloys and titanium.

Application of the activating flux process

The activating flux process can be applied in both manual and mechanised welding operations. However, because of the need to maintain a short arc length to achieve deep penetration, it is more often applied in mechanised applications.

Specific advantages claimed for the activating flux process, compared with the conventional TIG process, include:

• Increases depth of penetration e.g. up to 12mm thick stainless steel can be welded in a single pass compared with typically 3mm with conventional TIG
• Overcomes the problem of cast to cast variation e.g. deep penetration welds can be produced in low sulphur (less than 0.002%) content stainless steels which would normally form a wide and shallow weld bead with conventional TIG
• Reduces weld shrinkage and distortion e.g. the deep narrow weld in a square edge closed butt joint will produce less distortion than a multi-pass weld in the same thickness material but with a V-joint

The claims for a substantial increase in productivity are derived from the reduction in the welding time either through the reduction in the number, passes or the increase in welding speed.

Disadvantages of using a flux include the rougher surface appearance of the weld bead and the need to clean the weld after welding. In mechanised welding operations, the as-welded surface is significantly less smooth than is normally produced with the conventional TIG process but in manual welding operations, the surface roughness is similar. On welding, there is a light slag residue on the surface of the weld which often requires rigorous wire brushing to remove.

Welding of sheet and plate structures

Sheet and plate structures may be welded in all welding positions using both manual and mechanised techniques. Mechanised techniques are preferred when seeking to exploit the capacity to weld a greater wall thickness because of the need to control the arc length. For example, in the butt welding of C Mn and stainless steel plate in the PA position, section thickness up 12 mm can be welded in a single pass using a mechanised technique. It should be noted, however, that for section thickness of greater than typically 6mm, the weld pool must be supported to prevent excessive sagging of the weld bead.

For section thickness greater than 12mm, it is recommended that a two pass weld i.e. one pass from each side, is used. Alternatively, if access is only possible from one side, the activated flux should be used to increase the size of the root face and then the number of passes required to fill the joint with the conventional filler wire technique can be substantially reduced. For example, when welding 20mm thick material, a 7mm root face will reduce the amount of filler wire by approximately 30% compared with a conventional TIG root face thickness of typically 3mm, Fig 2 ^[4] .

Fig.2. Section through 20mm thick stainless steel with a 7mm thick root face welded using the PWI activating flux

When welding in positions other than the PA position, or using a manual welding technique, the maximum thickness which can be welded in a single pass will be limited to less than 6mm by the need to achieve an acceptable weld bead profile with an unsupported weld pool.

Welding of Tubes

Tube Butt Joints

Tubes may be welded in all welding positions using both manual and mechanised techniques but as with plate material, mechanised techniques are preferred when seeking to exploit the capacity to weld a greater wall thickness. For example, when welding stainless steel in the PA (1G) position, the tube wall thickness can be as great as 9mm providing a mechanised technique is used. However, when the wall thickness is greater than approximately 6mm, it is necessary to support the weld pool to avoid sagging and profile defects due to the size of the weld pool.

When welding in the PC (5G) welding position, conventional orbital welding equipment can be used. A typical orbital welding head used for welding with an activating flux is shown in Fig 3 (note, the flux coating applied to the tube). In this welding position, the maximum wall thickness is substantially reduced, typically to less than 5mm, to produce an acceptable weld bead profile. The principal factor limiting the wall thickness is the occurrence of root concavity/shrinkage groove imperfection, commonly known as 'suck-back', in the more difficult 3 to 9 o'clock positions. Current pulsing is effective in controlling the weld bead profile and ensuring that root concavity is not too excessive and below the minimum limit of 0.5mm for quality level B (stringent), in accordance with the requirements of EN 25817 (ISO 25817). The typical weld bead appearance and weld bead penetration profile for 70mm diameter, 5.0mm wall thickness, type 316 stainless steel tube which was welded without an insert or filler wire, is shown in Fig 4. The welding procedure was approved in accordance with the requirements of EN 288.

Welding procedure:
Welding current, pulse Welding current, background Arc voltage Welding speed Start position	150A/300ms 30A/300ms 9.5V 60mm/min 4 o'clock, up

Fig.3. MESSER Polysoude welding equipment used for the orbital welding of tube. Note the flux coating on the tube prior to welding

Fig.4. 70mm diameter, 5mm wall thickness stainless steel tube, welded using the PATIG-SA activating flux in the (5G) position:

a) Weld bead appearance,

b) Section through weld

The A TIG procedure should be compared with conventional TIG welding procedures where a substantially greater number of weld passes would have been required. For example, in welding 60 mm diameter, 5.7mm wall thickness stainless steel, a stringer welding procedure will require eight passes with filler wire addition to fill the joint preparation. However, the number of passes can be reduced to four by using an electrode weaving procedure.

Welding procedure (8 stringer passes):
Welding current, pulse Welding current, background Welding speed Wire feed speed Shielding gas	72-199A/800ms 30-40A/400ms 160s/rev 0.2-0.3m/min argon 1% H2

Welding procedure (4 weaving passes):
Welding current, Weave rate Weave amplitude Weave frequency End dwell Welding speed Wire feed speed Shielding gas	81-105A 10mm/s 2, 2.5, 3, 5mm 0.36-0.52Hz 0.3-0.4s 165-200s/rev 0.75-0.9m/min argon

The joint preparation and the sections through the welds for the two procedures are given in Fig 5.

Fig.5. Orbital tube welds in 60mm diameter, 5.7mm wall thickness, type 304 stainless steel tube using conventional TIG process with stringer and weaving welding procedures:

a) Joint preparation,

b) Weld with a stringer bead procedure and

c) Weld with an electrode weaving procedure

The maximum wall thickness in C Mn steel tube which can be successfully welded is slightly less than with stainless steel. The maximum wall thickness was limited to 4mm to avoid the risk of root concavity. The welding procedure was likewise approved in accordance with the requirements of EN288.

Welding procedure:
Welding current, pulse Welding current, background Arc voltage Welding speed Start position	170A/300ms 18A/300ms 9.5V 65mm/min 4 o'clock, up

In certain types of steel and stainless steel with a more fluid weld pool, the maximum wall thickness of the tube was found to be limited to less than 4.0mm in order to meet the quality level B requirements of EN 25817 i.e. not greater than 0.5mm root concavity. However, the degree of root concavity can be substantially reduced by employing a welding procedure in which a second partial penetration pass (without flux) is used to forge the material into the root of the weld.

Tube to tubeplate joints

Activating flux has been used to stiffen the arc and increase the depth of penetration in tube to tubeplate face welds, Fig 6. The constricted arc produced a narrow, 3mm deep weld with no joint preparation. The constricted arc also eliminated the tendency to melt away the edge of the tube which is frequently observed when welding with the wider conventional TIG arc. Typical welding parameters for face welds in a stainless steel tube to tube sheet component were:

 

Welding current, Welding speed Arc voltage Shielding gas

90A 160mm/min 8V argon

Fig.6. General appearance of the weld and section showing the depth of penetration for a face weld in a tube to tubeplate component produced using the PATIG-SA activating flux

The flux is also effective when welding fillet joints in tube to tubeplate welds as a means of increasing the throat thickness, Fig 7. The problem of poor root fusion was attributed to the low sulphur content (<0.002%) of the type 316L stainless steel tube material. In addition to an illustration of how an activating flux can improve root fusion in a difficult cast of stainless steel, the example also serves to demonstrate that an activating flux is effective in welding fillet joints when a filler wire is used.

Fig.7. Section through tube to tubeplate fillet weld in difficult cast of type 316 stainless steel (<0.002% S) showing how the PWI activating flux increases the throat thickness

Slow exploitation of fluxes

There is no doubt that activating fluxes have the potential to substantially reduce production costs. However, because the flux is seen as an additional consumable cost and the application of the flux an additional operation, industry has to date been slow to exploit the benefits from the increased weld bead penetration. It is hoped that the welding techniques and procedures described here will stimulate welding engineers to consider the benefits of activating fluxes for specific applications in their companies.

References

1	Gurevich S.M. et al,	Improving the penetration of titanium alloys when they are welded by argon tungsten arc process, Avt. Svarka, 9, 1965
2	Paskell T., Lundin, C. and Castner H.,	GTAW flux increases weld joint penetration, Welding Journal, April, 1997
3	Lucas W and Howse D.S.,	Activating flux- increasing the performance and productivity of the TIG and plasma processes, Welding and Metal Fabrication, Vol.64, No.1, 1996
4	Howse D. S.,	Developments in A-TIG welding, Proc. Int. Conf., TWI, 1998.