Innovation in Weld Protection
The last fifty years has witnessed unprecedented development in welding technology. Major advances in metallurgy resulted in the introduction of new alloys capable of meeting the increasing demands for high strength performance.
Concurrent with this was the evolution of specialised welding materials by international companies such as ESAB in Sweden and Hobart in the USA.
For some time, full exploitation of these developments was limited by the lack of welding accessories but in due course leading manufacturers met the challenge of producing sophisticated programmable systems such as robots and real-time monitoring of weld quality. In the context of this presentation, accessories which provide reliable and cost-effective solutions to the problems associated with prevention of defects caused by poor inert gas coverage were also developed.
As manual welding has given way progressively to mechanised and fully automated fabrication the significance of facilities which can be integrated with the welding cycle to help ensure high levels of repeatability and quality has become recognised.
Glass fibre tape materials – the cost-effective solution to weld backing
The fundamental requirement of a quality joint is to provide continuous fusion between the two materials along their length and this can be achieved in a number of different ways.
The most direct technique is for the welder to exercise total control over the deposit, producing an acceptably smooth underbead of constant width with no significant surface oxidation products. The root gap plays an important part here in ensuring consistency of penetration; too wide and there will be over-penetration, too small and there will be inadequate penetration. Achieving consistency requires a level of skill on the part of the welder that is not always available.
To counter this need for high levels of welder skill, mechanical methods of weld-bead control have been developed. In circumstances where access to the rear of the joint is possible, the external bead profile can be machined or ground to an acceptable form – perhaps even to produce a smooth, flat continuous surface. Alternatively the bead can be deposited from the rear of the joint and the internal profile treated. Where access to the rear of the joint is not possible or desirable, recourse to physical weld bead or backing support is the only alternative.
The following represents the range of the more popular backing solutions available:
1 Ceramic tile backing strip
2 Permanent backing bar
3 Temporary backing bar
4 Consumable inserts
5 Inert gas
6 Glass reinforced fibre tape
Glass reinforced tape
Self-adhesive, thermally stable and inert tape is simply applied to the underside of the joint. The tape provides good mechanical support to the weld and prevents contamination. It is easy to use, requiring no special skills. This tape solution remains surprisingly underused and relatively unknown.
One example of the fibre tape is provided by the 'Argweld®' product. This tape is suitable for most common materials such as carbon, alloy and stainless steels and it can be used in conjunction with the MMA, TIG and MIG processes. The lower duty version can be used up to 80 Amps and heavier duty tapes are available for application up to 240 Amps.
Glass fibre tape is essentially a strip of adhesively-backed aluminium foil centrally overlaid with a band of woven continuous filament glass fibre cloth and is supplied in rolls. The tape is simply cut to length and stuck to the underside of the joint to be welded. The thermally stable woven fibre not only prevents over-penetration of the fusion zone but shapes the underbead to produce a positive re-inforcement which is continuous and uniform and which blends smoothly to the parent material on each side. The tape can be removed by peeling it away from the weld zone.
Composition of fibre: oxides of silicon, aluminium, calcium, boron and magnesium fused in an amorphous vitreous form.
Fibre diameter: filament diameters between
6 and 25 microns
Boiling point 800 ºC
Non-flammable and non-burning
Cost savings are well illustrated by comparing tape products with gas purging on a 3 metre diameter tank 5 metres long. The minimum recommended purge would consume double the tank volume in gas at 70 litres/min. The purge process would thus take 16 hours and the gas alone would cost in excess of £200 (71 000 litres = 7 bottles @£30 ).
The Argweld® Backing Tape™, does allow welders to weld faster than normal without any detriment to the final result.
A typical cost for backing tape for this requirement would be:
Preparation for single-V butt joint. Bevel angle
Included angle Root face Joint gap
Typical sequence of welding for multi-pass
single-V butt weld Capping run Filling runs Root run
Ceramic tile backing Clamping
Permanent backing Tack welds
Temporary backing Clamping
Consumable insert Tacking into place
Backing Tape™ Applying adhesively
Gas purge backing Attaching gas hoses
Weld Trailing Shields® – Cost-Effective Inert Gas Protection of Welds
Prevention of oxidation during fusion welding is a problem facing a large number of fabricators. The standard gas shielding routinely used with arc welding processes is adequate with carbon and low alloy steels for constructional purposes. However additional protection has to be provided when welding the more reactive materials such as titanium alloys and some stainless steels.
The ultimate level of protection can be provided by using sealed welding enclosures within which the gas environment can be rigidly controlled. This solution may be inappropriate for larger items being welded. A compromise, but highly effective, alternative is to use one or more Weld Trailing Shields® since these will provide excellent inert gas coverage with minimal set-up time.
Trailing shields have been designed specifically for use with GTAW (TIG) or PAW (plasma) welding torches and provide a high level of additional inert gas shielding to supplement that supplied by the basic torch. The shields are available to match a wide variety of forms from flat to diameters as small as 25 mm (1 inch).
The welding torch is mounted on the leading end of the shield and inert gas fed through one or more ports behind the fusion zone. A seal between the shield and the work can be assured through the use of a flexible and durable skirt made typically from silicone. Turbulence inside the cavity is avoided by passing the gas through a mesh filter above the fusion zone.
Choosing a shield
Selection of shield length is dependent upon a number of factors, but the following need to be considered;
· the extent to which oxidation is to be limited along the weld length
· thickness of material being welded (thicker sections will remain hotter longer)
· sensitivity of the material to oxidation
· welding process being used (PAW has a different heat input than GTAW)
· welding speed and current
For curved shapes, eg pipes, tubes and cylindrical vessels, trailing shields are available for common diameters of pipes and vessels. Internal models are less widely used but can be manufactured to meet specified needs.
Flat trailing shields for plate, sheet and rectangular tanks are available in small lightweight versions for manual welders.
Larger, more robust, versions for attaching to automatic/mechanised welding systems are also available.
Special versions can be designed and manufactured to provide increased shielding for extra protection if required. This may arise from a need to cover hot metal on each side of the fusion zone or behind the fusion zone when heat input during welding is particularly high.
NOTE: Welding speed is normally greater when using automatic or mechanised welding so it's difficult to keep the top surface clean without the extra protection provided by trailing shields.
The benefits offered by trailing shields can be exploited on all joints where weld quality coupled with protection against oxidation and discolouration is required. Industries where this is particularly important are those making extensive use of sensitive materials such as stainless steels, nickel based alloys and titanium alloys. Prominent among these are the racing and sports car manufacturers, aerospace, nuclear, petrochemical, food and pharmaceutical sectors.
Weld Trailing Shields® can also be used for shielding the back side of critical welds where it is possible for them to be held against the weld and moved along in time with the welder.
Such trailing shields are also available under the Argweld® banner.
There are many lively conversations about weld protection in the linked in group "Weld Purging Best Practices".
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