The Benefits offered by Weld Trailing Shields®
Whilst the gas shield provided by a standard GTAW or PAW torch is adequate for protection of the majority of welds, certain applications require enhanced inert gas coverage.
For example, the biofuels sector is now recommending a 50 ppm oxygen level for shielding the molten weld pool.
Alloys such as those based on titanium and some stainless steels are particularly sensitive to contamination by oxygen when exposed to air behind the arc shield even though temperatures here may have fallen well below that of the molten metal.
Additionally, where heat input during welding is higher than routinely observed, arc gas coverage even with less sensitive alloys can be inadequate.
This may arise for example during the use of high current processes where heat input is increased.
The ultimate level of protection can be provided by using sealed welding enclosures within which the gas environment can be rigidly controlled, (see Huntingdon Fusion's Technical Note TN-10 Inflatable Purge Enclosures). This solution is, however, somewhat expensive in respect of equipment and gas and is time-consuming. A compromise, but highly effective, alternative is to use a trailing shield since this will provide excellent coverage with minimal set-up time.
(See Technical Note TN-5 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 aerospace, semiconductor, nuclear, petrochemical, food, biofuels and pharmaceutical sectors.
Inert Gas Selection and Flow Rates
Shielding is most commonly provided by argon. It is denser than the helium alternative and thus less likely to be dissipated if turbulence is present. It is also much less expensive in most countries. As a rule of thumb 7 l/m of inert gas flow is regarded as adequate for effective shielding.
Trailing Shield Design
Trailing Shields® are generally custom-made to fit a particular torch and a particular welding operation. The shields manufactured by HFT® under the Argweld® name have in-built flexibility; they can accommodate most torches through the use of an adjustable clamp. For curved shapes, eg pipes, tubes and cylindrical vessels, trailing shields are manufactured to suit a specific diameter. They can be configured for either internal or external welding. Flat trailing shields for plate, sheet and rectangular tanks are available in small lightweight versions for manual welders or larger, more robust, versions for attaching to automatic/mechanised welding systems.
Modifications can be made by HFT® 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.
Argweld® Trailing Shields® from Huntingdon Fusion Techniques 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.
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 is ensured through the use of a flexible, pre-formed and easily replaceable silicone skirt. Turbulence inside the cavity is avoided by passing the gas through a mesh filter pack combination above the fusion zone.
Choosing a Shield
Selection 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 (titanium alloys and some stainless steels are particularly sensitive to contamination by residual oxygen).
- welding process being used (PAW has a different heat input than GTAW).
- welding speed and current.
Selection of Argweld® Trailing Shields® from HFT®.
The product range covers all applications between flat and 25 mm diameter for external and internal weld configurations.
Internal design showing welding torch clamped in position.
External weld design showing robust silicone skirt to ensure efficient gas sealing and
underside of mesh fitter pack for even gas distribution.
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