Alloys based on nickel are used widely in industry sectors such as petrochemical, aerospace and power generation where resistance to chemicals and mechanical strength at high temperatures is required. The Nimonic and Monel alloys are the most common proprietary metals, but the total range is extensive, covering nickel contents from 30% to 99%.
Applied with care, all the conventional welding processes can be used to weld nickel and its alloys1,2 and the basic technical aspects have been understood for many years. It is however essential to ensure that contamination does not occur—the nickel alloys are particularly susceptible to cracking and porosity if the welding environment is not properly controlled.
Machining or vigorous stainless steel wire brushing followed by thorough degreasing with a suitable solvent is necessary prior to welding, with the welding taking place within about eight hours to reduce the risk of contamination.
The presence of nitrogen in the shielding gas can give rise to porosity so care must be taken to ensure that the weld area is sufficiently protected and this is particularly relevant in site welding applications. With the gas shielded processes, gas purity and the efficiency of the gas shield needs careful monitoring. Gas hoses should be checked for damage and leaks at regular intervals and, with the TIG (GTAW) process, the largest ceramic gas shroud available should be used together with a gas lens.
It goes without saying that gas purging of the root is essential when depositing a TIG (GTAW) root pass. Failure to control purging can result not only in the introduction of weld metal inclusions, but also reduce corrosion resistance if left on exposed surfaces. Post weld cleaning to remove these undesirable contaminants can be time-consuming and expensive.
CONTROLLING PURGE GAS COVERAGE
For Nickel Alloy Tube and Pipe welding, Huntingdon Fusion Techniques HFT® has developed a wide range of ancillary equipment designed specifically to ensure optimised coverage of the weld zone with inert purge gas. From simple expandable plugs to fully integrated inflatable devices the products can accommodate pipe sizes from 10 to 2,500 mm.
Pipe purging principle. The first requirement is to provide gas entry and exit points. Argon gas (D) is fed through one end seal (C) with exits to prevent an undesirable build-up of pressure (B). Alternatively the gas may be expelled through the joint line (A). Argon has a greater density than air and the gas inlet should be at a lower elevation than the bleed end so that air is expelled effectively from the pipe bore. The converse applies when using helium.
These plugs are a popular choice and are available with nylon, steel and aluminium bodies. Covering the pipe diameter range up to 2,400 mm.
A central hole in the plug provides access for testing, gas admission, gas or liquid release but may be sealed when not required.
Surrounding each body is a flexible seal that can be expanded by applying a radial force through a manually operated wing nut on the shaft. Tube diameters between 12 and 160 mm can be accommodated with the nylon body, up to 900 mm with the aluminium type and up to 2,400 mm with the steel body variants.
These mechanical plugs can be used for purging and sealing pipework fabrications where a variety of openings are present and where it is easier to purge the complete assembly.
Range of nylon body expandable products with synthetic rubber seals. They are inert and available up to 160 mm.
Alternative devices with metal bodies cover the pipe range up to 2400 mm.
These systems have been developed to help speed up the welding process for engineers involved in the fabrication of pipes and tube lines and assemblies up to 2,400 mm diameter, where a single joint can be isolated and purged.
The revolutionary PurgElite® range is manufactured and marketed world-wide by HFT®. It is now widely recognised as a robust, easy to use, welding ancillary that offers considerable savings in time and inert gas.
The ‘Elite’ development incorporates many advances in engineering technology and includes products as small as 25 mm diameter.
The two inflatable dams are connected by a synthetic flexible hose that will not scratch the inside of polished tube or pipe work. The hose is made of a self sealing intumescent material that resists even hot metal being dropped on it and will not disturb the purge gas flow.
Complicated and time consuming metal valves have been eliminated by the development of “IntaCal®“, an inline device that allows both dams to be inflated and the interspace purged automatically.
Bursting of dams is prevented with an accessory called PurgeGate®.
Examples of the PurgElite® product range. They include the worlds’ smallest integrated purge system at 25 mm diameter.
HFT®’s QuickPurge® Family of Inflatable Tube, Pipe and Pipeline Weld Purging Systems has become the internationally preferred product for pipe welding between 150 and 2,400 mm. The design is based on many years development by HFT® engineers and exploits advances in materials technology and user feedback to create systems offering ease and speed of use without sacrificing effective inert gas coverage.
Key features of the QuickPurge®III system include a central sleeve that dramatically reduces purge volume between the two dams, a PurgeGate® II device fitted to each unit to automatically control gas flow irrespective of cylinder gas pressure and flow and totally prevents any risk of bursting, as a result of over pressurisation.
RootGlo® is also fitted to each system and this allows the welder to see the centre of the purge system through the root gap for accurate positioning purposes.
Both QuickPurge® and PurgElite® products are multi-use systems and include high temperature options for pre- and post- weld operations as high as 760ºC.
Figure 4. Fully integrated QuickPurge equipment, developed specifically for the rugged demands of site welding and made up to 2400 mm diameter.
The inflatable components employ low vapour pressure synthetic fabrics with low outgassing rates throughout and incorporate a volume-reducing sleeve to resist thermal damage. Hoses used to transmit inert gas to and from the purge cavity and to provide inflation pressure are made from engineering grade nylon.
High temperature versions of purge systems – the HotPurge® - are available where pre- or post-weld protection is required for heat treatment up to 760ºC for 24 hours.
Having looked at weld purging devices for the underside of the weld, options such as trailing shields are available for the topside.
Trailing Shields® offer highly effective weld purge option. They provide excellent coverage with minimal set-up time.
Argweld® Trailing Shields® from Huntingdon Fusion Techniques HFT® 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. These provide an additional 100 to 150 mm coverage length to protect the weld while it is cooling during which time the welding torch continues to progress.
The shields are available to match a wide variety of forms from flat to diameters as small as 25 mm. Both internal and external models are available. (External models above 8” internal diameter).
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 series of mesh filters above the fusion zone.
Trailing shields manufactured for use with GTAW and PAW torches.
Other designs are available for internal , external and flat joints.
For component welding, Flexible Enclosures® overcome all the disadvantages of glove boxes and vacuum systems but at a fraction of the cost. They occupy considerably less floor space and all parts of the welded component finish bright and shiny with no oxidation or discolouration. Argon gas costs are reduced significantly and cleaning costs are eliminated.
Ultra-violet stabilised engineering polymers are used throughout during manufacture. The vertical sides are made from translucent material and the top is constructed using optically clear sheet.
High integrity tapes and adhesives are employed for permanent sealing of the various panels and leak-tight zips incorporated to facilitate entry and removal of equipment or parts for fabrication.
|Figure 6. Cost-effective enclosures are used extensively in research and development but the larger range (up to 27 cubic meters) introduced recently makes these systems suitable for manufacturing. They are currently used during Wire and Arc Additive Manufacture (WAAM) with nickel alloys.|
CONTROLLING PURGE GAS QUALITY
Using specialised weld purging equipment does not guarantee defect free welds. Control of the oxygen content of the purge gas is crucial to success and a monitor that measures residual oxygen content reliably and accurately at the low levels considered necessary when welding nickel alloys.
Very few manufacturers offer instruments to meet the specific requirements of the welding industry. Following over 40 years of experience and many months of field testing Huntingdon Fusion Techniques HFT® has recently introduced a revolutionary product range, the PurgEye®.
These are capable of measuring oxygen content accurately as low as 10 ppm - more than adequate to satisfy the residual oxygen of 50 ppm recommended for nickel alloy welding.
Seven variants of PurgEye® are available but the PurgEye® 600, with a sensitivity down to 10 ppm oxygen, raises the bar in monitoring. Mains or battery driven, the instrument has integral switching software to control external devices like power supplies or alarms and software to give quality control prints of results for each weld. It also incorporates an internal pump to extract gas samples from the purged volume.
PurgEye® 600 has touch screen controls. It also records purge data for every weld and makes it available for downloading for subsequent evaluation as a quality control document. A further feature is on/off switching capability for alarms or welding equipment in the event of rise in oxygen levels during welding outside operator pre-set limits.
- Welding of nickel alloys - Part 1 - Job Knowledge 22, 107, 108 TWI.
- Welding and brazing of nickel and nickel based alloys. Mortland J E et al. NASA report S5101 1972.
- huntingdonfusion.com. Burry Port UK
The following information is available from HFT® on request;
Technical Notes; Expandable Plugs, PurgElite® Systems, Trailing Shields®, QuickPurge® Systems, Flexible Welding Enclosures®, Weld Purge Monitors®.
White Papers; Flexible Enclosures, Advanced Purge Monitor Technology, Trailing Shields, Principles of Tube and Pipe Welding, Orbital Weld Purging, Why You Should Use Gas Purging When Welding.
By Dr. Michael J. Fletcher M.Sc. Metallurgy
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