Nickel and its alloys are used in a very wide range of applications - from high temperature oxidation and creep resistance service to aggressive corrosive environments and very low temperature cryogenic applications. The Nimonic and Monel alloys are the most common proprietary metals, but the total range is extensive, covering nickel contents from 30% to 99%.
All the conventional welding processes can be used to weld nickel and its alloys and matching welding consumables are available. Care must be taken however throughout the welding process 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.
The most serious cracking problem with nickel alloys is hot cracking in either the weld metal or close to the fusion line in the HAZ with the latter being the more frequent. The main source of this problem is sulphur. Both weld metal and HAZ cracking are generally the result of contamination by grease.
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.
Porosity can be a problem with the nickel alloys, the main culprit being nitrogen. 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 must be as good as possible. Gas hoses should be checked for damage and leaks at regular intervals and, with the TIG process, as large a ceramic shroud as is 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 root pass
Failure to control the weld underbead and in particular to avoid ‘grapes’ and contamination from poor purge gas quality can result not only in the introduction of weld metal inclusions, but also reduce corrosion resistance if left on exposed surfaces. Total welding times can therefore be substantially extended when post-weld cleaning is required to remove these undesirable contaminants. This can clearly be time-consuming and expensive.
Controlling Purge Gas Coverage
Huntingdon Fusion Techniques 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 1800 mm.
The first requirement is to provide gas entry and exit points. Gas is fed through one end seal (C) with an exit 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 are available with nylon, steel and aluminium bodies. A central hole in the plug provides access for testing or gas admission, but may be closed when not used with a screwon cap and sealing washer. 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. These seals are available in a variety of materials to accommodate different requirements. Tube diameters between 12 and 160 mm can be accommodated with the nylon body and up to 900 mm with the metal body variants.
Inflatable devices 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 300 mm diameter.
This revolutionary PurgElite® product range replaces an earlier one that has been manufactured and marketed successfully by HFT worldwide for many years. 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 extends the size range to include purge systems as small as 25 mm.
The inflatable components employ low vapour pressure synthetic fabrics with low outgassing rates throughout and incorporate a connecting hose with a protection 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.
A compromise, but highly effective, alternative is to use a trailing shield since this will provide excellent coverage with minimal set-up time.
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.
Both internal and external models are available.
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, preformed and easily replaceable silicone skirt. Turbulence inside the cavity is avoided by passing the gas through a mesh filter above the fusion zone.
Vacuum enclosures and glove boxes are very effective in controlling residual oxidising gas content but are heavy, expensive and occupy considerable floor space. Flexible enclosures overcome all the disadvantages of vacuum systems.
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.
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.
Several manufacturers supply oxygen monitors but generally these are calibrated to measure much higher levels of oxygen, typically 20%. Only HFT has designed and developed monitors to meet the specific requirements of the welding industry. Following many months of field testing the company 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.
PurgEye® 100 is a hand held or tripod mounted instrument to display from 20.9% to 100 ppm oxygen.
PurgEye® 300 reads down to 10 ppm. Mains driven, the instrument has integral switching software to control external devices like power supplies or alarms and software to give QC prints of results for each weld.
PurgEye® 500 has all of the features of the PurgEye® 300 but additionally incorporates an internal pump to extract gas samples from the purge enclosure.
The fully integrated PurgEye® 600 incorporates touch screen controls. It also records all weld purge data for every weld and makes it available for downloading for subsequent evaluation as a quality control document. The ‘600’ also has on/off switching capability for alarms or welding equipment in the event of rise in oxygen levels during welding outside pre-set limits as well as an integral electronic sampling pump
For further details and information, ask the Company for HFT Technical Note TN-15 Weld Purge Monitors.