Nickel is the base metal in a wide range of alloys developed primarily to provide high temperature strength and excellent corrosion resistance. Common amongst these alloys is the Hastelloy series containing chromium and molybdenum. The primary applications are in the aerospace, power generation, petrochemical, offshore and automotive sectors.
Although few problems arise with the majority of welding applications, porosity can occur and as little as 0.025% nitrogen will form pores in the solidifying weld metal. Draughts can disrupt the gas shield and atmospheric contamination will occur resulting in porosity.
Care must therefore 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 carefully controlled.
Gas hoses should be checked for damage and leaks at regular intervals and, with the GTAW process, a large ceramic shroud should be used together with a gas lens. It goes without saying that gas purging of the root is essential when depositing a GTAW root pass.
Multi-pass welding can present difficulties. Any oxide build-up due to poor protection can inhibit fusion so attention to cleaning by grinding or wire brushing after each pass may be necessary. One feature of nickel alloys that is often encountered is the formation on the surface of the weld pool of a viscous and adherent scum. This can be difficult to remove and can result in inclusions and lack of inter-run fusion if not removed prior to depositing the next pass. Wire brushing is frequently not sufficient to remove this layer and it then becomes necessary to grind the weld surface.
Both weld metal and HAZ cracking are generally the result of contamination such as grease, oil and dirt left behind following inadequate cleaning; excess sulphur in the parent or weld filler metals causing a problem is a rare event. Machining or vigorous stainless steel wire brushing followed by thorough degreasing with an appropriate solvent is necessary prior to welding, with the welding taking place within about eight hours to reduce the risk of contamination. Thorough cleaning also applies to the filler material.
Inert gas purging
Failure to control the weld underbead (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.
Choosing a weld purge method for tubes and pipes
A) Pipe purging
The first requirement is to provide gas entry and exit points. Gas is fed through one end seal (C) with an exit (B) for feeding to a Weld Purge Monitor® for measurement of the outgoing oxygen and in addition to prevent an undesirable build-up of pressure.
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.
Integrated Inflatable Pipe Purging Systems
- Argon is the most widely used inert gas for purging, because of its low cost and ease of availability, so for “inert gas” read ‘Argon’.
- Professionally designed and manufactured Pipe Purging Systems have the two end dams placed a certain distance apart, to keep the weld purge space as small as possible, whilst at the same time, calculated to be far enough from the joint line to avoid overheating by the weld heat input.
This style of multiple use inflatable devices has been developed to help speed up the welding process for engineers involved in the fabrication of pipe and tube lines and assemblies up to 610 mm diameter.
The revolutionary PurgElite® product range replaces an earlier one that has been manufactured and supplied successfully 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 integral, IntaCal® automatic gas flow regulation and purging device along with the optional PurgeGate® anti burst system are amongst the advanced engineering developments provided The inflatable components employ low vapour pressure synthetic fabrics with low outgassing rates throughout and incorporate an intumescent connecting hose to resist thermal damage, particularly from molten weld droplets, to prevent rupture of the purge line and subsequent loss of weld through heavy oxidation.
Hoses used to transmit inert gas to and from the purge cavity and to provide inflation pressure are made of engineering grade nylon to prevent any outgassing that would otherwise raise the oxygen level suddenly.
Single Inflatable Dams
For situations where a double ended pipe purging system is not suitable, inflatable end dams can be obtained as single units to seal, purge and exhaust the unwanted air/gas.
These are available for diameters of 6” to 88”, (152 to 2235 mm).
High speed pipe purging systems
Rugged pipe purging systems are available from 6” to 88” (152 to 2235 mm) in diameter for tough, on site or shop activities,
The inflatable dams are made of low outgassing rate materials . The dams are closer together than normal and the systems are designed to purge the interspace much faster giving a very quick purge down to the required ultimate oxygen level.
These various types of inflatable purging products have no metal parts that will interfere with joint x-rays.
Although these QuickPurge® systems are manufactured from heat resistant materials to resist the welding heat, an additional range called HotPurge® is available for welding high strength steels that need pre- and post- weld, heat treatment cycles up to 300ºC (572ºF).
Water Soluble Weld Purge Film®
In cases where there is no post weld access, water soluble materials are available.
Although there is a water soluble paper variety, it is thought that as paper has a high content of water vapour, it is preferable to use a water soluble plastic film that is impervious to water vapour and will not absorb it.
Once it comes into contact with the water used for washing out the pipe or for hydrostatic testing of the joint(s) the film will simply dissolve to molecular level without leaving any trace, thus also eliminating the risk of any semi-dissolved paper sludge behind.
Argweld® Trailing Shields® have been designed specifically for use with GTAW torches. 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 for outside and inside circumferential welding.
The welding torch is mounted on the leading end of the shield and inert gas fed separately from the torch, through one or more ports behind the fusion zone.
A seal between the shield and the work is ensured using 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.
Wider and longer models are manufactured for automatic welding applications so that the hot metal is under an argon shield for a longer period to allow for the higher welding speed.
Flexible Welding Enclosures®
For batches of components, vacuum enclosures and glove boxes are very effective in controlling residual oxidising gas content but are heavy, expensive and occupy considerable floor space.
The low cost Flexible Welding Enclosure® overcomes all the disadvantages of vacuum systems and glove boxes.
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.
Expandable plugs and stoppers
Mechanical plugs 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 screw-on 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 162 mm can be accommodated with the nylon body, up to 914 mm with the metal body variants and up to 2440 mm with the inflatable models.
Control of purge gas oxygen level
Using specialised weld purging equipment does not guarantee defect free welds. Control of the oxygen content of the purge gas at all times is crucial to success.
It is easy to acquire oxygen monitors but generally these are for personal safety use and are calibrated to measure atmospheric levels of oxygen.
There is one design of an oxygen monitor developed specifically for the welding industry and that is the Weld Purge Monitor®.
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.
Other Weld Purge Monitors® with additional features are:
PurgEye® 300 Nano is a low cost entry level Weld Purge Monitor® reading down to 10 ppm to give very accurate readings.
PurgEye® 500 Desk has all the features of the PurgEye® 300 Plus but additionally incorporates an internal pump to extract gas samples from the purge enclosure.
The fully integrated PurgEye® 600 Touch 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 PurgEye® 600 Touch 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 and is also equipped with PurgeNet™.
Further Information and reading
Extensive information on all the weld purge techniques discussed above is available in the form of Technical Notes, White Papers, Conference and Seminar Presentations and International Publications. These are available free of charge on request from Huntingdon Fusion Techniques HFT®, https://www.huntingdonfusion.com/index.php/en_gb/technical-support
By Dr. Michael J. Fletcher M.Sc. Metallurgy
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