Nickel alloys are usually selected due to their excellent versatility, corrosion resistance and performance under high temperatures. Not surprisingly, this makes nickel alloys a popular choice for use in extreme environments, particularly in aircraft turbines, steam turbines, nuclear power plants, and the petrochemical and chemical industries.
Given its use in extreme environments, the weld zones of nickel alloys must have consistent properties—this is the only way that the finished, welded product will withstand the extreme environment. In addition, it is important that the weld is of high quality and contains very few flaws, as these could also affect performance in harsh environments.
What Are Nickel Alloys?
Nickel alloys are generally defined as alloys that feature nickel, a versatile element, as their principal element. Historically, nickel alloys were defined as those with more than 50% nickel. However, the nickel alloys used today general have a higher nickel content than 50%. For example:
- Inconel 718: 19Cr 3Mo .9Ti 5.1Cb .5Al 18Fe balance Ni
- Hastelloy X: 22Cr 1.5Co 9Mo .6W 18.5Fe balance Ni
Welding Nickel Alloys
All conventional welding processes are suitable for the welding of nickel alloys. Nickel alloys are, in fact, quite similar to austenitic stainless steels—the processes for welding the two materials are similar.
The main difference is in the thermal expansion. Nickel alloys have a lower coefficient of thermal expansion than stainless steels, and control methods for distortion are actually similar to what you would employ for carbon steel.
Common Issues When Welding Nickel Alloys
The most common and serious issue that occurs when welding nickel alloys is hot cracking. This occurs in either the fusion line, in the HAZ, or in the weld metal (fusion zone), although the fusion line is the most commonly affected area.
Usually, sulphur in the alloy or on the surface creates this cracking, although bismuth, lead, phosphorous and boron can also have a negative effect.
To prevent this, it is essential that both the HAZ and the weld metal are completely clear of oil, grease, dirt and other contaminants. Excess sulphur in the weld filler or parent materials can also cause issues.
To prepare the material, degreasing, followed by thorough stainless steel or machine wire brushing is required. Make sure you use a solvent that is designed for nickel alloys and that the welding takes place within eight hours of cleaning to prevent subsequent contamination.
Heat treatment should only be carried out with an electric furnace or with sulphur-free fuel, in a vacuum or inert environment.
If the material has already been used or is being repaired, it should be ground or machined to remove any contaminants that may have become trapped on the surface of the weld repair area.
Porosity is also an issue, particularly when oxygen or hydrogen cause surface contamination in the form of air entrapment in the weld pool. To counter this, an efficient gas purge and shielding are required, on face and root sides of the weld, and all gas hoses need to be in perfect condition. The welding area also needs to be sealed from any draughts.
Weld Preparation for Nickel Alloys
Weld preparation is essential when welding nickel alloys. The most important aspect of the design is ensuring there is sufficient access for the welding torch, and that full penetration may be achieved if it is required.
The best butt joint design is a square butt, but this is limited by thickness due to the inability to penetrate the joint. Thus, a U or V preparation is often used, with a 30° to 40° angle at 10mm thick, to allow for penetration and subsequent fill passes.
Regarding gas preparation, it can be useful to add up to 10% hydrogen to the inert gas mix, as this improves fluidity in the weld pool.
No preheating is required for nickel alloy welding unless there is a requirement to remove condensation. Welding nickel usually requires a maximum inter-pass temperature of 250 ̊C, although certain alloys should only use a maximum of 100 ̊C.
Some grinding post-weld may be required to remove an adherent oxide layer that can form on the surface of the weld pool. Sometimes, wire brushing will not be enough to remove this post-weld residue.
Welding Nickel Alloys with K-TIG
K-TIG can be used to weld nickel alloys up to 13mm thick, in a single weld pass, without the need for a gap or edge bevelling/grooved joint design. The K-TIG process can perform welds in both 1G and 2G positions and perform circumferential and longitudinal welding. The fact that there are no problems with closing out welds at overlaps also makes K-TIG beneficial for circumferential welding.
How Does K-TIG’s Keyhole Work?
K-TIG (Keyhole TIG) is a highly refined version of TIG/GTAW developed by the Australian Government’s Commonwealth Science and Industrial Research Organisation (CSIRO).
This welding process, based on years of research into the gas-tungsten-arc process, features countless innovations that create process efficiencies, stabilise the weld pool and remove heat from the HAZ.
It is ideally suited to the welding of nickel alloys and offers fabricators numerous advantages. It offers quality and single pass welds on nickel alloys up to 13mm (1/2 inch) thick, at speeds of up to 100 times that of conventional TIG/GTAW.
The increased penetration afforded by the keyhole also removes the need for edge bevelling or grooved joint preparations, while the efficiency of the process slashes gas consumption and wire consumption by 90%.
The keyhole is opened via the high energy density in the welding arc, which creates powerful penetrative capabilities and allows for high-speed welding.
The stable weld pool is created by a minimisation of surface energy, as well as the ability for gases to exit out of the weld zone via the keyhole. The surface tension is managed by the K-TIG torch, which converts the high current arc into a plasma jet, allowing the torch to manage surface tension and stabilise the weight of the molten material while the welding is taking place.
The best part is, all a nickel alloy welds will require is simple square butt preparation, instead of costly and time-intensive V or U preparation.
How to Get the Most Out of Your K-TIG System When Welding Nickel Alloys
Automation is required to unlock the real potential of K-TIG. This delivers a consistent travel speed, and all that is required in terms of automation is rigid torch mount, and steady travel speed. The machine can be incorporated into existing equipment such as seamers, rollers, rotators, manipulators, column and booms and robots.
The sophistication of the K-TIG system and K-TIG Evolve 1000 controller, combined with the consistent quality of an automated process, means both productivity and quality will typically increase significantly when you transition to the K-TIG process.
Edge Preparation and Set-Up
You will save both money and time by only having to work with a square butt joint. There is no need to achieve a perfect square butt joint, as K-TIG can maintain stability in the weld pool even with gaps and misalignment in the preparation. You can even add filler wire to a full penetration pass, increasing the fit-up tolerance further.
Purging requirements for K-TIG are the same as what you would use when using conventional TIG welding for nickel alloy welds. This means there is no need to re-learn purging requirements, and your workforce will feel comfortable with the process.
The preferred shielding gas for K-TIG welding with nickel alloys is 90% Argon and 10% Hydrogen. It is possible to perform welds with 100% argon, although the addition of hydrogen is preferred as it helps to improve weld pool fluidity.
What Training or Knowledge Do I Require?
K-TIG makes welding nickel alloys easy, and it follows that you don’t require extensive training to get your workshop up to speed with using the process. In fact, training can be completed in just a couple of hours, and only one or two days is required for comprehensive workshop supervisor training.
This ease of use and the reduced need to have workshops full of highly-experienced and credentialed welders is essential in remedying the fact that developed nations are facing a chronic skills shortage in the field of welding. You can increase your nickel alloy welding production, without adding to your workforce.
K-TIG worked with a company that was quoting a job that required the welding of a 9.5mm (3/8 inch) thick joint, using Alloy 20— a nickel-chromium-molybdenum alloy developed for applications involving sulfuric acid. The job was complicated by the fact that two different joints had to be welded, which were quite large, at 3.8m (150 inches) in length.
The estimators at the company gave the following assessments for the job carried out using TIG or GTAW for root passes, followed by flux-cored GMAW for fill and cover passes:
- 12 passes per joint
- 12 hours for set up time
- 67 hours for arc-on time
- 13 hours for interpass, cleaning and grinding
K-TIG was able to complete this same welding in just 48 minutes, which produced an arc-on time saving of 98.8%.
This is undeniable proof of what you can achieve when using K-TIG to perform nickel alloy welds.