Nickel alloys are 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 their 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 few flaws, as these could also affect performance in harsh environments.
Welding nickel alloys
Nickel alloys feature nickel, a versatile element, as their principal element. Historically, nickel alloys were defined as containing more than 50 percent nickel. However, the nickel alloys used today generally have a higher nickel content than 50 percent. For example:
- Inconel 718: 19Cr 3Mo .9Ti 5.1Cb .5Al 18Fe balance Ni
- Hastelloy X: 22Cr 1.5Co 9Mo .6W 18.5Fe balance Ni
All conventional welding processes are suitable for the welding of nickel alloys. Nickel alloys are, in fact, quite similar to austenitic stainless steels and 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 those used for carbon steel.
The most common and serious issue that occurs when welding nickel alloys is hot cracking. This occurs in the fusion line, in the heat-affected zone (HAZ) or in the weld metal (fusion zone), although the fusion line is the most commonly affected area.
Typically, 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 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. A solvent designed for nickel alloys should be used and the welding must take 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 creates surface contamination in the form of air entrapment in the weld pool. To counter this, an efficient gas purge and shielding are required on the 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.
Prepping 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 degree angle at 10 mm thick, to allow for penetration and subsequent fill passes.
Regarding gas preparation, it can be useful to add up to 10 percent 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 typically requires a maximum interpass temperature of 250 degrees C, although for certain alloys, only a maximum of 100 degrees C should be used.
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 is not enough to remove this post-weld residue.
Key to welding nickel
To weld nickel alloys up to 13 mm thick in a single weld pass without the need for a gap or edge beveling/grooved joint design, the K-TIG process can be quite useful. K-TIG, short for keyhole TIG, is a highly refined version of TIG welding developed by the Australian Government’s Commonwealth Science and Industrial Research Organization.
The K-TIG process can perform welds in 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 it beneficial for circumferential welding.
This welding process, based on years of research into the gas-tungsten-arc process, features countless innovations that create process efficiencies, stabilize the weld pool and remove heat from the HAZ. And it produces welds 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 beveling or grooved joint preparations, while the efficiency of the process slashes gas and wire consumption by 90 percent. 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 minimization 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 stabilize the weight of the molten material while the welding is taking place.
The only requirement for nickel alloy welding is simple square butt preparation as opposed to costly and time-intensive V or U preparation.
Finding full potential
There are many benefits to using the K-TIG process, including these five listed.
Automation: The key to unlocking the real potential of K-TIG welding is automation. All that is required is a rigid torch mount and steady travel speed. The machine can be incorporated into existing equipment, such as seamers, rollers, rotators, manipulators, columns, and booms and robots.
Edge preparation and setup: There is no need to achieve a perfect square butt joint as the K-TIG process can maintain stability in the weld pool even with gaps and misalignment in the preparation. Filler wire can be added to a full penetration pass, increasing the fit-up tolerance further.
Purging: Purging requirements for K-TIG are the same as conventional TIG welding for nickel alloy welds. This means there is no need to relearn purging requirements, which helps welders feel comfortable with the new process.
Shielding gas: The preferred shielding gas for K-TIG welding with nickel alloys is 90 percent argon and 10 percent hydrogen. It is possible to perform welds with 100 percent argon, although the addition of hydrogen is preferred as it helps to improve weld pool fluidity.
Training and knowledge: K-TIG makes welding nickel alloys easy, and it follows that there’s no need for extensive training to bring welders 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 light of the skills shortage in the field of welding. Essentially, nickel alloy welding production can be increased without the need for more welders.
As a real-world example, K-TIG worked with a company quoting a job that required welding of a 9.5-mm (3/8-in.)-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 3.8 m (150 in.) in length.
The estimators at the company gave the following assessments for the job carried out using TIG welding for root passes followed by flux-cored MIG welding for fill and cover passes:
- 12 passes per joint
- 12 hours for setup time
- 67 hours for arc-on time
- 13 hours for interpass, cleaning and grinding
The K-TIG process was able to complete this same welding in just 48 min., which produced an arc-on time saving of 98.8 percent. These results serve as undeniable proof of what can be achieved when using K-TIG to perform nickel alloy welds.