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 generally 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
Nickel welding is performed using one of the many available nickel alloys. Arc welding can be used including Stick, MIG, and TIG processes. Using stick welding, in particular, will create a weld stronger than the base metal. For nickel MIG welding, the shielding gas is a 50/50 helium and argon mix.
All conventional welding processes are suitable for the welding of nickel alloys. The main difference is in the thermal expansion. Nickel alloys have a lower coefficient of thermal expansion than stainless steel, and control methods for distortion are actually similar to what you would employ for carbon steel.
Nickel alloys can be joined reliably by all types of welding processes or methods, with the exception of forge welding and oxyacetylene welding. The wrought nickel alloys can be welded under conditions similar to those used to weld austenitic stainless steel. Cast nickel alloys, particularly those with a high silicon content, present difficulties in welding.
The most widely employed processes for welding the non-age-hard enable (solid-solution-strengthened) wrought nickel alloys are gas-tungsten arc welding (GTAW), gas-metal arc welding (GMAW), and shielded metal arc welding (SMAW). Submerged arc welding (SAW) and electro slag welding (ESW) have limited applicability, as does arc plasma welding (PAW). Although the GTAW process is preferred for welding the precipitation-hardenable alloys, both the GMAW and SMAW processes are also used.
Nickel alloys are usually welded in solution-treated conditions. Precipitation-hard enables (PH) alloys should be annealed before welding if they have undergone any operations that introduce high residual stresses.
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 causes 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.
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.
Post weld Treatment
No post-weld treatment, either thermal or chemical, is required to maintain or restore corrosion resistance, although in some cases a full solution anneal will improve corrosion resistance.
Heat treatment may be necessary to meet specification requirements, such as stress relief of a fabricated structure to avoid age hardening or stress-corrosion cracking (SCC) of the weldment in hydrofluoric acid vapor or caustic soda. If welding induces moderate-to-high residual stresses, then the PH alloys would require a stress-relief anneal after welding and before aging.
Nickel and nickel alloys are susceptible to embrittlement by lead, sulfur, phosphorus, and other low-melting-point elements. These materials can exist in grease, oil, paint, marking crayons or inks, forming lubricants, cutting fluids, shop dirt, and processing chemicals.
Work-pieces must be completely free of foreign material before they are heated or welded. Shop dirt, oil, and grease can be removed by either vapor degreasing or swabbing with acetone or another nontoxic solvent.
Paint and other materials that are not soluble in degreasing solvents may require the use of methylene chloride, alkaline cleaners, or special proprietary compounds. If alkaline cleaners that contain sodium carbonate are used, then the cleaners themselves must be removed prior to welding. Spraying or scrubbing with hot water is recommended. Marking ink can usually be removed with alcohol.
Processing material that has become embedded in the work metal can be removed by grinding, abrasive blasting, and swabbing with 10% HCl solution, followed by a thorough water wash. Oxides must also be removed from the area involved in the welding operation, primarily because of the difference between the oxide and base metal melting points. Oxides are normally removed by grinding, machining, abrasive blasting, or pickling.
Nickel alloys, both cast and wrought and either solid-solution-strengthened or precipitation-hardenable can be welded by the GTAW process. The addition of filler is usually recommended. Direct current electrode negative (DCEN) is recommended for both manual and machine welding.
Either argon or helium, or a mixture of the two, is used as a shielding gas for welding nickel and nickel alloys. Additions of oxygen, carbon dioxide, or nitrogen to argon gas will usually cause porosity or erosion of the electrode. Argon with small quantities of hydrogen (typically 5%) can be used and may help avoid porosity in pure nickel, as well as aid in reducing oxide formation during welding.
Welding of Cast Nickel Alloys
Cast nickel alloys can be joined by the GTAW, GMAW, and SMAW processes. For optimum results, casting should be solution annealed before welding to relieve some of the casting stresses and provide some homogenization of the cast structure.
Light peening of solidified metal after the first pass will relieve stresses and, thus, reduce cracking at the junction of the weld metal and the cast metal. The peening of the subsequent passes is of little, if any, benefit. Stress relieving after welding is also desirable.