Nickel-based superalloys combine high strength with exceptional corrosion resistance. These super properties make them a popular material of choice across numerous industries.
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The INCONEL family of products that we stock here at Corrotherm has been specifically designed to handle the most extreme of environments. Now used in everything from chemical processing and aerospace, to the extraction and storage of oil and gas, INCONEL nickel alloys represent the premium standard in working metals.
All the different grades of INCONEL have high levels of nickel, with the flagship INCONEL alloy 625 containing a minimum of 58% nickel in its composition. When combined with significant levels of chromium, these nickel alloys offer better resistance to corrosion and high temperatures than other competing alloys.
Machining nickel superalloys
However, when choosing to work with one of these nickel-based superalloys, it&#;s important to know how to handle them. Such highly specialised and technical materials need to be worked and machined by an engineer who understands the unique properties of the INCONEL range. Nickel-based products can be hard to machine without the right approach and processes.
For a start, you&#;ll need to keep an eye on your machining speeds. Working with such a durable metal alloy at high speed using a solid carbide end mill will shorten your tool life and lead to work hardening. This means you need to be prepared to resharpen or change your tool regularly. However, you can use these high-speed drills at slower speeds if that&#;s the best option for your machines.
You may opt to work with cobalt drills, especially when using twist drill bits. If it&#;s feasible to do so, it&#;s also worth using C grade inserts, which are more expensive but can make machining these alloys easier. All of these tooling options make a difference to your manufacturing productivity, so play around with the combination that&#;s right for your business.
Another thing to consider when machining these high-spec nickel alloys is your use of coolants. Machining these nickel superalloys creates a lot of heat, which can warp the material as you work it. You may need to experiment with the right combination and quantity of coolants and lubricants, depending on the project. For example, for CNC machining, some engineers find using water-miscible vegetable oil-based fluids to be productive.
Welding and fillers for nickel superalloys
It&#;s also possible to weld nickel superalloys, but as with machining it takes a bit of practice, experience and knowledge. Because these high-nickel alloys are designed to withstand high temperatures, this can make it more difficult to weld the material and the welds can crack.
Picking the right filler material is important. Something like INCONEL alloy 625 is a good option for a filler to join two pieces of these superalloys together. If these welds are done with precision and skills, these welds should be both strong and highly corrosion resistant.
When it comes to welding methods, the most commonly recommended is Gas Tungsten Arc Welding (GTAW, also known as Tungsten Inert Gas or TIG). This is preferrable to other welding processes, such as Gas Metal Arc Welding (GMAW) or Shielded Metal Arc Welding (SMAW), for the best results.
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About Nickel-based Alloys
Nickel-based alloys are also referred to as ni-based superalloys due to their outstanding strength, heat resistance and corrosion resistance. The face-centered crystal structure is a distinctive feature of ni-based alloys, since nickel operates as a stabilizer for the austenite.
Common additional chemical elements to nickel-based alloys are chromium, cobalt, molybdenum, iron and tungsten.
Inconel® and Hastelloy® Nickel-based Alloys
Two of the most established nickel-based alloys families are Inconel® and Hastelloy®. Other notable manufacturers are Waspaloy®, Allvac® and General Electric®.
The most common Inconel® nickel-based alloys are:
- Inconel® 600, 2. (72% Ni, 14-17% Cr, 6-10% Fe, 1% Mn, 0.5%Cu): A nickel-chrome-iron alloy that displays excellent stability on a broad temperature scale. Stable against chlorine and chlorine water.
- Inconel® 617, 2. (Nickel balance, 20-23% Cr, 2% Fe, 10-13%Co, 8-10% Mo, 1.5% Al, 0.7% Mn, 0.7 % Si): This alloy made largely of nickel, chrome, cobalt and molybdenum displays high strength and heat resistance.
- Inconel® 718 2. (50-55% Ni, 17-21% Cr, iron balance, 4.75-5.5% Nb, 2.8-3.3% Mo, 1%Co, ): A hardenable nickel-chrome-iron-molybdenum alloy that is known for its good workability and excellent mechanical properties at low temperatures.
Hastelloy® nickel-based alloys are known for their resistance against acids. The most common ones are:
- Hastelloy® C-4, 2. (Nickel balance, 14.5 &#; 17.5% Cr, 0 &#; 2% Co, 14 &#; 17% Mo, 0 &#; 3% Fe, 0 &#; 1% Mn): C-4 is a nickel-chrome-molybdenum alloy that is applied in environments with inorganic acids.
- Hastelloy® C-22, 2. (Nickel balance, 20 -22.5% Cr, 0 &#; 2.5% Co, 12.5 &#; 14.5% Mo, 0 &#; 3% Fe, 0-0.5% Mn, 2.5 -3.5 W): C-22 is a corrosion-resistant nickel-chrome-molybdenum-tungsten alloy that displays good persistence against acids.
- Hastelloy® C-, 2. (Nickel balance, 23% Cr, 2% Co, 16% Mo, 3% Fe): C- is used in environments with aggressive oxidants, such as sulfuric acid and ferric chloride.
Improving durability of nickel-based work pieces
Nickel-based alloys are known for their excellent mechanical properties, such as corrosion resistance and high temperature stability. However, almost no work piece can last forever, no matter how splendid the material. To prolong lifespan of parts, nickel based alloys can be treated with BoroCoat®, our diffusion treatment to significantly improve corrosion and wear resistance as well as provide stability against oxidants.
The diffusion layers of BoroCoat® improve surface hardness to up to HV while maintaining a diffusion layer of 60 µm. The wear resistance is considerably improved, as is proven by the pin on disc test. While the wear depth of untreated nickel-based alloys increases the longer the pin rotates, ni-based alloys with BoroCoat® display consistent low wear depth throughout the test.
Areas of Application
Alloys with a nickel basis are often used in challenging environments that demand good resistance against high and low temperatures, oxidation/corrosion and high strength. This is why applications include but are not limited to: turbine engineering, power plant technology, chemical industry, aerospace engineering and valves/fittings.
If you want to learn more, please visit our website Soft Magnetic Alloy.
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