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.
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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.
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Wrought Nickel
Pure nickel UNS N is used in the chemical industry for its corrosion resistance - particularly to alkalis. It is also used for its properties in shielding against electromagnetic interference and in transducers.
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Nickel-Iron Alloys
These are used as soft magnetic materials, as glass-to-metal seals and as materials with defined thermal expansion properties.
Invar® (UNS K), with 36% nickel and the remainder iron, is unique in having an almost zero coefficient of thermal expansion around room temperature. This makes it valuable where high dimensional stability is required, such as in precision measuring instruments and thermostat rods. It is also used at cryogenic temperatures because of its very low thermal expansion rates.
Alloys containing 72-83% nickel have the best soft magnetic properties and are used in transformers, inductors, magnetic amplifiers, magnetic shields and memory storage devices.
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Nickel-Copper Alloys
These are highly resistant to corrosion by alkaline solutions, non-oxidizing salts and seawater. The best-known is Alloy 400.
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Nickel-Molybdenum Alloys
These are highly resistant to reducing acids in the absence of oxidizing ions, such as ferric and cupric or dissolved oxygen. The best-known is Alloy B-2.
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Nickel-Chromium Alloys
These are characterised by their high resistance to corrosion at both normal and high temperatures (resistance to scaling), good high-temperature strength and high electrical resistance. There are three main groups of alloys:
- Ni-Cr (and also Ni-Cr-Fe) alloys with high electrical resistance for heating elements, such as 70-30 (UNS N) and C-Grade (UNS N)
- Ni-Cr alloys (with Fe and other alloying elements) with good corrosion resistance. The best-known are Alloy 600 (UNS N) and Alloy 601 (UNS N)
- Ni-Cr alloys with high-temperature strength and creep resistance, mostly age-hardenable, such as Alloy X-750 (UNS N)
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Nickel-Chromium-Iron Alloys
There are basically two groups of alloys:
- Ni &#; Cr &#; Fe alloys with excellent strength at high temperature and the ability to resist oxidation, carburisation and other types of high-temperature corrosion. The best-known is alloy 800 (UNS N) and its variants 800H (UNS N) and 800HT (UNS N). (Recently, these alloys were classified as stainless steels reflecting their high Fe content)
- Ni &#; Cr &#; Fe (with Mo and Cu) alloys with excellent corrosion resistance in specific applications. Probably the best-known is alloy 825 (UNS N), which offers exceptional resistance to sulphuric acid. Alloy G-3 (UNS N) offers exceptional corrosion resistance to commercial phosphoric acids as well as many complex solutions containing highly oxidizing acids.
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Nickel-Chromium-Molybdenum Alloys
These are highly corrosion-resistant, of which Alloy C-276 (N) is the best-known. They offer exceptional resistance to reducing acids such as hydrochloric and sulphuric. There are a number of variants based on this composition, which have modified the Cr and Mo levels and, in some cases, added Cu or W in order to extend the corrosion resistance to conditions that are more oxidising or more reducing. These include Alloy C-22 (N), Alloy 59 (N), Alloy C- (UNS N), and Alloy 686 (N).
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Nickel-Chromium-Cobalt Alloys
The addition of cobalt and molybdenum imparts solid-solution strengthening and high levels of creep-rupture strength to alloy 617 (UNS N). The addition of cobalt to HR-160 (N) provides outstanding resistance to various forms of high-temperature corrosion attacks, such as sulphidation and chloride attack in both reducing and oxidizing atmospheres.
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Nickel-Titanium Alloys
55% nickel-titanium alloy (UNS N) (also known as Nitinol) has shape-memory properties. When formed at one temperature and then deformed at a lower one, it regains its original form when reheated. The transition temperatures can be adjusted through careful control of the composition. Medical devices and specialised connectors are two of specific the applications. The same alloy can also undergo considerable elastic deformation and still return to its original shape (super-elastic property). This property has been exploited for applications as diverse as spectacle frames and shock absorbers that provide earthquake resistance in historic stone buildings.
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