Inconel® Alloys: A Comprehensive Overview
Inconel® Alloys: A Comprehensive Overview
A Brief History of Inconel® Alloys
The Inconel® trademark was registered in 1932 by the International Nickel Company of Delaware and New York in the U.S. Inconel® alloys played a significant role in the development of the Whittle Jet engine in the early 1940s, thanks to research by Henry Wiggin & Co in Hereford, England. The Inconel® trademarks were later acquired in 1998 by Special Metals Corporation, further advancing the alloy's prominence.
What is Inconel?
Inconel® alloys refer to a family of austenitic alloys of nickel and chrome, these are trademarked high-strength superalloys* with exceptional corrosion resistance and heat resistance properties. Inconel® is a nickel/chromium, iron/steel superalloy with the addition of other elements such as molybdenum, cobalt, tantalum, titanium, and copper. Inconel® is frequently the first choice for demanding applications in geothermal, nuclear, aerospace, shipbuilding, and chemical processing, where components are subjected to high temperature, pressure or mechanical loads.
How Inconel® Performs Under High Temperatures
Inconel® alloys are oxidation and corrosion resistant. At high temperatures, Inconel® forms a stable passivation layer which protects the surface from further attack. Inconel® retains strength over a wide temperature range, attractive for high-temperature applications where aluminium and steel will succumb to creep. Inconel's high-temperature strength is developed by solid solution strengthening** or precipitation hardening***, depending on the alloy.
*A superalloy is an alloy with the ability to operate at a high fraction of its melting point. The main characteristics of a superalloy include mechanical strength, corrosion and oxidation resistance, thermal creep deformation resistance and surface stability.
** Solid Solution Strengthening is a method which involves dissolving a solute into a solvent to form a solid solution, which obstructs dislocation movement within the metal, thereby increasing strength.
*** Precipitation Hardening (also called age hardening or particle hardening) is a heat treatment technique in which extremely tiny particles are uniformly spread within the original material to increase its strength and hardness.
Common Trade Names of Inconel® Alloys
- UNS N06600 - Alloy 600: Inconel® 600, NA14, BS3076, NiCr15Fe (FR), NiCr15Fe (EU), NiCr15Fe8 (DE), 2.4816.
- UNS N06601 - Alloy 601: Inconel® 601, Nickel 601, 2.4851.
- UNS N06617 - Alloy 617: Inconel® 617, NiCr22Co12Mo9, 2.4663.
- UNS N06625 - Alloy 625: Inconel® 625, Chronin 625, Altemp 625, Haynes 625, Nickelvac 625 Nicrofer 6020, 2.4856.
- UNS N07718 - Alloy 718: Nicrofer 5219, Superimphy 718, Haynes 718, Pyromet 718, Supermet 718, Udimet 718, 2.4668.
- UNS N07750 – Alloy X-750: Inconel X750, Nickel X750, NiCr15Fe7TiAl, 2.4669.
Alloy Compositions - Detailed Analysis of Key Inconel® Alloys
The Inconel® alloy family has been specifically designed to operate in some of the most demanding environments.
With high nickel content, Inconel® Alloy 600 is virtually immune to chloride-induced stress corrosion cracking. The alloy has superb corrosion resistance to caustic environments and resistance to dry chlorine up to 538°C.
This alloy has excellent oxidation resistance up to 1093°C combined with high carburisation resistance and demonstrates good performance in high-temperature nitriding environments.
Typical Applications:
- Heat treating muffles and retorts
- Vacuum furnace fixtures
- Nitriding furnaces and baskets
- Chlorination equipment to 538°C
- Titanium dioxide plants
Alloy 601 alloy develops a tightly adherent oxide scale which resists spalling even under severe thermal cycling. This alloy has good high-temperature strength and retains its ductility after long service exposure, with good corrosion resistance under oxidising conditions.
Typical Applications:
- Copper brazing, annealing and sintering
- Muffles and retorts
- Radiant tubes
- Strand annealing tubes
- Steam superheater tube supports
- Rotary kilns and calciners
- Thermocouple protection tubes
- Nuclear waste transport tank lining
- Vitrification of nuclear waste
Inconel® 617 is a nickel-chromium-cobalt-molybdenum alloy with an exceptional combination of high-temperature strength and oxidation resistance.
In addition to high temperature applications in the aerospace and power generation markets, the alloy is also used in high temperature petrochemical plants for applications such as nitric acid catalyst grids where metallurgical stability is critical.
Typical Applications:
- Combustion cans and ducting for gas turbines
- Transition liners for gas turbines
- Casing for industrial gas turbines
- Industrial furnace components
- Nitric acid catalyst grids
A highly versatile alloy, Inconel® 625 demonstrates an excellent combination of corrosion resistance and high strength. Due to its high nickel content, it is immune to chloride-induced stress corrosion cracking and has good resistance to pitting and crevice corrosion.
NeoNickel offers Alloy 625 sheet certified to both UNS N06625, 625 LCF® and ATI 625 HP® (UNS N06626). The 625 sheet meets the low-cycle fatigue properties specified in UNS N06626 and is also produced and certified as DDQ (deep drawing quality). ATI 625 HP® is a registered trademark of ATI Allegheny. 625 LCF® is a registered trademark of Special Metals Corporation.
Typical Applications:
- Aircraft ducting and exhaust systems
- Engine thrust-reverser systems
- Turbine shroud rings
- Bellows, expansion joints
- Gaskets and damper seals
- Motorsport exhaust and clamps
- Weld overlay and spacers for heat recovery systems
- Furnace muffles
- Valve seats and components
Inconel® 718 is precipitation-hardened to give maximum strength and high creep-stress rupture strength. The alloy can be supplied in the annealed condition to enhance machinability followed by subsequent appropriate heat treatments. Capability tests are completed on test samples which have had the precipitation hardening process completed.
Alloy 718 demonstrates outstanding weldability including resistance to post-weld cracking.
Typical Applications:
- Gas turbine engine parts
- Liquid fuel rocket motor components
- Springs, fasteners
- Cryogenic tanks
- Pumps and valves
- Tooling
Inconel® X-750 has good mechanical properties at high temperatures up to 704°C. The alloy was developed from Alloy 600 by adding aluminium and titanium to make the alloy precipitation hardenable. Alloy X-750 also demonstrates good formability and excellent high-temperature oxidation resistance.
Alloy X-750 offers outstanding performance in highly stressed environments, which is why it’s often used in gas turbines, nuclear reactors and aircraft structures.
Typical Applications:
- Rocket engines
- Gas turbine engines
- Nuclear reactors
- Nuclear valves
- Pressure vessels
- Tooling
- Aircraft structure
Manufacturing and Welding Inconel® Alloys
Inconel® machinability
Machinability is the ease with which a metal can be cut/shaped (machined) with a satisfactory finish at a low cost. Materials with good machinability require little power to cut, can be cut quickly, easily obtain a good finish, and do not cause significant wear on the tooling.
In today’s world of advanced engineering, certain materials stand out from the crowd due to their excellent properties like toughness and strength in extreme environments. Factors that typically improve a material's performance often degrade its machinability, presenting a significant engineering challenge. These beneficial properties also make the machining of Inconel® alloys a complex task which requires precision and expertise as these alloys rapidly work-harden during machining.
A material's machinability is measured using the AISI Machinability index. The higher the index number; the easier the material is to machine. For example, AISI 1212 steel has an AISI Machinability index of 100% (the benchmark), let’s compare this with Aluminium (cast) which has a Machinability index of 450% while Inconel® X-750 (aged) scores a very moderate 12%.
The six main material factors which impact machinability are: Chemical composition, microstructure, grain size, hardness, heat treatment and fabrication method - hot rolled, cold drawn, etc. Additionally, selecting the correct cutting tools, coolants, cutting speed, feed rate and depth of cut are all critical factors too.
Welding of Inconel® Alloys
Inconel® alloys are very strong and expensive. Given the properties of Inconel and its expense, it is probably safe to say that any use of Inconel® is very likely an important one where the needs must justify the cost. In the demanding industries where Inconel® is used, such as nuclear and aerospace, a leak in a pipe or vessel isn’t just a leak—it’s potentially a headline-making catastrophe.
There is a plethora of information available online regarding welding Inconel® alloys so here we shall focus on 3 tips.
- Considering the level of practice and knowledge required to manually weld Inconel, it is recommended by many that automated welding machines are used. These machines increase safety, consistency and quality.
- Many professional welders recommend adding filler material to the TIG welding process. Due to the extremely high melting points of most Inconel® alloys it is impractical to directly join two Inconel® pieces, (the larger the pieces the harder the task). Instead, it is recommended to use a welding process that combines high temperature with a filler material.
- Use Inconel® alloy 625 as your filler material. This is one of the easiest Inconel® alloys for welding and is compatible with the other Inconel® grades used in manufacturing. A filler metal that is dissimilar to the alloys being welded will often cause a host of problems like galvanic corrosion and extreme volume expansion during hot welds.
Useful Links:
https://www.neonickel.com/alloys/all-alloys/alloy-600 https://www.neonickel.com/alloys/all-alloys/alloy-601 https://www.neonickel.com/alloys/all-alloys/alloy-617 https://www.neonickel.com/alloys/all-alloys/alloy-625 https://www.neonickel.com/alloys/all-alloys/alloy-718 https://www.neonickel.com/alloys/all-alloys/alloy-X-750 https://www.neonickel.com/technical-resources/welding-high-alloy-materials
