All about Nickel alloys
By Chris Waltos
Why Nickel Alloys, as a step up from Stainless steels
Nickel based alloys are closely related to Stainless steels by many virtues.
Where Stainless alloys, as versatile as they are, stop being effective, Nickel alloys tend to take on the baton, and work to resist even more demanding corrosive environments.
But navigating the family of Nickel alloys can be daunting, due to there being so many different versions and variations. Additionally, as alloying increases, the application of these grades tends to narrow to very specific uses.
In this blog, we focus on the common types available and used in Australia, and how they differ from each other.
A short history lesson
Nickel and its alloyed forms have been around for at least a century.
Alloy 400, launched as Monel 400 by INCO, has been around for almost 120 years. Haynes invented Hastelloy C, the very first nickel-chromium-molybdenum based alloy which would spawn a plethora of versions including the ubiquitous alloy 276 in 1926. Inconel 600 was introduced in the 1930’s.
But it wasn’t until the 1950’s until the development of these alloys exploded. This was partly driven by the adoption of Argon/Vacuum Oxygen Decarburisation, the process of removing impurities during steelmaking, such that tight chemistry control allowed alloying to really be pushed. Managing carbon and silicon allowed better weldability and forming, while also minimising detrimental phase formation. More generally, as industrial technologies developed, more advanced materials suitable for these demands were required.
Haynes International, Special Metals and VDM in particular have since been duking it out in further stretching the capabilities of Nickel alloys but, ultimately, the grades we tend to use today have been around for multiple decades; Alloy 22 was launched in 1985, while 625 is at least 60 years old!
Let’s work through the different families, by the dominant chemical elements present:
Pure Nickel
As you guessed it, almost completely (at least 99%) Nickel. Alloy 200 and 201 are the most common, used in chloralkali plants (which produce chlorine) and where hot, concentrated caustics (like sodium hydroxide) are present. Also used in electrical applications, particularly Alloy 205.
Nickel – Copper
Very good and extensively used in marine environments, resisting seawater as well as super duplexes. Also great in reducing acid environments, such as hydrofluoric acid or hydrochloric acid. Think Alloy 400 and its stronger (K-500 alternative for pump shafting and mechanical actuation componentry) moving into majority copper based 90:10 and 70:30, where resistance to biofouling (where sea organism growth can inhibit flow and induce corrosion) in low flow seawater is very useful.
Nickel – Molybdenum
Mostly Nickel with about 30% molybdenum, and made specifically for reducing acid corrosion resistances - plus are higher performing than nickel-copper alloys. Hastelloy B3 is the current generation grade, with B2 similar in performance but more readily available.
Nickel – Chromium – Iron
Based around the well-known Alloy 600 (also known by its brand name Inconel 600), here we introduce Chromium: a game changer for resistance to oxidising acids, localised corrosion and cracking. Adding more chromium with some Aluminium gives Alloy 601, a staple in high temperature environments. Further additions of Al plus at least 10% Cobalt gives Alloy 617, which offers better strength at high temperatures than 601.
A very similar group of alloys, slightly less alloyed and generally more iron than nickel, are the Incoloy branded 800’s: think Alloy 800/H/HT. This alloy is well known for resistance to high temperatures, used in furnace equipment, petrochemical cracking tubes and electrical resistance heating elements.
Adding Molybdenum and Copper gives Alloy 825, Alloy 20 and Alloy 28 which are typically used in aqueous environments where stainless steels tend to reach their limits. Very good Stress Corrosion Cracking resistance, and also to localised corrosion however super duplexes and 6 Mo type stainless steels encroach on these capabilities considerably.
Nickel – Chromium – Molybdenum
Adding Molybdenum to the previous family, and keeping iron to a minimum resulted in step changes in corrosion resistance. Grades such as Alloy 276, Alloy 22, 625, 686 and Alloy 59 are used in very hot, concentrated acids, bearing high chloride levels. Flue Gas Desulfurization equipment, used to scrub exhaust gases for ore smelting, as well as tanks, heat exchangers, pressure vessels, mixers, piping and valving for chemicals manufacturing and hydrometallurgical plants to name a few all use some grade variant in this family. Tungsten and Copper can also be added to optimise resistance to specific acids and applications.
P-H Alloys
As the name implies, precipitation hardened nickel alloys are used where mechanical properties are particularly important. Adding small amounts of Niobium, Titanium or Aluminium allows the formation of a secondary intermetallic microstructural phase – gamma prime (Ni3X (X=Al, Ti, Nb)) - which is an amazing strengthening phase, coining the term ‘Superalloys’. In short, the mechanisms which cause creep (loss of strength, even in supporting its own weight) at high temperatures are slowed considerably, a problem with almost every material. This is critical for applications requiring immense strength and oxidation resistance – think jet turbine blades which operate up to 1500 degrees C! Alloys here are from the Nimonic or Rene families.
Closer to our world, we see the aforementioned Alloy K-500, Alloy 617 and Inconel X-750.
Other less common types
Nickel – Iron: Soft magnetic materials with defined thermal expansion properties, used in electromagnetic devices.
Nickel – Chromium: Known as Nichrome and used extensively as wire in heating elements such as toasters and electric kettles.
Nickel – Titanium: Possess the ability to “remember” their shape, when heated after deformation. NASA plans to use Nitinol in future space exploration missions as ball bearings and other componentry.
Conclusion
Whilst many hundreds of variants have been commercially produced, in Australia there are between 10-15 grades which do the heavy lifting. Stirlings Performance Steels are expert distributors of all Nickel alloys mentioned, and can recommend the optimal material for any application.
A rhyme for your time
Beyond the limits of steel use a Nickel alloy, read our expert blog to see which to employ!