Quick: What’s the first word that comes to mind when you see the word Inconel?
Across the Internet, conversations rage about the benefits and downsides of the nickel-chromium high-performance alloy and we at OnlineMetals tend to think that both sides make some mighty valid points. Common sentiments include praising Inconel’s ability to handle the extremes while simultaneously generating strong language from metalworkers who know all about the reputation that Inconel has when it comes to both low-speed/high-maintenance machining needs and nickel allergy issues that tend to come standard with Inconel work. In this article, let’s discuss the pros and cons of working with Inconel and learn a thing or two about a superalloy that can handle a thing or two (up to and including your end mill).
First Off, What Exactly is Inconel?
Developed in the 1940s during a time in which alloys and aerospace worked hand in hand to produce engineering breakthroughs for civilian and military applications, Inconel is a nickel-chromium based superalloy designed to handle extreme services where pressure, heat, corrosion and oxidation hazards are present. While Inconel alloys vary in terms of elemental composition, nickel is the base, generally comprising between 44% to 72% of total composition alongside chromium, which makes up between 14% to 30% by mass. This combination is what gives Inconel its traits – a nickel base working in tandem with chromium, providing the complementary effect of forming stable metal carbides and improved resistance to corrosion.
Inconel has been used in refinery and utility process applications such as heat exchanger tubing, gas turbine blades, pressure vessels and steam generation systems where the need for continuous operation require materials that can withstand the temperature fluctuations and kinetic energy demands. In addition, it is also used extensively in high-end automotive exhaust valves and systems ranging from Formula One, NASCAR, NHRA and even in the public sector with Tesla’s incorporation of the alloy in its battery systems. However, Inconel was created with aerospace in mind and to this day, its usage in the industry still remains strong.
• For instance, did you know that the fabled X-15 hypersonic aircraft (below) utilized an Inconel-clad skin which provided the stability to reach its record-setting max speed of Mach 6.72?
• Better yet, the Saturn V rocket that helped put a man on the Moon used a similar version of the Inconel X used on the X-15 (Inconel X-750) for its F-1 rocket engine, handling close to 7000 kN of thrust and over 1000 psi of chamber pressure (7 MPa) and cementing its place as the most powerful single-nozzle liquid-fueled rocket engine ever flown.
• If that’s not relevant enough for you, SpaceX currently uses Inconel 718 in the engine manifold of its Merlin rocket (left) which powers the Falcon 9 space vehicle, pushing 934 kN of thrust on the Merlin 1D vacuum version.
What are the Compositions of Inconel Alloys and their General Features?
Inconel is diverse by design – with origins dating back to the 1940s, variations on the first alloy have been developed as industrial needs demanded better solutions. For instance, Inconel 625 was borne with the purpose of facilitating steam piping needs and because of the modifications that came after its original creation that improved weldability and creep-resistance, 625 Inconel’s use expanded into chemical process, marine and nuclear process/applications where high-pressure pumps and materials are used.
Here is a basic outline of major alloy options, along with percentage of nickel and chromium denoted in parentheses (% by mass), along with their key features:
• Inconel 600 (72% Ni/15-17% Cr) offers resistance to hydrochloric gases and is seen as a useful alternative to 200 and 201 grade nickel when used in the service of sulfur compounds or ammonium hydroxide.
• Inconel 625 (58% Ni/20-23% Cr) provides superior aqueous corrosion resistance, excellent weldability and is noted for its versatility.
• Inconel 690 (59% NI/30% Cr) features high chromium content perfect for aqueous corrosion resistance and metallurgical stability.
• Inconel X-750 (70% Ni/14-17% Cr), designed for the North American X-15. Composition differs from 600 with incorporation of niobium (1%), cobalt (1%), aluminum (0.4-1.0%) and titanium (2%). X-750 is also good at formability and retains its mechanical properties at high temperatures and is used for oil/gas production systems, gas turbine engines and in nuclear systems.
Working With Inconel – A Lesson in Speed
Take it from us – we’ve heard it all when it comes to projects using Inconel. Many of our online and will call shoppers praise Inconel for its durability but wince at the prospect of having to machine these alloys. Work hardening and welding problems can arise when not careful, and above all, watch your speed!
We keep harping about speed, but the plain truth is that machining Inconel with high speeds is a great way to ruin your equipment. While it is completely feasible to use a solid carbide end mill operating at higher speeds, the cutting speeds result in work hardening, resulting in shortened tool life that diminishes quality. There are many options outside of high speed steel; Cobalt drills and adjusting coolant, speed and feed seem to be the consensus choices for machining and working with Inconel.
As problematic as machining, welding Inconel is doable once you understand the issues you’ll be going up against. First off, as nickel alloy is known for its high heat tolerances, Inconel in welding can be brutal as the welds have a tendency to crack. But fear not, for there are alloys designed for welding usage. We recommend using Inconel 625 filler metal when joining two pieces of Inconel alloy and in many cases, when the joined materials are of dissimilar composition. As the strong welds take shape, it’s perfectly normal for the weld pool and fillers to be poorly defined and unattractive in appearance. When done right, Inconel welds are corrosion resistant and hold up as you would expect.
We also recommend TIG (tungsten inert gas) welding as it provides greater control when utilized by a skilled welder compared to GMAW and SMAW welding methods. If this is your first time to weld Inconel, practice, practice, practice first. Compared to stainless steel, welding Inconel requires less heat. Ensure that you avoid tight crevices and treat the welding part like stainless but the termination of the weld like aluminum. Weld with a 90 degree drag angle and slowly taper heat at the end of the bead to keep it from sucking back and cracking.
Shopping for Inconel & Nickel Alloys
OnlineMetals online metal shop carries 625 Inconel Equivalent sheet and we also carry Monel alloys for when you need a nickel-copper alloy that can handle salt water and petroleum applications. We provide cut-to-size specifications and can accommodate sizing needs and ship worldwide. Give us a call at (800) 704-2157 and get started on your next Inconel project today.