History of Inconel and Superalloys
Inconel, a registered trademark for a family of nickel-chromium based superalloys, was originally designed to handle extreme situations where pressure, heat, corrosion and oxidation hazards are present. Aerospace applications and engineering breakthroughs for civilian and military applications in the World War II effort were driving factors in the creation of this new class of materials.
Additional applications for these superalloys include refinery and utility process applications such as: heat exchanger tubing, pressure vessels, and steam generation systems where materials were required to withstand large temperature fluctuations and kinetic energy demands. The hard work of these materials is highly visible in high-end automotive exhaust valves and systems ranging from Formula One, NASCAR, NHRA and in the public sector with Tesla’s incorporation of the alloy in its battery systems. That said, aerospace applications continue to carry the majority of these specialty alloys usage.
Nickel, the primary element, generally comprises between 44% to 72% of total composition. While chromium, the secondary element, makes up between 14% to 30% by mass. This combination provides the complementary effect of forming stable metal carbides, improved resistance to corrosion, and scaling at elevated temperatures.
In this article, we will discuss the pros and cons of working with nickel based superalloys and demystify machining and welding with them!
What is the Chemical Makeup?
Below we breakdown the four major nickel-based superalloys, with percentage of nickel and chromium denoted in parentheses (% by mass), along with their key features:
- 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. Offered in Round Bar or Sheet/Plate.
- 625 (58% Ni/20-23% Cr) provides superior aqueous corrosion resistance, excellent weldability and is noted for its versatility. Offered in Round Bar or Sheet/Plate.
- 690 (59% NI/30% Cr) features high chromium content perfect for aqueous corrosion resistance and metallurgical stability.
- 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 Nickel-Based Superalloys
Many of our customers praise nickel-chromium alloys for their durability, but wince at the prospect of having to machine these alloys. Based on their sage advice we’ve summarized the most common themes we’ve heard when work hardening and welding these alloys.
Machining Tip: Watch Your Speed!
If you leave knowing one thing, it is that machining Inconel and its equivalent materials with high speeds can 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 these alloys.
Welding Tips: Fillers and Practice
Welding Inconel or any nickel-based superalloy is doable once you understand the challenges. Known for there high heat tolerances, welding nickel products can be brutal as the welds have a tendency to crack. We recommend using a 625 filler metal when joining two pieces of these alloys and in many cases, when the joined materials are of dissimilar composition. As a 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 625 filler welds are corrosion resistant and hold up to expectations.
Customers 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 these superalloys, practice, practice, practice first. Compared to stainless steel, the welding 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.
Did you know?
- That the fabled X-15 hypersonic aircraft (slide 1 in black & white) utilized an Inconel-clad skin which provided the stability to reach its record-setting max speed of Mach 6.72
- Or that SpaceX currently uses Inconel 718 in the engine manifold of its Merlin rocket (slide 2) which powers the Falcon 9 space vehicle, pushing 934 kN of thrust on the Merlin 1D vacuum version
- Even 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