Inconel 718 is a superalloy used in aerospace applications. Melting point: 1,260 °C. Tensile strength: 1,240 MPa.
When a rocket engine roars to life, some of its most punished parts are made from a metal you have probably never heard of: Inconel 718. It is the quiet workhorse that holds engines together where the heat, pressure, and shaking would tear ordinary metals apart.
Quick facts
- What it is: A nickel-based “superalloy” (a metal mixture built to stay strong at very high temperatures), also called Alloy 718 and coded UNS N07718.
- Main ingredients: About 50-55% nickel, ~19% chromium, ~18% iron, ~5% niobium and tantalum, ~3% molybdenum, plus small amounts of titanium, aluminum, and carbon.
- Density: 8.19 grams per cubic centimeter (0.296 pounds per cubic inch) – heavier than steel.
- Melting range: roughly 1260-1336 C (2300-2437 F).
- Strength: after heat treatment, its ultimate tensile strength (the pull it can take before breaking) often tops 1275 MPa (185 ksi).
- Useful range: stays strong from cryogenic (extremely cold) temperatures up to about 700 C (1300 F), and resists oxidation up to about 982 C (1800 F).
What it is and how it works
Inconel 718 gets its toughness from a trick called precipitation hardening, sometimes called “aging.” A part is first shaped and welded while the metal is soft. Then it is heated in a controlled way, and tiny hard particles grow inside the metal’s crystal structure – mainly a phase called gamma-double-prime (Ni3Nb), plus a little gamma-prime.
Think of those nanoscale particles like grains of sand mixed into wet concrete: they get in the way of the metal trying to slide and stretch, so it stays stiff and keeps its shape even when red-hot and heavily loaded. Resisting that slow sag under heat and load is called creep resistance.
What makes 718 special is that this hardening is driven by niobium rather than the faster-reacting aluminum and titanium used in many superalloys. Because of that, the metal can be welded and then aged without cracking – a problem known as strain-age cracking that plagues many high-strength alloys. Meanwhile, its nickel-chromium body naturally grows a thin protective oxide skin that shields it from corrosion.
Why it matters
A rocket engine is one of the harshest places an object can exist. Materials face extreme heat, crushing pressure, violent vibration, swings from cryogenic propellant to combustion-zone fire, and contact with reactive fuels and hydrogen, which makes many metals brittle. Inconel 718 endures all of this while staying weldable and, for a superalloy, comparatively affordable.
That is why it is chosen for load-bearing engine structure: combustion-chamber shells, turbopump rotors and housings, nozzle and manifold parts, ducts, valves, and fasteners. It is also one of the best-understood alloys for metal 3D printing (also called additive manufacturing). Printing lets engineers build complex, internally cooled parts as a single piece, cutting the number of components and shrinking production from months to weeks – a key enabler of modern reusable, rapidly improved rocket engines.
Trade-offs
Inconel 718 is not perfect. It is dense, adding weight wherever it is used, and it is notoriously hard and slow to machine the traditional way – it dulls cutting tools fast, which is a big reason 3D printing is so attractive for it. Its strengthening particles begin to dissolve above about 650-700 C, so its structural temperature ceiling is lower than some rival superalloys, even though it can briefly survive oxidation near 980 C. For the very hottest spots, engineers pair it with active cooling or higher-temperature metals.
Notable examples
- SpaceX SuperDraco: The launch-escape engine on the Crew Dragon spacecraft uses a combustion chamber 3D-printed from Inconel – the first flight-qualified, fully 3D-printed rocket engine chamber (each engine produces about 71 kN / 16,000 lbf of thrust and throttles between 20-100%).
- RS-25 (Space Shuttle Main Engine): Its main combustion chamber uses a structural shell of Inconel 718, lined with a copper alloy (NARloy-Z) that is cooled by liquid hydrogen. Alloy 718 is used extensively throughout the engine.
- NASA 3D-printed turbopumps: NASA has built turbopump inducers and other rotating parts from Inconel 718 as part of its additive-manufacturing engine research.
- RL10 upper-stage engine: The long-serving Aerojet Rocketdyne RL10, used on Centaur and Vulcan upper stages, relies on nickel superalloy hardware and growing use of additive manufacturing.
Ni 50-55%, Cr 17-21%, Fe bal, Nb 4.75-5.5%, Mo 2.8-3.3%, Ti 0.65-1.15%, Al 0.2-0.8%
| DENSITY | 8 kg/m³ |
| TENSILE STRENGTH | 1,240 MPa |
| YIELD STRENGTH | 1,036 MPa |
| STRENGTH-TO-WEIGHT | 151404.2 kN·m/kg |
| MELTING POINT | 1,260 °C |
| MAX SERVICE TEMPERATURE | 700 °C |
| THERMAL CONDUCTIVITY | 11.4 W/m·K |
| THERMAL EXPANSION | 13.0 µm/m·K |
| CATEGORY | Superalloy |
| DESIGNATIONS | UNS N07718, AMS 5662, AMS 5663 |
| MANUFACTURER | Special Metals Corporation |
| DENSITY | 8 kg/m³ |
| TENSILE STRENGTH | 1,240 MPa |
| YIELD STRENGTH | 1,036 MPa |
| MELTING POINT | 1,260 °C |
| MAX SERVICE TEMP | 700 °C |
| THERMAL CONDUCTIVITY | 11.4 W/m·K |
| THERMAL EXPANSION | 13.0 µm/m·K |
| CORROSION RESISTANCE | Excellent |
| WELDABILITY | Good |
| MACHINABILITY | Moderate |
| COST RATING | High |
