Inconel 718: The Complete Technical Handbook on Chemical Composition and Physical Properties

Introduction: A Critical Superalloy

Inconel 718 is a top material in the world of high-performance metals. It belongs to the nickel-chromium superalloy family and goes by the names UNS N07718 and nickel alloy 718.

This alloy was made for the toughest environments out there. It stands out because it stays strong at high temperatures, fights off corrosion, and can be welded much better than other materials with similar strength.

These qualities make it essential in important industries. The alloy shows up in aerospace parts, gas turbine engines, nuclear reactors, and oil and gas equipment where failure simply isn't an option.

This handbook gives you a full technical breakdown. We will look at its exact chemical makeup, the material properties that come from this mix, and what this means for the engineers, designers, and manufacturers who depend on this remarkable superalloy.

The Blueprint of Strength

The amazing performance of Inconel 718 doesn't happen by chance. Its power comes from a carefully balanced chemical composition where each element has an important job.

The way the nickel-iron-chromium base works with key strengthening elements transforms it from a simple alloy into a true superalloy.

Composition Breakdown (AMS 5662/5663)

The makeup of Inconel 718 follows strict rules under standards like AMS 5662 and AMS 5663. This ensures it performs the same way every time.

Element	Symbol	Weight Percentage (%)
Nickel	Ni	50.0 - 55.0
Chromium	Cr	17.0 - 21.0
Niobium (+ Tantalum)	Nb (+ Ta)	4.75 - 5.50
Molybdenum	Mo	2.80 - 3.30
Iron	Fe	Balance
Titanium	Ti	0.65 - 1.15
Aluminum	Al	0.20 - 0.80
Cobalt	Co	1.0 max
Carbon	C	0.08 max
Manganese	Mn	0.35 max
Silicon	Si	0.35 max
Phosphorus	P	0.015 max
Sulfur	S	0.015 max
Boron	B	0.006 max
Copper	Cu	0.30 max

Role of Key Elements

Each element in the Inconel 718 composition has a specific purpose.

Nickel forms the backbone of the alloy. It creates a stable, flexible matrix that stays strong at high temperatures and resists many types of corrosion.

Chromium protects the alloy from harsh environments. The element forms a thin oxide layer on the surface that shields the metal from oxidation and corrosion when things get hot.

Niobium is the most important strengthening element in Inconel 718. When heat-treated, it joins with nickel to form tiny particles that give the alloy its amazing strength.

Molybdenum makes the basic structure stronger. It also helps the alloy fight off pitting and crevice corrosion, especially in environments with lots of chlorides.

Iron makes up the rest of the mix. It works with nickel and chromium to form the basic structure and helps keep costs down without hurting performance.

The Strengthening Mechanism

The high strength of Inconel 718 comes from a process called precipitation hardening. This is where its special metallurgy really shines.

During a specific heat treatment cycle, tiny particles form throughout the alloy. These act as microscopic roadblocks that prevent the metal from deforming.

The main strengthening phase is gamma double prime with the formula Ni₃Nb. This phase fits perfectly within the metal's structure and is what gives the alloy its high strength up to about 650°C (1200°F).

A second strengthening phase, gamma prime, also forms. While not as powerful as gamma double prime, it helps maintain strength at higher temperatures.

The combination of these phases is carefully controlled by the heat treatment process. This is why recent studies on its microstructure are so important for getting the best performance from parts made with this alloy.

Translating Composition into Performance

The alloy's chemical blueprint directly creates a world-class set of inconel 718 properties. Engineers use these measurable traits when designing, analyzing, and choosing materials for important applications.

This table shows the typical properties for heat-treated Inconel 718.

Property	Metric Value	Imperial Value
Density	8.19 g/cm³	0.296 lb/in³
Melting Range	1260 - 1336 °C	2300 - 2437 °F
Tensile Strength (Min)	1275 MPa	185 ksi
Yield Strength (0.2% Offset, Min)	1034 MPa	150 ksi
Elongation (Min)	12%	12%
Hardness (Typical)	~36 HRC / ~331 HBW	~36 HRC / ~331 HBW
Thermal Conductivity (at 100°C)	11.4 W/m·K	6.58 BTU·in/hr·ft²·°F
Coeff. of Thermal Expansion (20-100°C)	13.0 µm/m·°C	7.2 µin/in·°F

Mechanical Properties

The standout feature of Inconel 718 is its mechanical strength, which stays high across a wide range of temperatures.

The tensile and yield strength are extremely high for a nickel alloy. The impressive inconel 718 yield strength, usually above 1034 MPa (150 ksi), comes directly from those tiny gamma double prime particles.

The typical inconel 718 hardness is around 36 HRC after full heat treatment. This high hardness gives excellent resistance to wear but makes the alloy hard to machine.

Its resistance to creep and fatigue failure is remarkable. The stable particles effectively lock the metal's structure in place, making it reliable for spinning parts in turbines that face high stress at temperatures up to 650°C (1200°F).

Thermal Properties

The thermal characteristics of Inconel 718 matter greatly for both design and manufacturing.

The inconel 718 thermal conductivity is quite low. This has two sides to it. While good for containing heat in things like combustors, it means that heat from machining doesn't spread quickly, leading to very hot cutting tools that wear out fast.

Its thermal expansion rate is predictable and works well with many steels and other high-temperature alloys. This predictability is vital for parts in complex machines, like jet engines, that heat up and cool down a lot during use.

Corrosion Resistance

Thanks to its high nickel and chromium content, Inconel 718 fights off corrosion in many different environments.

It resists oxidation and corrosion in the hot gases found in turbines and jet engines. The chromium provides strong protection against high-temperature oxidation.

The alloy is especially valued because it doesn't crack after welding, which is a common problem in other high-strength alloys. It also stands up well to stress-corrosion cracking from chlorides, making it good for certain marine and chemical processing uses.

Practical Implications

Understanding the data is one thing; applying that knowledge in real life is another. The properties of Inconel 718 have direct and important effects on how it's made and used.

The Machinability Challenge

In the real world of manufacturing, Inconel 718 is famously difficult to machine. This difficulty comes directly from its desirable properties.

The alloy stays strong and hard even at the high temperatures created at the cutting tool's edge. Its poor heat conduction focuses heat at the cutting point, and it tends to get harder as you work it, creating an even tougher surface for the next cut.

To successfully machine this alloy, you need a solid setup. We recommend these best practices:
* Use strong, rigid machine tools to reduce vibration.
* Work at low cutting speeds but keep feed rates high and steady to stay ahead of the hardened zone.
* Use plenty of high-pressure coolant to manage heat and clear chips.
* When possible, do rough machining before the final hardening heat treatment.

Research into advanced manufacturing processes is ongoing, trying to create parts that need minimal machining afterward.

Critical Applications

The unique mix of strength, temperature resistance, and corrosion resistance makes Inconel 718 the go-to material for parts where performance and reliability are essential.

• Aerospace & Gas Turbines: It's the workhorse alloy for jet engine and land-based gas turbine components. This includes critical parts like turbine discs, blades, shafts, casings, and high-strength fasteners.

• Oil & Gas Industry: In downhole and wellhead equipment, Inconel 718 provides the necessary strength and corrosion resistance to handle sour gas (H₂S) and high-pressure environments.

• Nuclear & Power Generation: Its strength and resistance to water corrosion make it good for reactor core parts, control rods, instruments, and steam generator components in nuclear power plants.

• Tooling & Cryogenics: The alloy is also used for high-strength bolts, cryogenic storage tanks, and components in rockets, where it maintains excellent properties at extremely low temperatures.

Conclusion: An Engineered Solution

The Inconel 718 composition shows brilliant metallurgical design. The careful balance of nickel, chromium, iron, and especially niobium creates a material that pushes performance boundaries.

Its ability to be strengthened through precipitation hardening lets it deliver a combination of high tensile strength, creep resistance, and weldability that few materials can match. This makes it essential for industries that define modern technology.

While it shares a family connection with other nickel alloys like the highly corrosion-resistant Inconel 625, the unique strengthening mechanism of Inconel 718 creates its own essential role. For engineers and designers facing extreme temperatures and high mechanical loads, understanding the composition and properties of UNS N07718 isn't just helpful—it's fundamental.