INCONEL 718 (UNS N07718/W.Nr. 2.4668) is a high-strength, corrosion-resistant nickel chromium alloy often used in the aerospace industry due to its superior mechanical properties and ability to withstand extreme temperatures ranging from -423°F to 1300°F (-253°C to 704°C). This age-hardenable material can be easily fabricated into complex shapes thanks to its excellent welding characteristics and support to post weld cracking. The combination of Ultimate tensile strength, fatigue, creep, and rupture strength makes INCONEL 718 an ideal choice for many applications in a myriad of industries including aerospace, chemical processing, power generation, oil & gas extraction and medical devices.
Table of Contents
- Inconel 718 Chemical Composition
- Physical and Mechanical Properties of Inconel 718
- Heat Treatment of Inconel 718
- Corrosion Resistance of Inconel 718
- Applications of Inconel 718
- Welding and Fabrication of Inconel 718
- Machining and Forming
Inconel 718 Chemical Composition
Overview
INCONEL 718 (UNS N07718/W.Nr. 2.4668) is a nickel-chromium alloy renowned for its corrosion resistance and strength at high temperatures. Inconel 718 finds extensive application across various industries, encompassing aerospace and medical sectors owing to its exceptional resilience even in the harshest environments.
Chemical composition of the Inconel 718
INCONEL 718 (UNS N07718/W.Nr. 2.4668) contains a range of elements, such as nickel, chromium, iron, cobalt, molybdenum, niobium and tantalum, with nickel being the primary component. Nickel 718 is known for its ability to perform at high temperatures without embrittlement or loss of strength. The chemical composition limits for this alloy are outlined in Table 1 below:
Table 1 – Inconel 718 Chemical Composition Limits (%)
| Element Name | Element Content (%) |
| Nickel | 50.0–55.0 |
| Chromium | 17.0–21.0 |
| Iron | 6.0–10.0 |
| Cobalt | ≤1.00 |
| Molybdenum | 2.8–3.3 |
| Niobium | 4.75–5.50 |
| Tantalum | 0.2–0.80 |
Physical and Mechanical Properties Inconel 718
INCONEL 718 is known for its excellent physical and mechanical properties. It is a precipitation-hardenable nickel-chromium alloy that can be readily fabricated, even into complex parts. Its welding characteristics, especially its support to post-weld cracking, are outstanding.
Density, Melting Point, and Thermal Conductivity
The density of Alloy 718 is 8.2 g/cm3, and it has a melting point of 1350°C (2462°F). Its thermal conductivity is relatively low, at approximately 11 W/mK in the temperature range from 20 to 100 °C (68–212 °F). This makes it highly suitable for applications that require high rise temperature resistance without sacrificing electrical insulation properties.
With its superior mechanical strength and corrosion resistance, INCONEL 718 stands out among its nickel-chromium counterparts. It can withstand higher stresses and temperatures, making it an excellent choice for critical components exposed to extreme conditions. The material also has good fatigue strength and creep rupture abilities, which ensures long-lasting durability even in arduous industrial settings. Additionally, its impressive oxidation resistance ensures reliable performance under high rise temperatures for extended periods.
Mechanical properties (tensile strength, yield strength, elongation)
INCONEL 718 exhibits excellent mechanical properties and Physical Properties over a wide range of temperatures. Its ultimate tensile strength ranges from 827 to 965 MPa (120 to 140 ksi) at room temperature and is maintained up to 648°C (1200°F). It also features superior yield strength, ranging from 450 to 655 MPa (66 to 95 ksi) at room temperature and is also retained up to 648°C (1200°F). Furthermore, INCONEL 718 has remarkable elongation values of 20–30% in both the annealed and solution-treated conditions.
Heat Treatment of Inconel 718
INCONEL 718 is an age-hardenable nickel-chromium alloy that can be readily formed and welded. It can be strengthened through heat treatment, which involves subjecting the material to a specific temperature to achieve desired properties. The two main types of heat treatments typically used in combination with INCONEL® alloy 718 are Solution Treatment and Age Hardening.
Solution Treatment and Ageing Processes
Solution Treatment is a process that involves heating the material at high rise temperatures for an extended period of time to dissolve any impurities within the material. This process results in a homogenous microstructure and increases ductility, toughness, and strength. The typical solution treatment temperature range for Alloy 718 is 927–1038°C (1700–1900°F). After this process, it is necessary to perform a rapid cooling procedure to avoid over-ageing due to uneven cooling rates.
Age Hardening is performed following the solution treatment process and involves further increasing the strength of the material by exposing it to temperatures between 540–760°C (1000–1400°F) for additional ageing times ranging from several hours up to 2000 hours or more depending on the required properties. This step also eliminates residual stresses created during fabrication as well as improving corrosion resistance properties.
Precipitation hardening of Inconel 718
Precipitation hardening is a heat treatment process that is used to improve the strength of superalloys such as INCONEL 718. It involves slow cooling of the material after solution treatment, which encourages the formation of nano-sized precipitates that disperse throughout the material and reinforce its structure. This process also increases the corrosion resistance properties of the material.
Corrosion Resistance of Inconel 718
Overview of corrosion resistance
INCONEL 718 is a highly corrosion resistant to most environments, making it an ideal material for components exposed to harsh conditions. It is especially resistant to oxidation and carburization in high rise temperature atmospheres. It also has good pitting resistance in low-pH environments such as seawater and acidic.
Resistance to oxidation and high-temperature environments
INCONEL 718 has excellent resistance to oxidation and carburization in high rise temperature atmospheres. It can withstand temperatures up to 1300°F without significant loss of properties or corrosion resistance. Its good oxidation resistance is due to the formation of a protective chromium oxide layer that prevents further oxidation and corrosion.
Corrosion resistance in various acids, alkalis, and aqueous solutions
INCONEL 718 also has superior corrosion resistance in various acids, alkalis and aqueous solutions. Its excellent resistance to hydrochloric acid makes it ideal for components exposed to marine atmospheres. It also exhibits good resistance to nitric acid, sulfuric acid, phosphoric acid, and formic acid. Additionally, it has excellent resistance to alkaline media such as sodium hydroxide and calcium hydroxide solutions. In conclusion, INCONEL® alloy 718 is a highly corrosion-resistant nickel-chromium material that can withstand extreme temperatures and harsh environments without significant loss of mechanical properties or corrosion resistance.
Applications of Inconel 718
Industrial Applications of Nickel 718
INCONEL 718 is an ideal material for applications requiring extreme durability due to its outstanding physical and properties. It is commonly used in industrial applications such as gas turbine components, nuclear reactors, pressure vessels, and aerospace components. Its superior creep strength, fatigue resistance, and oxidation resistance make it suitable for structural components subject to thermal shock, vibration, and cyclic loading.
Aerospace and gas turbine engine components
INCONEL 718 is widely used in aircraft engines due to its superior strength at elevated temperatures. It is often found in exhaust systems due to its good heat-resisting properties which increase engine efficiency by reducing back pressure buildup from trapped exhaust gases. The material is also commonly used in high rise temperature components for nuclear engineering such as control rod drives, fuel elements and safety rods due to its ability to withstand radioactive environments without significant loss of mechanical properties or corrosion resistance.
Oil and gas industry applications Inconel 718
INCONEL 718 is a highly sought-after material in the oil and gas industry due to its superior strength and corrosion resistance properties. It has excellent resistance to hydrochloric acid, making it ideal for components exposed to marine atmospheres. The material’s good creep strength, fatigue resistance, and oxidation resistance make it suitable for components subject to cyclic loading, vibration, and thermal shock. It is commonly used for pressure vessels and valves in oil refineries due to its ability to withstand extreme temperatures and harsh chemicals without significant loss of mechanical properties or corrosion resistance.
Welding and Fabrication of Inconel 718
Welding Processes and Techniques
UNS N07718 is an age-hardenable nickel-chromium alloy that can be readily welded using a variety of welding processes, including TIG, MIG and spot welding. In order to achieve the desired properties, it is important to use the proper welding technique and preheat temperatures.
Precautions and Challenges in Welding Inconel 718
When welding Alloy 718 it is important to be aware of certain challenges and precautions. It is essential to use the right welding process and preheat temperature, as well as choose the appropriate filler material and matching filler metal. As INCONEL® alloy 718 is an age-hardenable superalloy, it requires careful control of weld heat input and cooling rate to achieve optimum mechanical properties. Inconel 718 is a High Performance Alloy
Machining and Forming of Alloy 718
Machining and Forming INCONEL® alloy 718 is an age-hardenable superalloy that can be machined and formed into complex parts. However, due to its high strength and hardness, it is more difficult to machine than many other alloys. Careful consideration must be taken when selecting the appropriate cutting tools and speeds to avoid catastrophic failure.
