A Comprehensive Guide to Nickel-Based Superalloys: Inconel 718 (Alloy 718/UNS N07718)
What is Inconel 718?
Inconel 718 is also known as "Alloy 718" and is designated UNS N07718 or W.Nr.2.4668 (China Grade: GH4169). It is an age-hardenable Ni-Cr-Fe-based superalloy containing significant amounts of iron, niobium, and molybdenum, as well as smaller amounts of aluminum and titanium. Titanium and niobium are added to overcome strain-age cracking problems during welding and weld repair. Inconel 718 combines corrosion resistance and high strength with excellent weldability, including post-weld cracking resistance. The alloy has excellent creep rupture strength at temperatures up to 1300 °F [700 °C].
Inconel 718 alloy has a good combination of properties in the temperature range of -253 °C to 700 °C through the precipitation of body-centered tetragonal γ' phase and FCC γ' phase. The yield strength below 650℃ ranks first among deformed high-temperature alloys. It has good fatigue resistance, radiation resistance, oxidation resistance and corrosion resistance, and has good machinability, weldability and long-term structural stability. It can manufacture various complex-shaped parts and has been widely used in aerospace, nuclear energy and petroleum industries within the above temperature range.
Characteristics of Inconel 718 (Alloy 718/UNS N06718)
Inconel718 precipitation-hardened molybdenum alloy, containing niobium and nickel-chromium alloy, has high strength and good toughness at 700℃, and has good corrosion resistance in high and low temperature environments. The delivery state can be solution treated or precipitation hardened.
Easy to process;
High tensile strength, fatigue strength, creep strength and fracture strength at 700℃;
Oxidation resistance at 1000℃;
Stable low-temperature chemical properties;
Characteristic is good welding performance.
Whether in high or low temperature environments, 718 alloy has excellent resistance to stress corrosion cracking and pitting. Alloy 718 has particularly outstanding oxidation resistance at high temperatures.
Corrosion resistance of Inconel 718 (alloy 718/UNS N06718)
The chemical composition of the alloy can be divided into three categories: standard composition, high-quality composition and high-purity composition. High-quality composition can reduce carbon and increase niobium on the basis of standard composition, thereby reducing the amount of niobium carbide, reducing fatigue sources and increasing the number of strengthening phases, improving fatigue resistance and material strength. At the same time, reduce the content of harmful impurities and gases. High-purity composition is to reduce the content of sulfur and harmful impurities on the basis of high quality standards, improve the purity and comprehensive performance of the material.
The boron content of Inconel 718 alloy for nuclear energy needs to be controlled (other elements remain unchanged), and the specific content is determined by negotiation between the two parties. When ω(b)≤0.002%, the alloy grade is Inconel 718A, which is different from the Inconel 718 alloy used in the aerospace industry.
Heat treatment system Nickel-based alloy Incoloy 718
The alloy has different heat treatment systems to control the grain size, shape, distribution and quantity of the δ phase, thereby obtaining different mechanical properties. The heat treatment system of the alloy can be divided into three types:
(1) (1010 – 1065)℃±10℃, 1h, oil cooling, air cooling or water cooling +720℃±5℃, 8h, furnace cooling 50℃/h to 620℃±5℃, 8h, air cooling. The results show that the grain size of the material is coarsened after treatment, the δ phase does not exist in the grain boundaries and grains, and there is notch sensitivity, but it is beneficial to improve the impact performance and resistance to low-temperature hydrogen embrittlement.
(2) (950 – 980)℃±10℃, 1h, oil cooling, air cooling or water cooling +720℃±5℃, 8h, furnace cooling 50℃/h to 620℃±5℃, 8h, air cooling. It is the most commonly used heat treatment system, also known as the standard heat treatment system.
(3) Cool to 5℃±8℃ at 620℃/h, hold for 50h, and air cool for 8h. After treatment, the material contains less δ phase, which can improve the strength and impact properties of the material. This system is also called direct aging heat treatment system.
