Alloy 718, often referred to as Inconel 718, is a high-strength, corrosion-resistant nickel chromium alloy supplied in the hot worked, solution annealed and age hardened condition. As with other types of Inconel, the Special Materials Corporation owns the trademark for Incoloy Alloy 718.
The addition of Nb, Ti, Al and Ni forms a series of precipitates during the controlled solution annealing and ageing process steps. This results in a significant increase in strength and hardness compared with alternative nickel alloys. These tensile and creep rupture properties are maintained from cryogenic to elevated temperatures. These properties make it suitable for applications between -250oC – 650oC.
As a nickel-chromium alloy, Inconel 718 possesses acceptable corrosion resistance in a range of environments including sea water, hydrogen sulphide, CO2, elemental sulphur and chloride ions over various temperatures. It is resistant to sulphide stress cracking and is capable of passing the sulphide stress corrosion test in accordance with NACE TM 0177 Method C solution A.
Furthermore, Alloy 718 also features exceptional welding properties – even in comparison to other Inconel alloys. As a result of these exceptional welding properties, Inconel Alloy 718 is suitable for a wide range of applications. We will go into further detail about these uses for Alloy 718 below.
Inconel 718 was introduced in the 1960s and initially used in the aerospace industry. However, due to its ready availability in the market, it was taken up by the oil and gas industry. This was associated with wells moving offshore, drilling deeper with higher temperatures, pressures and corrosive contaminants.
Other nickel-based alloys such as Hastelloy C276 and other Inconel alloys (such as Inconel 625) provided sufficient corrosion resistance, but insufficient strength without extensive cold working. Martensitic stainless steels were limited by their potential for hydrogen embrittlement and stress corrosion cracking. Therefore, Inconel 718 quickly became a default choice for many pressure-containing and load-bearing components in aggressive environments.
Nickel alloys such as Inconel 718 are well-suited to high-temperature applications as most other metals are either brittle or oxidize when exposed to elevated temperatures. However, nickel-based alloys will quickly form a protective oxide layer when heated in an atmosphere containing oxygen, thereby preventing further oxidation. It also has a relatively high melting point.
This means that only a handful of more exotic alloys are able to better the performance of nickel-based alloys at high temperatures. Inconel 718 retains its ambient temperature mechanical properties up to 650degC.
Inconel 718 offers very high levels of performance, both corrosion resistance and high strength. As such, applications will tend towards more aggressive environments as load-bearing components. The list of applications in oil and gas, plus the chemical process industry, is extensive but includes wellhead and downhole components, sheathing, fasteners, gate valves, choke stems, packers, tubing hangers and other items for corrosive/sour service.
Langley Alloys stocks Alloy 718 as solid bars between 5/8” (15.875mm) and 10” (254mm) diameter. Our stock is certified to API 6A CRA and NACE MR0175 / ISO 15156-3.
|Age hardened 120 ksi||Age hardened 140 ksi|
|Bars up to and including 10” (254) diameter or section||Bars greater than 10” (254mm) diameter or section||Bars up to and including 10” (254) diameter or section||Bars greater than 10” (254mm) diameter or section|
|0.2% Proof Stress, min||827 N/mm2||125 ksi||827 N/mm2||125 ksi||965 N/mm2||140 ksi||965 N/mm2||140 ksi|
|0.2% Proof Stress, max||1000 N/mm2||145 ksi||1000 N/mm2||145 ksi||1034 N/mm2||150 ksi||1034 N/mm2||150 ksi|
|Tensile strength, min||1034 N/mm2||150 ksi||1034 N/mm2||150 ksi||1138 N/mm2||165 ksi||1103 N/mm2||160 ksi|
|Elongation, 5.65√S0 and 4D||20%||20%||20%||20%|
|Reduction of Area||35%||25%||35%||25%|
|Hardness (Rockwell C)||32-40HRC||32-40HRC||34-40 HRC||34-40HRC|
|Impact Strength (-60oC) Longitudinal||<3”||<3”||<3”||<3”|
|68J av (61J min)||50 ft.lb av (45 ft.lb min)||68J av (61J min)||50 ft.lb av (45 ft.lb min)|
|Impact Strength (-60oC) Transverse||≥3” – 10”||≥3” – 10”||41J av (37J min)||30 ft.lb av (27 ft.lb min)||≥3” – 10”||≥3” – 10”||41J av (37J min)||30 ft.lb av (27 ft.lb min)|
|47J av (41J min)||35 ft.lb av (30 ft.lb min)||47J av (41J min)||35 ft.lb av (30 ft.lb min|
|Magnetic Permeability (20°C)||<1.002|
|Young’s Modulus (kN/mm2)||200|
|Specific Electrical Resistance, 20°C (µΩ-m)||1.25|
|Mean coefficient of thermal expansion, 20-100°C (m/m/degC)||12.1 x 10-6|
|Specific Heat, 20°C (J/kg.K)||435|
|Thermal Conductivity, 20°C (W/m.K)||11.1|
|Curie Temperature (degC)||-112 (aged)|