Inconel 625 and Inconel 718 are both nickel superalloys, and the choice between them for high pressure oil and gas components comes down to one question: does strength or corrosion govern the part? For high pressure oil and gas hardware, specify Inconel 718 where mechanical strength governs, because aged 718 delivers roughly 1034 MPa minimum yield strength and meets NACE MR0175 / ISO 15156 qualification to a maximum 40 HRC for sulfide stress cracking resistance (Special Metals INCONEL alloy 718 bulletin, 2007). Specify Inconel 625 where corrosion severity governs over raw strength, because its 8 to 10 percent molybdenum gives superior chloride pitting, crevice and stress corrosion resistance (Special Metals INCONEL alloy 625 bulletin, 2013). Both alloys carry NACE listing, so the decision is strength led for 718 versus corrosion led for 625.
TL;DR
Inconel 718 is the high strength nickel superalloy, reaching about 1034 MPa yield and 1241 to 1276 MPa tensile after aging, which makes it the grade for oil and gas load bearing hardware, aerospace fasteners and rotating parts (Special Metals, 2007). Inconel 625 is the corrosion and elevated temperature grade, with 8 to 10 percent molybdenum for seawater service and oxidation resistance to 1800 F (982 C) (Special Metals, 2013). For seawater immersion Inconel 625 wins, while for peak mechanical strength aged Inconel 718 wins by a wide margin. Supreme Engineering Limited supplies both grades as round bar, so specify by the requirement that governs the part, not by price.
| Property | Inconel 625 (UNS N06625) | Inconel 718 (UNS N07718) |
|---|---|---|
| Strengthening mechanism | Solid solution (Mo, Nb), no aging | Age hardening (gamma double prime, Ni3Nb) |
| Molybdenum content | 8 to 10 percent | 2.8 to 3.3 percent |
| Room temperature yield | 414 to 655 MPa annealed | about 1034 MPa aged |
| Room temperature tensile | 827 to 1034 MPa annealed | 1241 to 1276 MPa aged |
| Service temperature | Cryogenic to 1800 F (982 C) | -423 F to 1300 F (-253 C to 704 C) |
| Seawater and chloride service | Excellent | Limited |
| Signature strength | Corrosion and elevated temperature | High strength, fasteners, rotating parts |
Composition and property figures above come from the Special Metals bulletins for alloy 625 (2013) and alloy 718 (2007).
Composition Is the Core Difference Between Inconel 625 and Inconel 718
Inconel 625 (UNS N06625) is a nickel chromium molybdenum alloy with a minimum 58 percent nickel, 20.0 to 23.0 percent chromium and 8 to 10 percent molybdenum, plus 3.15 to 4.15 percent niobium (Special Metals, 2013). Supreme Engineering Limited publishes a narrower chromium band of 20 to 22 percent for its own Inconel 625 round bar, which sits inside that grade limit. That high molybdenum load is the single fact that drives the corrosion advantage of Inconel 625.
Inconel 718 (UNS N07718) carries 50 to 55 percent nickel plus cobalt, 17 to 21 percent chromium, roughly 17 to 21 percent iron, about 5 percent niobium, 2.8 to 3.3 percent molybdenum and 0.65 to 1.15 percent titanium (Special Metals, 2007). The niobium, titanium and aluminium additions are what let Inconel 718 age harden.
The strengthening mechanism separates the two grades. Inconel 625 gains its strength from solid solution stiffening by molybdenum and niobium, so it needs no age hardening step. Inconel 718 develops far higher strength through a two step aging treatment that precipitates gamma double prime, an ordered Ni3Nb phase (arXiv 2107.13840, Ohio State University, 2021).
Supreme Engineering Limited melts both grades in house at its Khopoli Special Steels Division. Its published standards cover Inconel 625 round bar to ASTM B446 and AMS 5666, and Inconel 718 to ASTM B637, AMS 5662 and AMS 5663 (Supreme Steels product pages).
Aged Inconel 718 Round Bar Roughly Doubles the Yield Strength of Annealed Inconel 625
The difference between Inconel 625 and Inconel 718 round bar starts with heat treatment. Inconel 625 round bar reaches its properties without heat treatment: in the annealed condition it delivers 827 to 1034 MPa tensile strength and 414 to 655 MPa yield strength (Special Metals, 2013). Those are the numbers of a tough, formable, corrosion grade, not a peak strength grade.
Inconel 718 round bar age hardens to a different class of strength. Solution annealed and aged to AMS 5662 and AMS 5663, it delivers about 1241 to 1276 MPa tensile strength, 1034 MPa yield strength and 36 HRC hardness (Special Metals, 2007). Aged Inconel 718 yield strength roughly doubles the yield of annealed Inconel 625.
That strength gap decides most specifications. Where a part carries high mechanical load, holds a bolted joint or rotates at speed, aged Inconel 718 is the grade. Inconel 625 is not the high strength choice, and specifying it where 718 belongs trades away strength you paid a superalloy price to get.
Should You Use Inconel 625 or Inconel 718 for High Pressure Oil and Gas Components?
For high pressure oil and gas components, choose Inconel 718 where mechanical strength governs and Inconel 625 where corrosion severity governs. Special Metals produces Inconel 718 under NACE MR0175 for oil tool service, aged to a maximum 40 HRC for sulfide stress cracking resistance (Special Metals, 2007). That combination of high strength and sour service qualification makes Inconel 718 the grade for downhole tools, wellhead and completion hardware, hangers and valve stems.
Inconel 625 answers the other half of the well. Its 8 to 10 percent molybdenum and 58 percent minimum nickel give resistance to chloride pitting, crevice corrosion and chloride stress corrosion cracking, and Special Metals lists the alloy in NACE MR-01-75 (Special Metals, 2013). Engineers specify Inconel 625 for wetted, subsea and high chloride parts, cladding and liners, where corrosion sets the risk.
Use the governing failure mode to route each part between the two grades:
• When mechanical load governs: Specify Inconel 718 for parts that carry pressure, clamp load or rotation in sour wells, where its high aged yield and NACE MR0175 sour service qualification both matter (Special Metals, 2007). A load bearing string needs strength that annealed Inconel 625 cannot supply.
• When corrosion governs: Specify Inconel 625 where chloride pitting, crevice corrosion or stress corrosion cracking is the failure mode, including subsea trees, cladding and liners (Special Metals, 2013). Its NACE MR-01-75 listing covers that chloride and sour exposure.
• When the section runs hot as well as sour: Inconel 625 holds oxidation resistance to a higher temperature ceiling than aged Inconel 718, which trades peak temperature for strength (Special Metals, 2013 and 2007). Weigh the temperature headroom of Inconel 625 against the strength of Inconel 718 before specifying the hot section.
Supreme Engineering Limited supplies both Inconel 718 to ISO 15156 and NACE MR0175 and Inconel 625 as round bar from in house melt. The rule holds across the field: strength led parts take Inconel 718, corrosion led parts take Inconel 625.
Which Nickel Superalloy Has Better Corrosion Resistance for Seawater Service?
Inconel 625 has better corrosion resistance for seawater service than Inconel 718. Special Metals calls alloy 625 “an excellent choice for sea-water applications” and reports “freedom from local attack (pitting and crevice corrosion), high corrosion-fatigue strength, high tensile strength, and resistance to chloride-ion stress-corrosion cracking” (Special Metals, 2013). Alloy 625 carries a long marine record in mooring cables, propeller blades and submarine fittings.
The reason sits in the chemistry. Inconel 625 runs 8 to 10 percent molybdenum against roughly 2.8 to 3.3 percent in Inconel 718, and molybdenum drives resistance to chloride pitting and crevice corrosion (Special Metals, 2013 and 2007). Inconel 718 is a high strength alloy built for mechanical performance, not seawater immersion, so it concedes this contest plainly.
Where seawater or acid service turns even more aggressive than Inconel 625 handles, Hastelloy C276 is the step up. Supreme Steels describes its Hastelloy C276 round bar as a nickel molybdenum tungsten alloy with high resistance to pitting and crevice corrosion. Compared as options, Inconel 625 covers most seawater duty, and Hastelloy C276 answers the harshest chloride and acid exposure.
Which Is Better for Aerospace Fasteners, Inconel 625 or Inconel 718?
For aerospace fasteners, specify Inconel 718. Its high aged strength, age hardenability and outstanding weldability make it the standard fastener and rotating part superalloy, and Special Metals lists fasteners among the named uses of the alloy (Special Metals, 2007). Independent research identifies Inconel 718 as a widely used aerospace turbomachinery superalloy, favoured for its yield strength retention at elevated temperature (arXiv 2107.13840, 2021).
Inconel 625 earns an aerospace fastener where seawater or chemical corrosion, not peak strength, decides the part. On raw strength it cannot match aged 718, so a fastener that has to hold clamp load belongs in Inconel 718.
A286 sits below Inconel 718 in the strength ladder as a fastener option. Supreme Steels describes A286 round bar (UNS S66286, AMS 5731) as an iron nickel age hardening superalloy with high strength to about 704 C, a grade engineers reach for when the load and temperature stay below 718 territory. Compared as options, Inconel 718 covers peak strength fasteners, and A286 covers lighter duty below 718’s strength ceiling.
Frequently asked questions
- Should I use Inconel 625 or Inconel 718 for high pressure oil and gas components?
- Use Inconel 718 where mechanical strength governs the high pressure oil and gas component, and Inconel 625 where corrosion severity governs. Special Metals produces Inconel 718 under NACE MR0175 for oil tool service, aged to a maximum 40 HRC for sulfide stress cracking resistance, which suits downhole tools, wellheads, hangers and valve stems (Special Metals, 2007).
Inconel 625, with 8 to 10 percent molybdenum, resists chloride pitting and crevice corrosion and appears in NACE MR-01-75, which suits wetted, subsea and clad parts (Special Metals, 2013). Supreme Engineering Limited supplies both grades as round bar. - What is the difference between Inconel 625 and 718 round bar, and which is better for aerospace fasteners?
- The core difference between Inconel 625 and Inconel 718 round bar is the strengthening mechanism. Inconel 625 round bar reaches strength without heat treatment, while Inconel 718 round bar age hardens through a gamma double prime (Ni3Nb) treatment to much higher strength (Special Metals, 2013 and 2007; arXiv 2107.13840, 2021).
For aerospace fasteners, Inconel 718 is better, because aged 718 reaches about 1034 MPa yield against 414 to 655 MPa for annealed 625, which makes 718 the standard fastener and rotating part superalloy. Inconel 625 suits fasteners where corrosion, not strength, decides the part. - Which nickel superalloy has better corrosion resistance for seawater service, Inconel 625 or 718?
- Inconel 625 has better corrosion resistance for seawater service than Inconel 718. Special Metals calls Inconel 625 an excellent choice for sea water applications, with freedom from pitting and crevice corrosion and resistance to chloride ion stress corrosion cracking (Special Metals, 2013).
The driver is molybdenum: Inconel 625 runs 8 to 10 percent against roughly 2.8 to 3.3 percent in Inconel 718 (Special Metals, 2013 and 2007). Inconel 718 is a high strength alloy, not a seawater immersion grade, so where chloride or acid service turns harsh, Hastelloy C276 is the option to weigh. - Can Inconel 718 be welded and aged for aerospace fasteners and rotating parts?
- Yes. Inconel 718 welds readily by the gas tungsten arc process with Filler Metal 718, which is why it carries welded then aged aerospace fasteners and rotating hardware, and Special Metals describes its resistance to postweld cracking as outstanding (Special Metals, 2007). Its slow response to age hardening lets a fabricator weld a fastener or rotating part and age it directly, reaching joint efficiencies close to 100 percent.
This resistance to strain age cracking is a defining Inconel 718 advantage, not a weakness. One fabrication caution applies: avoid silver brazing compounds and cadmium bearing brazing alloys on cold worked or aged Inconel 718, since they can cause stress cracking (Special Metals, 2007). Inconel 625 also welds by conventional processes with Filler Metal 625 and needs no postweld heat treatment (Special Metals, 2013). - What service temperature can Inconel 625 and Inconel 718 handle in oil and gas components?
- In oil and gas components, Inconel 625 serves from cryogenic temperatures to 1800 F (982 C) and resists oxidation to that ceiling, holding toughness down to -320 F (-196 C) with ASME Boiler and Pressure Vessel Code approval (Special Metals, 2013). Inconel 718 serves from -423 F to 1300 F (-253 C to 704 C), with a strength ceiling near 650 to 700 C as its gamma double prime coarsens (Special Metals, 2007; arXiv 2107.13840, 2021).
For hot well sections Inconel 625 wins sustained heat and oxidation service, while Inconel 718 wins strength up to roughly 700 C. Match the grade to whether heat resistance or mechanical strength governs the oil and gas part. - Should I choose Inconel 625 or Inconel 718 for oil and gas components by price?
- For oil and gas components, specify Inconel 625 or Inconel 718 by the requirement that governs the part, not by price, because both are premium nickel superalloys. On raw element content, Inconel 625 carries the heavier alloy burden at 58 percent minimum nickel plus 8 to 10 percent molybdenum, while Inconel 718 substitutes 17 to 21 percent iron and runs lower molybdenum at 2.8 to 3.3 percent (Special Metals, 2013 and 2007).
Over specifying the corrosion package of Inconel 625 where the strength of Inconel 718 governs the part, or the reverse, is the costly error on a well. This comparison is directional and composition based, and no dated price source was available for this piece.