Two Alloys, Two Strengthening Routes
Inconel 625 and Inconel 718 are nickel-based superalloys used in demanding high-temperature and corrosive service, but they are strengthened in different ways. Inconel 625 is a solid-solution-strengthened alloy; Inconel 718 is precipitation-hardened. That single difference drives most of the practical distinctions in strength, weldability and application.
Chemical Composition
Inconel 625 (UNS N06625) contains carbon 0.10% max, manganese 0.50% max, silicon 0.50% max, phosphorus and sulfur 0.015% max, chromium 20.0-23.0%, molybdenum 8.0-10.0%, niobium plus tantalum 3.15-4.15%, cobalt 1.00% max, aluminum 0.40% max, titanium 0.40% max and iron 5.00% max. Inconel 718 (UNS N07718) contains carbon 0.08% max, manganese 0.35% max, silicon 0.35% max, phosphorus and sulfur 0.015% max, nickel plus cobalt 50.0-55.0%, chromium 17.0-21.0%, cobalt 1.00% max, copper 0.15% max, aluminum 0.35-0.80%, molybdenum 2.80-3.30%, titanium 0.65-1.15%, niobium plus tantalum 4.75-5.50% and boron 0.001-0.006%.
Strengthening and Properties
Inconel 625 gains its strength from the solid-solution effects of molybdenum and niobium in a nickel-chromium matrix, delivering outstanding strength and toughness from cryogenic temperatures to 2000°F (1090°C), with excellent fatigue strength and resistance to chloride stress corrosion cracking. Inconel 718 is designed for very high yield, tensile and creep-rupture properties at temperatures up to 1300°F (705°C) through precipitation hardening of niobium-rich phases during aging, and has excellent weldability. Typical densities are 8.44 g/cm³ for 625 with a melting range of 1290-1350°C, and 8.19 g/cm³ for 718 with a melting range of 1260-1336°C.
Applications
Inconel 625 is used for heat shields, furnace hardware, gas turbine engine ducting, combustion liners and spray bars, chemical plant hardware and special seawater applications. Inconel 718 is used in jet engines and high-speed airframe parts, and in oil and gas drilling and production for valves, pump shafts and wellhead components, where its high strength and resistance to chlorides, stress corrosion and sulfide stress cracking are decisive.
Product Forms
Both alloys are available as pipe, tube, sheet, strip, plate, round bar, flat bar, forging stock, hexagon, wire and extruded sections, in sizes commonly covering round bar from 3 mm to 300 mm diameter, sheet from 0.1 mm to 5.0 mm, plate from 5.0 mm to 100 mm and above, strip from 0.05 mm to 2.0 mm, and seamless pipe from 6 mm to 355 mm OD, per standards such as ASTM B160, B166, B167 and related specifications. Each order can be supported by material test certificates and third-party inspection if required.
FAQ
Q: What is the main difference between Inconel 625 and 718?
A: 625 is solid-solution strengthened for corrosion resistance and toughness from cryogenic temperatures to 1090°C; 718 is precipitation hardened for very high strength up to 705°C.
Q: Which alloy is stronger?
A: Inconel 718, which develops very high yield and tensile strength through aging, typically exceeding 625 in room-temperature strength.
Q: Is Inconel 625 good for seawater?
A: Yes, its resistance to chlorides and stress corrosion cracking makes it a standard choice for seawater and marine hardware.
Q: What are the UNS numbers?
A: Inconel 625 is UNS N06625 and Inconel 718 is UNS N07718.
Q: Which alloy is used in oil and gas downhole service?
A: Inconel 718, for valves, pump shafts and wellhead components where high strength and sulfide stress cracking resistance are required.
Q: Are both alloys weldable?
A: Yes, both have excellent weldability; 718 is often welded in the solution-annealed condition and aged afterward.
