How Alloy 625 and Alloy 718 Tubing Compare
Alloy 625 and Alloy 718 are both nickel-chromium superalloys supplied as seamless and welded tube. They share a face-centred cubic matrix, a high nickel content and excellent toughness at low temperature, but they diverge in the way they reach strength. Alloy 625 (UNS N06625, W.Nr. 2.4856) gains its strength from solid-solution alloying, whereas Alloy 718 (UNS N07718, W.Nr. 2.4668) is strengthened by controlled precipitation hardening. That single metallurgical difference drives almost every selection decision: corrosion-dominated duty points to Alloy 625, while hot and heavily loaded duty points to Alloy 718.
Alloy 625 Tubing: Corrosion Resistance and Simplicity
Alloy 625 draws its properties from a nickel matrix carrying roughly 20–23% chromium, 8–10% molybdenum and 3.15–4.15% niobium plus tantalum. Molybdenum and niobium jointly stabilise a passive surface film, so the tube resists pitting, crevice attack and chloride-induced stress corrosion cracking even in seawater and acidic process streams. Because no aging treatment is required, the annealed tube keeps useful ductility and toughness while remaining strong in both cryogenic and elevated-temperature service.
Typical tensile strength: 620–930 MPa, dependent on product form and condition
Typical 0.2% yield strength: 275–620 MPa
Elongation in 50 mm: 30–40%
Useful service range: cryogenic up to about 982 °C for short-term exposure
Common tubing specifications: ASTM B444 for seamless tube, plus ASME SB 444
Alloy 718 Tubing: High Strength at Temperature
Alloy 718 trades part of the molybdenum for iron, titanium, aluminium and a higher niobium level. That chemistry supports gamma prime and gamma double prime precipitates during a two-step aging cycle, lifting static strength and creep-rupture resistance well above the annealed condition and holding them to about 650 °C. The price of that strength is a heat-treatment-sensitive route and reduced corrosion toughness in strongly reducing or heavily chlorinated environments.
Aged tensile strength: approximately 1,240–1,550 MPa
Aged 0.2% yield strength: approximately 760–1,310 MPa
Elongation: about 12–30%, varying between aged and annealed conditions
Continuous service to about 650 °C, with oxidation resistance to roughly 982 °C
Related specifications: ASTM B670 for strip and plate, AMS 5662 and AMS 5663 for heat-treatment control
Formability, Welding and Fabrication
The high nickel content gives both grades excellent formability, and both can be cold formed, stamped and rolled into wire profiles for aerospace and oil and gas hardware. Alloy 718 is particularly suited to forming while holding high strength, because its chemistry resists strain-age cracking during heat treatment. Neither grade is commonly deep drawn. Typical formed parts include bellows, springs, seal rings, honeycomb seals, sensor diaphragms and high-performance fasteners.
| Fabrication topic | Alloy 625 tubing | Alloy 718 tubing |
|---|---|---|
| Welding response | Highly weldable with matching filler; aging seldom required | Weldable, but post-weld heat treatment and aging are usually required |
| Cracking risk | Little sensitivity to post-weld embrittlement | Strain-age cracking possible without qualified procedures |
| Machining | Difficult, but generally easier than Alloy 718 | More difficult; work hardens faster and shortens tool life |
| Cold forming | Good cold formability | Good in the annealed condition, less ductile when aged |
Where Each Tubing Grade Is Used
Aerospace was the first major user of Alloy 718 tubing, and the alloy still makes up a substantial share of finished component mass in modern engine hardware, where it must survive extreme pressure, corrosive exhaust gas and thrust loading. Similar high-stress duty appears in gas turbine and automotive engines. Alloy 625 tubing serves marine and offshore equipment, submarine tie-back lines, heat exchangers, header bars, flexible metal hoses, propeller blades, wire rope and telecommunications cable in salt-laden conditions, as well as chemical process piping and pollution control hardware.
Frequently Asked Questions
Q: Which tubing grade should I choose for seawater service?
Choose Alloy 625 tubing. Its molybdenum and niobium content stabilises the passive film and resists pitting, crevice corrosion and chloride stress corrosion cracking without any aging treatment.
Q: Can Alloy 718 tubing be used above 650 °C?
Continuous service is normally limited to about 650 °C, although short-term oxidation resistance extends to roughly 982 °C. Above the continuous limit, creep behaviour and long-term phase stability must be reviewed for the specific section size.
Q: Does Alloy 625 tubing need heat treatment after welding?
Generally it does not. The alloy is not strengthened by precipitation, so matching filler metal produces sound joints without a post-weld aging cycle. Stress relief may still be specified for heavy sections.
Q: Why does Alloy 718 reach higher strength than Alloy 625?
Alloy 718 is precipitation hardened. Titanium, aluminium and niobium form gamma prime and gamma double prime precipitates during aging, and these precipitates block dislocation movement far more effectively than solid-solution alloying alone.
Q: Are the two grades interchangeable?
No. Substituting Alloy 718 for Alloy 625 in a chloride-rich line reduces corrosion margin, and substituting Alloy 625 for Alloy 718 in a high-load hot section reduces strength and creep resistance. Each grade serves a distinct duty.
Q: What tubing sizes are commonly available?
Seamless tube is typically offered from about 6 mm to 355 mm outside diameter in schedules 5S to XXS, while welded tube is supplied from 6 mm to 300 mm outside diameter in wall thicknesses from 0.5 mm to 10 mm.
