Alloy 625 and Alloy 718 are two of the most widely used nickel superalloys, but they reach their properties in different ways. One gains strength from molybdenum and niobium in solid solution plus cold work; the other is hardened by precipitation.
Composition and Strengthening
UNS N06625 contains a minimum of 58 percent nickel, 20 to 23 percent chromium, 8 to 10 percent molybdenum and 3.15 to 4.15 percent niobium plus tantalum. UNS N07718 contains about 50 to 55 percent nickel, 17 to 21 percent chromium, 4.75 to 5.5 percent niobium plus tantalum and 2.8 to 3.3 percent molybdenum.
625 is primarily solid-solution strengthened and can be further strengthened by cold work, while 718 is precipitation hardened by a controlled solution treatment and aging cycle that forms gamma double prime and gamma prime phases.
Strength and Temperature Capability
UNS N07718 reaches very high tensile and yield strength after aging, with excellent fatigue and creep resistance up to about 700 degrees C. UNS N06625 is strong in the annealed condition and can be cold worked to higher strength, but its strength at high temperature is lower than aged 718.
Where maximum strength and fatigue life at temperature are required, 718 is normally preferred; where corrosion resistance and weldability matter more, 625 wins.
Corrosion Resistance
UNS N06625 offers exceptional resistance to pitting, crevice corrosion and chloride stress corrosion cracking, and performs well in seawater, mixed acids and oxidising and reducing conditions. UNS N07718 offers good general corrosion resistance and performs well in many oil and gas and marine environments, but it is less resistant than 625 in severe chloride and acid service.
In sour gas service, choice depends on the specific environmental limits rather than on the alloy alone.
Weldability and Fabrication
UNS N06625 welds very well and is widely used as weld overlay, cladding and filler material because it keeps corrosion resistance in the as-welded condition. UNS N07718 can be welded, but the procedure is more demanding and post-weld heat treatment is often required to restore properties.
Both alloys need clean handling to avoid contamination, but 625 is generally the more forgiving of the two during fabrication.
Selection Guidance
Choose UNS N06625 where corrosion resistance, seawater and chloride exposure, or weld overlay duty dominate. Choose UNS N07718 where very high strength, fatigue resistance and high-temperature performance up to about 700 degrees C are the primary requirements.
Both are covered by their own product-form standards, so the alloy and the applicable standard should always be stated on the purchase order.
Frequently Asked Questions
Q: Which alloy is stronger, 625 or 718?
UNS N07718 is stronger after aging and maintains high strength at temperature up to about 700 degrees C.
Q: Which alloy resists chlorides better?
UNS N06625, thanks to its higher molybdenum content, resists pitting, crevice corrosion and chloride stress corrosion cracking better than 718.
Q: Which alloy is easier to weld?
UNS N06625 welds more readily and retains corrosion resistance in the as-welded condition, while 718 welding requires careful procedure control.
Q: Why is 718 precipitation hardened?
Niobium, titanium and aluminium form gamma double prime and gamma prime precipitates during aging, giving very high strength.
Q: Can 625 be used as weld overlay?
Yes, 625 is widely used as overlay and cladding material because it maintains corrosion resistance after welding.
