Alloy 625 and alloy 718 share the same nickel-based family and are frequently quoted against each other for aerospace, chemical and oil and gas work, yet they are strengthened by completely different mechanisms. That difference, more than any single element, drives where each alloy can be used.
The Key Difference in One Paragraph
Alloy 625 (UNS N06625) is a solid-solution strengthened nickel-chromium-molybdenum-niobium alloy. Its strength comes from molybdenum and niobium held in solution, so it is not hardened by a precipitation treatment. Alloy 718 (UNS N07718) is a precipitation-hardening nickel-iron-chromium alloy: titanium, aluminium and niobium form gamma-prime and gamma-double-prime phases during aging, which is why 718 develops far higher yield strength and is the workhorse for high-pressure rotating hardware.
Chemistry Compared
| Element | Alloy 625 (N06625) | Alloy 718 (N07718) |
|---|---|---|
| Nickel | 58.0 min | 50.0-55.0 |
| Chromium | 20.0-23.0 | 17.0-21.0 |
| Molybdenum | 8.0-10.0 | 2.80-3.30 |
| Niobium + tantalum | 3.15-4.15 | 4.75-5.50 |
| Titanium | 0.40 max | 0.65-1.15 |
| Aluminium | 0.40 max | 0.20-0.80 |
| Iron | 5.0 max | Balance |
Two numbers matter most to a buyer: 625 carries roughly three times the molybdenum of 718, which is why it wins in chloride and reducing-acid service, and 718 carries deliberate titanium and aluminium additions, which is why it can be aged to very high strength.
Mechanical Properties and Hardening
Alloy 625 Grade 1 in the solution-annealed condition is normally specified with a minimum tensile strength of 827 MPa and a minimum 0.2 percent yield strength of 414 MPa. Alloy 718 in the precipitation-hardened condition is specified to a minimum tensile strength of 1241 MPa (180 ksi) and a minimum yield strength of 1034 MPa (150 ksi). The yield strength ratio is therefore close to two and a half to one in favour of 718. The trade-off is that 718 must be solution treated and aged under controlled conditions, and its properties depend on heat-treat lot control, whereas 625 is used as delivered.
Corrosion Resistance
Because of its molybdenum content, 625 provides better resistance to pitting, crevice corrosion and reducing acids, and it is the common choice for seawater-exposed components and for mixed acid handling. Alloy 718 resists oxidising atmospheres and many industrial chemicals well, and its chromium level of 17.0-21.0 percent supports oxidation resistance, but in heavily chlorinated or strongly reducing environments it is not a substitute for 625. Neither alloy is intended for the most aggressive oxidising chloride media, where higher-molybdenum grades are used.
Temperature Capability
Alloy 625 keeps useful strength and oxidation resistance to about 980 °C in continuous service and is often selected for ducting, bellows and combustion components where thermal cycling is severe. Alloy 718 is normally exploited up to about 650 °C, where its age-hardened structure remains stable; above that range the strengthening phases coarsen and long-term creep performance falls away. For high-temperature low-stress components, 625 is often the more economical answer; for high-stress components below 650 °C, 718 dominates.
Welding, Machining and Supply
Both alloys are weldable. Alloy 625 is the more forgiving because it does not depend on a precipitation reaction, so it can be welded and put into service without a post-weld aging treatment. Alloy 718 is normally welded in the solution-treated condition and then aged, and it is more prone to work hardening during machining, requiring low cutting speeds, rigid tooling and generous coolant flow.
625: ASTM B444 for seamless pipe and tube, ASTM B705 for welded pipe, ASTM B443 for plate, sheet and strip, ASTM B446 for bar and rod.
718: ASTM B637 for bars, forgings and forging stock, and ASTM B670 for plate, sheet and strip.
Selection Checklist
Ask four questions before choosing. Is chloride or reducing-acid corrosion the limiting factor, in which case 625 leads. Is very high yield strength under a bolted or rotating load the limiting factor, in which case 718 leads. Is the service temperature above 650 °C, in which case 718 loses its advantage. Does the shop have qualified heat treatment and can the finished part be aged, because if not, 625 is the practical choice.
Frequently Asked Questions
Q: Is alloy 718 stronger than alloy 625?
A: Yes. Precipitation-hardened 718 is specified to a minimum yield strength of 1034 MPa against 414 MPa for solution-annealed 625, and to a minimum tensile strength of 1241 MPa against 827 MPa. The gap is a direct result of the gamma-prime and gamma-double-prime precipitation reaction in 718.
Q: Which alloy performs better in seawater?
A: Alloy 625. Its 8.0-10.0 percent molybdenum and 3.15-4.15 percent niobium give it much better resistance to pitting and crevice corrosion in chloride-bearing water, so it is the standard selection for marine and offshore hardware.
Q: Does alloy 625 need a heat treatment after welding?
A: No precipitation treatment is required. Solution-annealed 625 can be welded and placed in service, which is one of the main reasons it is chosen for large fabricated assemblies where post-weld heat treatment is impractical.
Q: Can alloy 718 be used above 650 °C?
A: It is normally not selected for continuous service above about 650 °C because the strengthening phases coarsen and creep resistance declines. For hotter duties a solid-solution alloy such as 625, or a higher-temperature precipitation-hardening alloy, is preferred.
Q: How are the two alloys ordered to a specification?
A: Quote UNS N06625 with ASTM B444 or B705 for 625, and UNS N07718 with ASTM B637 or B670 for 718. State the condition, such as solution annealed or solution treated and aged, plus any required elevated-temperature or stress-rupture testing.
