Chemistry and Standard Designations
Both grades belong to the nickel-chromium family, yet their compositions are set for different strengthening routes. Alloy 625 is identified by UNS N06625 and W.Nr. 2.4856, with wrought product specifications that include ASTM B443 for plate, sheet and strip, ASTM B444 for seamless pipe and tube, and ASTM B446 for rod and bar. Alloy 718 is identified by UNS N07718 and W.Nr. 2.4668, and is commonly supplied to AMS 5662 and AMS 5663 for heat-treated bar and forgings, ASTM B637 for bar and forging stock, and ASTM B670 for plate, sheet and strip.
| Element, wt% | Alloy 625 (UNS N06625) | Alloy 718 (UNS N07718) |
|---|---|---|
| Nickel (Ni) | 58.0 min | 50.0–55.0 |
| Chromium (Cr) | 20.0–23.0 | 17.0–21.0 |
| Iron (Fe) | 5.0 max | Balance |
| Molybdenum (Mo) | 8.0–10.0 | 2.8–3.3 |
| Niobium + Tantalum (Nb+Ta) | 3.15–4.15 | 4.75–5.50 |
| Aluminium (Al) | 0.40 max | 0.20–0.80 |
| Titanium (Ti) | 0.40 max | 0.65–1.15 |
| Cobalt (Co) | 1.0 max | 1.0 max |
| Carbon (C) | 0.10 max | 0.08 max |
| Manganese (Mn) | 0.50 max | 0.35 max |
| Silicon (Si) | 0.50 max | 0.35 max |
| Sulfur (S) | 0.015 max | 0.015 max |
| Phosphorus (P) | 0.015 max | 0.015 max |
These are standard specification limits for the two grades, shown for orientation. Residual elements and minor additions vary between heats and product forms, so the certified analysis of the actual lot governs any design that works to tight limits.
Strengthening Mechanisms and Microstructure
Alloy 625 keeps a face-centred cubic nickel matrix and gains strength from molybdenum and niobium held in solid solution. It does not depend on a substantial precipitation reaction for its normal strength, which simplifies fabrication and welding and produces a good balance of toughness and corrosion resistance at moderately high temperature. Alloy 718 is designed as an age-hardenable nickel-chromium alloy. Controlled solution treatment followed by a two-step age produces a fine dispersion of gamma prime and gamma double prime precipitates that raise yield and tensile strength sharply compared with the annealed condition. That mechanism gives Alloy 718 high creep and fatigue resistance in the 400–650 °C range, but it also introduces sensitivity to heat-treatment parameters and to post-weld aging cycles.
Mechanical and Physical Properties Compared
| Property | Alloy 625, solution annealed | Alloy 718, aged |
|---|---|---|
| Density | 8.44 g/cm³ | 8.19–8.25 g/cm³ |
| Melting range | 1,290–1,350 °C | 1,260–1,336 °C |
| Tensile strength | 620–930 MPa | 1,240–1,550 MPa |
| 0.2% yield strength | 275–620 MPa | 760–1,310 MPa |
| Elongation in 50 mm | 30–40% | 12–30% |
| Hardness | 70–95 HRB, about 170–260 HB | 250–360 HB, about 22–35 HRC |
| Creep capability | Good to about 350–370 °C | Strong to about 650 °C in suitable conditions |
| Useful service temperature | Cryogenic to about 982 °C peak short term | About -269 °C to 650 °C continuous |
Alloy 718 in the aged condition delivers substantially higher static strength and better creep behaviour, which makes it the usual choice for high-stress rotating parts, bolts and structural members in aerospace and power generation. Alloy 625 combines ductility with corrosion resistance and is preferred for corrosive piping, heat exchangers, chemical process equipment and marine hardware.
Machinability, Welding and Heat Treatment
Nickel-base superalloys are difficult to machine because of low thermal conductivity and rapid work hardening. Rigid setups and carbide or coated carbide tooling are essential for both grades, and Alloy 718 tightens the process window further because interrupted cuts can cause sudden tool failure.
| Aspect | Alloy 625 | Alloy 718 |
|---|---|---|
| Relative machinability | Medium; easier than Alloy 718 in most shops | More difficult; work hardens faster and shortens tool life |
| Tooling practice | Rigid setups, carbide or coated carbide inserts, high cutting forces | Similar tools with tighter process control; interrupted cuts risk tool failure |
| Welding | Readily welded with matching filler; little need for aging | Weldable; qualified procedures and post-weld aging usually required |
| Heat treatment | Supplied solution annealed; stabilization or stress relief for heavy sections | Controlled solution treatment plus two-step age to reach peak properties |
| Distortion control | Lower sensitivity to post-weld aging concerns | Strict control and post-weld heat treatment for critical parts |
Alloy 625 needs no complex aging to reach its baseline strength. Alloy 718 requires a controlled solution treatment and aging cycle, and for critical parts the schedule and cooling rates must be followed strictly. For sour service, order material certified to the applicable NACE and API requirements.
How to Choose Between Alloy 625 and Alloy 718
Choose Alloy 625 when the environment contains chlorides, seawater or acid, when weldability with simple post-weld treatment matters, and when moderate strength with higher ductility is acceptable.
Choose Alloy 718 when design loads demand maximum tensile and creep strength in the 300–650 °C range, when fatigue life under high tensile stress is critical, and when heat treatment to AMS or API controls can be carried out.
When corrosion resistance and high strength are both required, evaluate cladding, bimetallic construction, coatings or corrosion testing under representative conditions before committing to a single alloy.
Always capture heat-treatment condition, certification and inspection requirements in the purchase specification, because the same grade delivers very different properties in annealed and aged conditions.
Frequently Asked Questions
Q: Which alloy is stronger, Alloy 625 or Alloy 718?
Alloy 718 is considerably stronger in the aged condition, with tensile strength of roughly 1,240–1,550 MPa against 620–930 MPa for solution annealed Alloy 625.
Q: Which grade resists chlorides better?
Alloy 625. Its molybdenum level of 8–10% plus niobium produces a more stable passive film, giving it the edge against pitting, crevice corrosion and chloride stress corrosion cracking.
Q: Is Alloy 718 harder to machine than Alloy 625?
Yes. Alloy 718 work hardens faster and its aged condition is stronger, so tool life is shorter and cutting parameters must be held more tightly. Both grades need rigid setups and carbide tooling.
Q: Can Alloy 718 be welded without heat treatment?
For non-critical duty it can be welded, but post-weld heat treatment and aging are normally required to restore properties and to avoid strain-age cracking in the heat-affected zone.
Q: Which standards cover the two grades?
Alloy 625 is covered by UNS N06625 with ASTM B443, B444 and B446 for different product forms. Alloy 718 is covered by UNS N07718 with ASTM B637 and B670 and AMS 5662 / AMS 5663 for heat-treated products.
Q: Do the two alloys share the same density?
No. Alloy 625 is denser at about 8.44 g/cm³ against 8.19–8.25 g/cm³ for Alloy 718, which matters when component weight is controlled.
