Which is better: Inconel 625 or 718?
Aged Inconel 718 is usually the stronger starting point for highly loaded fasteners. Inconel 625 is often the first alloy to evaluate when seawater or chemical corrosion drives the choice. For hot service, compare the bolt temperature, load and required life as well as the alloy’s final condition. [1] [2]
| Decision | Starting point | Check before specifying |
|---|---|---|
| High load in a fixed bolt size | Appropriately aged 718 | Required strength and ductility at the bolt temperature, in the delivered condition. |
| Seawater or aggressive process liquid | 625 for corrosion review | Actual fluid, temperature and wet crevices beneath nuts and washers. |
| Sustained hot service | Compare both against the joint duty | Creep, rupture life and retained clamp load over the required time; an oxidation limit is insufficient. |
| Replacement of a specified fastener | The approved alloy and condition | Design-owner acceptance of any substitution, including mating parts and assembly requirements. |
Replacing an existing bolt? Keep its approved specification until the material change has been accepted. Matching the thread and dimensions does not establish equivalence.
What is the difference between Inconel 625 and 718?
718 develops much of its strength through fine gamma-prime and gamma-double-prime precipitates formed during controlled aging. These impede deformation within the metal. 625 relies principally on molybdenum and niobium in solid solution. The alloys have different compositions and heat-treatment responses, so aging 625 does not turn it into 718. [1] [2]
The material pages identify 625 / UNS N06625 and 718 / UNS N07718. An order also needs the required delivery condition.
How much does condition change strength?
An annealed 718 offer cannot stand in for an aged 718 requirement. The producer’s examples below show how much the supplied condition matters. Tensile strength is the maximum stress reached in a tensile test; 0.2% offset yield strength indicates a defined amount of permanent deformation.
| Condition | Tensile strength | 0.2% offset yield strength |
|---|---|---|
| Annealed | 827–1034 MPa | 414–655 MPa |
| Solution-treated | 724–896 MPa | 290–414 MPa |
| Treatment | Tensile strength | 0.2% offset yield strength |
|---|---|---|
| 1750°F / 1 h anneal | 117.5 ksi ≈ 810 MPa | 55.0 ksi ≈ 379 MPa |
| 1750°F / 1 h anneal + aging | 192.0 ksi ≈ 1324 MPa | 165.0 ksi ≈ 1138 MPa |
How to use these tables: the 625 figures are nominal ranges; the 718 figures are selected examples. They are separate reference data, not a matched-alloy experiment or finished-bolt acceptance limits. The 718 table represents five heats and does not identify the selected entries as a same-heat pair.
The published aging cycle for these 718 entries is 1325°F / 8 h, followed by furnace cooling to 1150°F, with a total aging time of 18 h. For a fatigue requirement, also consult the 718 fatigue-evidence guide: a material tensile result cannot describe the fatigue performance of a threaded joint.
What differs in the composition?
INCONEL is a registered trademark of the Special Metals group. The producer’s selected composition limits below help explain the difference between the two alloys. [1] [2]
| Element | 625 / N06625 | 718 / N07718 |
|---|---|---|
| Chromium | 20.0–23.0% | 17.0–21.0% |
| Molybdenum | 8.0–10.0% | 2.80–3.30% |
| Niobium plus tantalum | 3.15–4.15% | 4.75–5.50% |
Which is better for corrosion resistance?
625 is often the starting point for seawater and aggressive chemical exposure. Its producer bulletin documents resistance to chloride pitting and crevice corrosion; its relatively high molybdenum content helps explain that role. Higher mechanical strength in 718 does not establish equivalent resistance in the same fluid. [1]
For fasteners, examine liquid trapped beneath nuts and washers as well as the exposed shank. Fluid chemistry, temperature, deposits and shutdown conditions matter. The seawater guide develops the differences between immersion, splash zones and wet crevices.
Which is better at high temperature?
A loaded bolt needs adequate hot strength and retained clamp load. Oxidation resistance alone cannot answer that question. A lightly loaded hot component may have different priorities, so a broad alloy service-temperature range is insufficient to select a fastener. [1] [2]
A joint can remain unbroken while losing enough clamp load to leak or separate. Assess time at bolt temperature and the thermal expansion of the clamped members; see creep and preload retention for the joint-level explanation. [3]
Was strength measured hot or after cooling?
The 625 bulletin gives a useful example. For hot-rolled annealed bar exposed at 760°C for 2000 hours, Table 6 reports tensile strength of 1123.8 MPa at room temperature, versus 584.7 MPa in the exposure-temperature test column. The larger figure describes the room-temperature test after exposure, not strength while operating at 760°C. Neither is a bolt design allowable. [1]
Can 625 be age-hardened?
Yes. Although 625 relies principally on solid-solution strengthening, precipitation during some aging treatments can increase strength. In a 2024 study of specimens from a 60 mm forged rod, stronger or harder aged conditions did not always retain the same resistance to intergranular corrosion. The authors related the tradeoff to precipitation, including chromium-rich grain-boundary carbides and local depletion of corrosion-resisting elements. [4]
The study used ASTM G28 Method A in ferric sulfate–sulfuric acid, not seawater or a bolted joint. Its purchasing lesson is to specify both mechanical and corrosion acceptance requirements when reviewing a proposal to strengthen 625.
Which is easier to machine or weld?
Both alloys can be demanding to machine. Poor heat conduction concentrates cutting heat near the tool; high strength at cutting temperatures and work hardening raise the forces needed to remove material. Tool wear follows from those mechanisms. [5]
For 718, condition changes the tradeoff. Special Metals reports easier machining and longer tool life in the annealed condition, while aged material can give a slightly better finish and better chip-breaker action. Machining before aging therefore changes the cutting task; final dimensions and properties still need verification after processing. [2, p. 26]
Long threaded lengths, deep holes, thin sections and tight tolerances can change the manufacturing work. Compare quotations that include the route from supplied stock to finished condition, including treatment and inspection.
625 is well established for welding; 718 is also weldable. The 718 producer describes resistance to postweld cracking and a slow aging response that permits welding before hardening. A welded stud or repair needs a qualified procedure covering the joint, filler and subsequent treatment. [1] [2]
Can 718 replace a specified 625 stud?
Consider a hypothetical pump cover specified with 625 studs. A supplier offers 718 sooner and cites higher room-temperature strength. That leaves the controlling question unanswered: will the substitute resist the actual pump fluid, including wet crevices beneath nuts and washers?
Keep the specified alloy until the design owner accepts the change. Supply the fluid composition, metal temperature, operating and shutdown conditions, and mating materials for that review.
Can 625 replace an aged 718 bolt?
In a second hypothetical example, a compact joint relies on a particular aged 718 fastener’s properties. A same-size 625 bolt may fit while falling short of the mechanical requirement. Substitution may require a different section, preload or joint design.
Compare the proposed finished-part requirements with the design’s required properties at temperature. A typical bar value cannot replace a finished-fastener minimum. [1] [2]
Which costs less as a finished fastener?
There is no reliable fixed 625-to-718 price ratio for finished fasteners. Raw-stock cost, machining, heat treatment and testing all contribute. Compare the same drawing, quantity, final condition, mating parts and document scope.
For example, a quote for annealed feedstock and a quote for finished, heat-treated bolts with lot testing include different work. Ask suppliers to identify omissions and deviations before comparing the complete delivered price.
What should I send for a quotation?
- Have a drawing or specification? Send it with the quantity and any required tests or documents.
- Know the part but have no drawing? Share the fastener type, size, quantity and specified material.
- Still choosing the alloy? Add the service medium, bolt temperature, load and required life; the nickel-alloy selection guide helps organize that review.
Once approved, carry the alloy, UNS, final condition and finished-part acceptance requirements into the 625 purchase specification or 718 purchase specification.
For 625 hex bolts, 718 hex bolts and stud-and-nut sets, include mating-component requirements. Changing the nut or lubricant may require a new assembly review even when the bolt alloy stays the same.