Inconel 625 vs Inconel 718: Which Nickel Alloy Is Better for High-Temperature Applications?
Quick Answer
Inconel 625 is the corrosion alloy. Inconel 718 is the strength alloy. 625 is solid-solution strengthened, welds readily and holds up in seawater, acids and chloride environments, but its strength above roughly 650°C is modest. 718 is age-hardened and roughly twice as strong at room temperature, but it starts losing that strength above about 650°C and it has less chromium and molybdenum to fight corrosion with. Pick on whichever requirement is actually controlling your design.
Inconel 625 vs 718 in one minute
Both are nickel-chromium alloys. Both get called “Inconel,” which is a Special Metals trade name rather than a grade designation. Both cost far more than stainless. Beyond that they are genuinely different materials designed for different problems, and the mistake we see most often is treating them as interchangeable premium options. If your part is highly loaded (a bolt, a shaft, a turbine disc, a fastener, a spring) and the temperature sits below roughly 650°C, 718 is usually the answer. If your part sits in an aggressive fluid, gets welded into a fabrication, or runs hotter than 700°C without heavy mechanical load, 625 is usually the answer. Neither alloy is “better.” They fail in different directions.What are Inconel 625 and Inconel 718?
Inconel 625 (UNS N06625, W.Nr. 2.4856) is a nickel-chromium-molybdenum alloy with niobium. It’s strengthened by solid solution. The molybdenum and niobium atoms sit in the nickel matrix and make dislocation movement harder. No heat treatment is required to develop its properties. It arrives strong, it stays strong, and there is nothing to over-age. Inconel 718 (UNS N07718, W.Nr. 2.4668) is a nickel-chromium alloy with significant iron, plus niobium, titanium and aluminium. It is precipitation-hardened. Its strength comes from fine particles of gamma double prime (γ″, Ni₃Nb) that form during a controlled ageing treatment, with a secondary contribution from gamma prime (γ′, Ni₃(Al,Ti)). Without that heat treatment, 718 is a fairly ordinary material. That single difference, solid solution versus precipitation hardening, drives almost everything else in this comparison. It explains the strength gap, the temperature ceiling, the welding behaviour and the price difference. Both belong to the wider Inconel alloy family, alongside grades like 600, 601, 800 and 825, each aimed at a different combination of temperature, corrosion and strength.Inconel 625 vs 718 chemical composition
Nominal limits below reflect the widely referenced composition ranges for these alloys. Confirm against the current Special Metals technical bulletin or the governing material specification (ASTM B443/B444/B446 for 625; ASTM B637/B670 and the relevant AMS specifications for 718) before you release anything to manufacture.|
Element |
Inconel 625 (N06625) |
Inconel 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 (+Ta) | 3.15 – 4.15 | 4.75 – 5.50 |
| Iron | 5.0 max | Balance (typically 18 – 19) |
| Titanium | 0.40 max | 0.65 – 1.15 |
| Aluminium | 0.40 max | 0.20 – 0.80 |
| Cobalt | 1.0 max | 1.0 max |
| Carbon | 0.10 max | 0.08 max |
| Manganese | 0.50 max | 0.35 max |
| Silicon | 0.50 max | 0.35 max |
| Boron | — | 0.006 max |
| Copper | — | 0.30 max |
Mechanical strength: 625 vs 718
Any strength number for these alloys is meaningless without the material condition attached. Solution-annealed 718 and aged 718 are effectively two different materials sharing one UNS number. Typical room-temperature values, product form and condition stated:| Property | 625, annealed (bar/pipe) | 625, solution annealed | 718, solution annealed | 718, solution treated + aged |
|---|---|---|---|---|
| 0.2% yield strength | ~415 – 550 MPa | ~275 – 415 MPa | ~450 – 600 MPa | ~1030 – 1180 MPa |
| Tensile strength | ~830 – 1000 MPa | ~690 – 830 MPa | ~830 – 965 MPa | ~1240 – 1400 MPa |
| Elongation | ~40 – 55% | ~50 – 60% | ~40 – 50% | ~12 – 20% |
| Density | 8.44 g/cm³ | 8.44 g/cm³ | 8.19 g/cm³ | 8.19 g/cm³ |
High-temperature performance
“Which one handles higher temperature” is the wrong question, because high-temperature capability splits into three separate properties that behave differently.Oxidation and scaling resistance
This is about the alloy surviving hot gas without wasting away. Chromium content dominates, with aluminium and silicon contributing. 625, with 20–23% Cr, forms a stable protective oxide and resists scaling to roughly 980°C in air. 718, with 17–21% Cr, is respectable but has less margin. On oxidation resistance alone, 625 wins.Retained mechanical strength under load
This is where the ranking inverts, then inverts again. Up to about 650°C, aged 718 retains a very large share of its room-temperature strength and comfortably out-performs 625. It is the reason 718 dominates gas turbine discs, casings, shafts and fasteners. Above roughly 650°C, the γ″ precipitates that provide that strength begin to coarsen and eventually transform to the stable delta phase. The alloy over-ages in service. Strength falls away, and it doesn’t come back. Practical continuous-service limits for aged 718 are usually placed around 650°C, sometimes 700°C for short excursions. 625 has no such cliff, because it has no precipitates to lose. Its strength declines gradually and predictably with temperature. Above about 700°C, 625 in the solution-annealed condition is the more sensible structural choice of the two.Creep and stress rupture
Long-term behaviour under sustained load. Below 650°C, 718 again leads by a wide margin. Above that crossover, solution-annealed 625 takes over. The practical summary: below roughly 650°C, 718. Above roughly 700°C, 625. In the band between, it depends on the stress level, the duration and whether the environment is also corrosive.When Inconel 718 has the advantage
- Rotating and highly stressed components: discs, shafts, blades, spacers
- Fasteners, studs, bolts and springs where preload must be maintained hot
- Aerospace and gas turbine structural parts within its temperature range
- Downhole and high-pressure oil and gas components requiring high yield strength
- Anywhere the section size is constrained and the alloy has to carry the load
When Inconel 625 has the advantage
- Seawater, brine and high-chloride process service
- Chemical processing: acids, mixed oxidising and reducing conditions
- Flue gas desulphurisation, scrubbers, ducting and stack liners
- Welded fabrications and cladding, including weld overlay onto carbon steel
- Bellows, expansion joints and thin sections needing formability
- Service above about 700°C where load is moderate
Corrosion resistance comparison
This is the least contested part of the comparison. 625 is significantly the more corrosion-resistant alloy, and the composition table explains why. Chloride pitting and crevice corrosion. 625’s 8–10% molybdenum gives it a very high pitting resistance, which is why it’s a standard choice for seawater handling, offshore equipment and heat exchanger service in brackish water. 718’s ~3% molybdenum places it well below that. Chloride stress corrosion cracking. High-nickel alloys are broadly resistant to chloride SCC, the failure mode that plagues austenitic stainless like 304 and 316. Both alloys perform well here. 625 has the wider margin. Acids. 625 handles a broad span of both oxidising and reducing acids, which is unusual and is largely a chromium-plus-molybdenum effect. 718 isn’t normally specified for acid service. Sour service. 718 appears in NACE/ISO 15156-compliant applications, but only in tightly specified conditions and heat treatment envelopes, with hardness limits. This is specification-controlled territory and not something to assume. A caution worth stating plainly: corrosion data is environment-specific. Concentration, temperature, aeration, flow velocity, crevice geometry and contaminants all shift the result. Published general rankings are a starting point for material screening, not a substitute for testing or for the corrosion engineer’s sign-off.Weldability, forming and heat treatment
The received wisdom is “625 welds, 718 doesn’t.” That’s too crude, and getting it wrong in either direction causes problems. Inconel 625 is genuinely straightforward. It welds by GTAW, GMAW, SMAW and submerged arc using matching or ERNiCrMo-3 filler. It needs no pre-heat and no post-weld heat treatment for most service. It is widely used as a weld overlay consumable on carbon steel, precisely because it deposits so reliably. It also forms well, which is why bellows and thin-wall fabrications favour it. Inconel 718 is, in fact, one of the more weldable precipitation-hardened superalloys. Its γ″ strengthening reaction is sluggish, so it resists the strain-age cracking that makes alloys like Waspaloy and René 41 so difficult. That is a real advantage and it’s part of why 718 became so widely adopted. The complication is different. Welding disturbs the heat treatment. A weld puts a heat-affected zone through an uncontrolled thermal cycle, which locally over-ages or dissolves the precipitates. Recovering full properties normally requires a full solution treatment and re-age of the assembly after welding. That is a furnace operation with size limits, distortion risk and cost. Niobium segregation and Laves phase in the fusion zone add further control requirements. So the honest framing isn’t “718 can’t be welded.” It’s that welding 718 is a controlled metallurgical process with a heat treatment tail, while welding 625 is largely a fabrication activity. Machining. Both work-harden aggressively and demand rigid setups, sharp tooling, positive rake, generous coolant and low surface speeds. Aged 718 is the harder of the two to cut. Where possible, rough machine 718 in the solution-annealed condition, age, then finish.Inconel 625 vs 718 for pipes, tubes, bars and plates
Product form availability differs between the two alloys, and it’s worth knowing before you specify. 625 is available across essentially the full range: seamless and welded Inconel 625 pipe and tube (ASTM B444, B704, B705), Inconel 625 round bars (B446), and Inconel 625 plates and sheets (B443), plus fittings, flanges, wire and welding consumables. It is the more commonly stocked of the two in pipe and plate form. 718 is dominated by bar and forging stock, because that’s what its applications need. Inconel 718 round bar (B637 and the AMS specifications) is the mainstream form, with Inconel 718 plates and sheets (B670) also available. Inconel 718 pipe and tube exists but is a more specialised item with longer lead times and higher minimums. If your requirement is genuinely a pipeline or a pressure-containing line, 625 is far more likely to be the practical choice — and you can browse the broader range of Inconel pipes and tubes or the wider nickel alloy pipes and tubes category for adjacent grades.Cost and availability
We don’t publish fixed prices for these alloys, and you should be sceptical of anyone who does. Nickel and molybdenum are exchange-traded and volatile, and quotations move accordingly. Here’s what actually drives the number:- Alloy surcharge. Nickel, chromium, molybdenum and niobium content, priced against current metal markets. 625’s high molybdenum loading makes it sensitive to moly pricing.
- Heat treatment. 718 requires solution treatment and ageing, with furnace time, atmosphere control and property verification. That cost is built in before the material ships.
- Product form. Seamless pipe costs more than welded. Small-diameter thin-wall tube costs more per kilogram than bar.
- Size and quantity. Non-standard sizes may require a mill run with a minimum tonnage.
- Certification. EN 10204 3.1 versus 3.2, NACE compliance, third-party inspection, PMI, ultrasonic testing — each adds cost and lead time.
- Availability. 625 in common pipe and plate sizes is often ex-stock. 718 in pipe form frequently isn’t.
Which alloy should you choose?
| Governing requirement | Choose | Why |
|---|---|---|
| Seawater or high-chloride service | 625 | Molybdenum content gives far higher pitting and crevice resistance |
| Acid or mixed chemical process | 625 | Broad resistance across oxidising and reducing conditions |
| Maximum strength below 650°C | 718 | Age-hardened yield roughly 2.5× annealed 625 |
| Structural service above 700°C | 625 | 718 over-ages; 625 has no precipitates to lose |
| Welded fabrication, no PWHT possible | 625 | No heat treatment tail after welding |
| Weld overlay or cladding | 625 | Established, widely used overlay consumable |
| Bolting, fasteners, springs hot | 718 | Retains preload; high yield strength |
| Thin sections, bellows, forming | 625 | Superior formability in the annealed condition |
| Rotating machinery components | 718 | Strength-to-weight and fatigue performance |
| Pipe and tube availability | 625 | Far more widely produced and stocked in pipe form |
Frequently asked questions
718 in the solution-treated and aged condition, by a wide margin. Roughly 1030–1180 MPa yield against roughly 415–550 MPa for annealed 625 at room temperature. State the condition whenever you quote these figures, because solution-annealed 718 without ageing is much closer to 625
625, clearly. It carries about three times the molybdenum and meaningfully more chromium, with far less iron. For chloride, seawater and acid environments it’s the correct choice of the two.
It depends on which high-temperature property is controlling. For oxidation resistance in hot gas, 625. For strength under load below about 650°C, 718. For strength under load above about 700°C, 625, because 718 over-ages and loses its strengthening phase permanently.
For most fabrication purposes, yes. 625 welds without pre-heat or post-weld heat treatment. 718 is actually well-behaved during welding compared with other age-hardened superalloys, but restoring full properties afterwards normally requires a complete solution treatment and re-age cycle, which is an added operation with real constraints.
Not reliably in either direction, and it varies with market conditions and product form. 718 carries heat treatment cost; 625 carries higher molybdenum content. The bigger cost drivers in practice are product form, size, quantity and certification requirements. Ask for a current quotation rather than working from a rule of thumb.
Specifying the right nickel alloy
If you’re between these two, the fastest way to resolve it’s to identify which single requirement is controlling — corrosion, strength, temperature or fabrication — and let that decide. Trying to optimise all four at once is usually how projects end up over-specified and over-budget.
We supply both grades across pipe, tube, bar, plate and sheet, in the standard material conditions and with certification to project requirements. If you have a datasheet, a drawing or a service condition and want it checked against the right grade and condition before purchase, send it over.
Contact us with grade, product form, size, quantity, required condition, destination and certification, and we will come back with availability and a quotation.
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