Inconel 617 vs Inconel 625: High-Temperature Selection
Date: 2026年9月26日 Categories: News Views: 265
By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012
Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360
Quick Answer: Inconel 617 or Inconel 625 Above 800 °C?
Inconel 617 is the stronger choice above roughly 850 °C under sustained load or in oxidising and carburising furnace atmospheres, because its cobalt, aluminium and higher chromium content raises creep-rupture strength and oxidation resistance. Inconel 625 remains the better choice below that band, and wherever chlorides, mixed acids, seawater or severe thermal fatigue govern the design.
Key Takeaways
- 617 wins above roughly 850 °C under load. Inconel 617 is a Ni-Cr-Co-Mo alloy with 10-15 % cobalt and 0.8-1.5 % aluminium; those two additions are what lift its creep-rupture strength and oxidation resistance above the range where 625 begins to lose margin.
- 625 wins below that band and in aggressive chemistry. Inconel 625 is a Ni-Cr-Mo-Nb alloy whose niobium and molybdenum give it outstanding resistance to chlorides, pitting, crevice corrosion and a wide range of acids, which 617 does not match.
- They are not interchangeable, and the mill certificate will not tell you which you were sent. The two alloys are visually identical in bar, plate and tube form; nickel, chromium and molybdenum are broadly similar between them, so verification requires the cobalt and aluminium lines for 617 and the niobium-plus-tantalum line for 625.
- Welding practice differs. 617 is welded with a cobalt-bearing matching filler to preserve its high-temperature properties, while 625 is welded with ERNiCrMo-3, and substituting one filler for the other on a high-temperature joint is a common and expensive error.
- The price gap is real but secondary. 617 contains roughly 10-15 % cobalt and therefore costs appreciably more per kilogram than 625 in 2026, so 625 is the default economic choice whenever the duty temperature does not demand 617.
- Neither alloy is a substitute for a proper design allowable. Above 800 °C, selection must be based on the ASME Code allowables and stress-rupture data for the specific product form and the actual atmosphere, not on room-temperature tensile figures.
What Are Inconel 617 and Inconel 625?
Inconel 617 is a solid-solution strengthened nickel-chromium-cobalt-molybdenum alloy, designated UNS N06617 and W.Nr 2.4663, with approximately 44.5 % minimum nickel, 20.0-24.0 % chromium, 10.0-15.0 % cobalt, 8.0-10.0 % molybdenum and a deliberate aluminium addition of about 0.8-1.5 %. It was developed for very high temperature service where both creep strength and oxidation resistance are required at the same time, and it is supplied as bar, plate, sheet, seamless tube and pipe under the ASTM product standards and the corresponding AMS aerospace specifications. Our Inconel alloy supplier page lists the forms and certifications we hold for the high-temperature grades.
Inconel 625 is also a nickel-chromium-molybdenum alloy, designated UNS N06625 and W.Nr 2.4856, but its chemistry is organised differently: approximately 58 % minimum nickel, 20.0-23.0 % chromium, 8.0-10.0 % molybdenum and 3.15-4.15 % niobium plus tantalum, with essentially no cobalt and only residual aluminium. The niobium is the key element. It combines with molybdenum to strengthen the solid solution and, in the aged condition, to form the fine precipitates that give 625 its useful elevated-temperature strength, while the molybdenum and chromium together produce the outstanding corrosion resistance for which the alloy is best known. It is the standard material for seawater-exposed components, chemical process equipment handling chlorides, and high-temperature ducting and bellows where corrosion resistance and fatigue life matter more than maximum creep strength.
The two alloys are frequently treated as a generic "high-temperature nickel alloy" pair by specification writers, and the mistake is understandable because they look alike, machine alike and are both supplied under the same family of ASTM standards. They diverge once the service conditions are examined properly. Inconel 617 is the alloy you specify when the metal temperature is high enough that creep and oxidation become the design drivers. Inconel 625 is the alloy you specify when the environment is the design driver, and its temperature ceiling under load is lower than 617's. This article examines composition, mechanical properties, heat treatment, creep and oxidation behaviour, welding and selection in turn, and closes with explicit rules and a decision matrix.
It is worth stating the underlying metallurgy plainly at the start, because it explains every difference that follows. Both alloys are austenitic, face-centred-cubic, solid-solution strengthened materials that do not respond to the precipitation-hardening treatments used for Inconel 718 or Inconel X-750. Neither can be strengthened by a simple ageing cycle in the way that the gamma-prime and gamma-double-prime alloys are. Their strength at temperature therefore depends on the chemistry of the solid solution plus, for 617, the slow precipitation of M23C6-type carbides and fine gamma-prime at service temperature, which is why 617 continues to develop strength during long exposures. That mechanism also explains why 617 performs better in long-term stress-rupture tests than its short-term tensile data would suggest, and why a selection made from a datasheet tensile comparison alone is misleading.
Chemical Composition and the Logic Behind the Difference
The composition table below sets out the two chemistries side by side with the governing standard named for each. The single most important observation is that 617 contains cobalt and aluminium while 625 contains none, and that 625 contains niobium plus tantalum while 617 does not.
| Element (wt %) | Inconel 617 (N06617) | Inconel 625 (N06625) | Per standard | Function in the alloy |
|---|---|---|---|---|
| Nickel | balance (44.5 min) | 58.0 min | B166 / B168 (617); B446 / B443 (625) | matrix |
| Chromium | 20.0-24.0 | 20.0-23.0 | B166 / B168 (617); B446 / B443 (625) | oxidation and corrosion resistance |
| Cobalt | 10.0-15.0 | 1.0 max | B166 / B168 (617); B446 / B443 (625) | raises high-temperature strength in 617 |
| Molybdenum | 8.0-10.0 | 8.0-10.0 | B166 / B168 (617); B446 / B443 (625) | solid-solution strengthening, pitting resistance |
| Niobium + Tantalum | not specified | 3.15-4.15 | B166 / B168 (617); B446 / B443 (625) | strengthening and weldability in 625 |
| Aluminium | 0.8-1.5 | 0.40 max | B166 / B168 (617); B446 / B443 (625) | oxidation resistance and gamma-prime in 617 |
| Titanium | 0.6 max | 0.40 max | B166 / B168 (617); B446 / B443 (625) | residual control |
| Iron | 3.0 max | 5.0 max | B166 / B168 (617); B446 / B443 (625) | residual |
| Carbon | 0.05-0.15 | 0.10 max (0.02 max, Grade 1) | B166 / B168 (617); B446 / B443 (625) | carbide formation; controlled in 625 Grade 1 |
| Manganese | 0.5 max | 0.50 max | B166 / B168 (617); B446 / B443 (625) | deoxidation residual |
| Silicon | 0.5 max | 0.50 max | B166 / B168 (617); B446 / B443 (625) | deoxidation residual |
| Sulphur | 0.015 max | 0.015 max | B166 / B168 (617); B446 / B443 (625) | residual, controlled |
Table note: Values are the standard composition limits of the governing ASTM product standards, latest editions: ASTM B166 (rod, bar and wire) and ASTM B168 (plate, sheet and strip) for UNS N06617, and ASTM B446 (rod, bar and wire), ASTM B443 (plate, sheet and strip), ASTM B444 (seamless pipe and tube), ASTM B704 and B705 (welded tube and pipe) and ASTM B564 (forgings) for UNS N06625. The cobalt range in 617 and the niobium-plus-tantalum range in 625 are the two lines that identify the materials in a dispute, because the remaining elements are similar enough to be inconclusive on their own. Inconel 625 is supplied in two carbon grades: the standard grade and a low-carbon Grade 1 (0.02 % carbon maximum) intended for service above approximately 650 °C, where the lower carbon improves ductility and reduces the risk of carbide precipitation at the grain boundaries. Where the purchase order does not state the grade, the standard grade is supplied, so a high-temperature 625 duty should always name Grade 1 explicitly.
Two consequences follow from the table. First, cobalt and aluminium in 617 are not contaminants or residual elements; they are deliberate additions that cost money and that drive the creep and oxidation behaviour on which the alloy's reputation rests. Removing them, or substituting a cheaper chemistry that happens to meet the nickel and chromium lines, produces a material that satisfies a chemistry-only check and fails in service. Second, the absence of cobalt in 625 means that the two alloys can be separated quickly on a bench by portable optical emission spectrometry, which is why we recommend a PMI check to ASTM E1476 or ASTM E572 on any delivery where the two grades are stocked side by side.
The cross-system question deserves a caution that applies throughout this article. In the Chinese designation system, GH3625 is the widely used designation for the Inconel 625 type of alloy and it corresponds closely in composition; that designation is useful for commercial recognition but it is not an ASTM equivalent, and its limits are written in a different standard system. Inconel 617 has no direct GB equivalent in common use. Where a drawing originates in China and will be manufactured to an ASTM or AMS specification, or the reverse, the conversion must be made explicitly, with the source system identified in its own column on the documentation, and the material must be verified against the specification actually named on the purchase order rather than against the nearest-sounding designation.
Mechanical Properties and the Standard Basis
At room temperature the two alloys are closer than their reputations suggest, and it is this similarity that misleads buyers. Inconel 625 is slightly the stronger of the two in the annealed condition and is considerably the more ductile after long exposures at moderate temperature, while 617 develops its advantage only as the temperature rises. The table below gives typical published values with the standard basis stated; none of these figures is a substitute for the acceptance minima published in the governing product specification.
| Property (typical) | Inconel 617, annealed | Inconel 625, annealed | Basis |
|---|---|---|---|
| Tensile strength, 20 °C | ~750 MPa | ~850-930 MPa | typical, not a standard minimum |
| 0.2 % yield strength, 20 °C | ~350 MPa | ~415-460 MPa | typical, not a standard minimum |
| Elongation, 20 °C | ~50 % | ~30-40 % | typical, not a standard minimum |
| Hardness | ~85-95 HRB | ~90-100 HRB | typical, not a standard minimum |
| Tensile strength, 650 °C | ~450 MPa | ~500 MPa | typical, not a standard minimum |
| Reliable service ceiling, low stress | ~1,100 °C (oxidising) | ~980 °C (oxidising) | typical published guidance |
| Reliable service ceiling, sustained load | ~980 °C | ~650-700 °C | typical published guidance |
| Strengthening mechanism | M23C6 carbides and gamma-prime during service | solid solution, gamma-double-prime after ageing | published metallurgy |
Table note: The values shown are typical published annealed properties and are explicitly not standard minima. Property minima for a specific product form are fixed by the governing product standard named on the purchase order - ASTM B166/B168 for 617 and ASTM B446/B443/B444/B704/B705/B564 for 625, in each case together with the corresponding AMS specification for aerospace use. Room-temperature tensile testing follows ASTM E8/E8M, elevated-temperature tensile testing ASTM E21, and hardness testing ASTM E10 or E18 depending on the scale specified. The service ceilings shown are typical published guidance rather than standard limits: the permissible temperature depends on stress, atmosphere, cycling and required life, and for design work the ASME Code allowable stresses for the specific product form must be used instead of any datasheet figure.
The practical reading of the table is straightforward. If the design is limited by short-term tensile strength at room temperature, 625 is the slightly stronger and much more corrosion-resistant choice and there is no reason to pay for 617. If the design is limited by what the component must carry at 850 °C after 10,000 hours, the ranking reverses, because 617's creep-rupture strength in that regime is substantially higher and its oxidation resistance is better as well. The transition is not sharp and it is not the same for every product form, which is why we ask for the operating temperature, the stress and the atmosphere before quoting either alloy rather than simply quoting the cheaper of the two.
One qualification deserves emphasis because it is frequently missed in quotations. Because 617 continues to precipitate fine carbides and gamma-prime during service, its properties depend on thermal history, and a component that has been exposed above about 650 °C for a long period will show different properties from one that has not. This is generally beneficial for creep strength and it is one reason 617 is specified for long-life components. It also means that repair welding of a 617 component that has seen service requires a procedure that accounts for the aged microstructure, and that a post-service assessment should not rely on the original certificate values alone. For 625, the corresponding sensitivity is to the carbon grade and to the presence of secondary phases after exposure above approximately 650 °C, which is precisely why the low-carbon Grade 1 exists.
Heat Treatment, Forming and Joining
Both alloys are supplied and used in the annealed condition, and neither requires a hardening treatment, which makes fabrication simpler than for the precipitation-hardening nickel alloys. The annealing cycle nevertheless matters, because an incorrect cycle produces either a coarse or a partially recrystallised structure with poorer properties, and in Inconel 625 the cooling rate after annealing affects carbide precipitation at the grain boundaries in the same way that it does in the austenitic stainless steels.
| Parameter | Inconel 617 | Inconel 625 | Basis |
|---|---|---|---|
| Annealing / solution treatment (typical) | ~1,150-1,200 °C, rapid cool | ~1,040-1,120 °C, rapid cool | typical commercial practice, not a standard minimum |
| Purpose of the cycle | recrystallisation, dissolve carbides | recrystallisation, carbide dissolution | published metallurgy |
| Ageing treatment required | none as a specification requirement | none (Grade 1 preferred above ~650 °C) | ASTM B166 / B168 |
| Post-weld heat treatment | not normally required | not normally required | fabrication practice |
| Matching welding consumable | ERNiCrCoMo-1 type | ERNiCrMo-3 | AWS A5.14 |
| Hot forming range (typical) | ~1,050-1,200 °C | ~870-1,150 °C | typical practice |
| Cold formability | high work-hardening rate, anneal between passes | high work-hardening rate, anneal between passes | published practice |
| Cleaning requirement | sulphur and lead contamination excluded | sulphur and lead contamination excluded | fabrication requirement |
| Common welding process | GTAW, GMAW, SMAW with matching filler | GTAW, GMAW, SMAW with matching filler | published practice |
Table note: The annealing ranges shown are typical commercial cycles; the governing specification on the purchase order fixes the actual requirement, and the producer's certified heat treatment record for the delivered heat is the document that proves it. Welding consumables are selected to match the base material under AWS A5.14 and, for aerospace work, under the applicable AMS filler-metal specification. The requirement to exclude sulphur and lead from marking materials, lubricants and shop contamination is a fabrication discipline rather than a standard limit, but it is not optional: both alloys are sensitive to contamination-induced cracking at elevated temperature.
Two fabrication points decide the success of most jobs in these alloys. The first concerns the welding consumable on a high-temperature 617 joint. Because 617 relies on cobalt and aluminium for its creep and oxidation behaviour, the filler must carry those elements as well; a cobalt-free nickel-chromium-molybdenum filler produces a weld that passes radiography and looks sound while having substantially lower creep-rupture strength than the parent metal in exactly the temperature band the component was designed for. Creep damage in such a joint appears as cracking in the weld metal or the heat-affected zone after extended service, long after the inspection documents have been accepted. We specify the consumable requirement explicitly on the drawing whenever a 617 fabrication is quoted.
The second concerns Inconel 625 in high-temperature service. Because 625's principal strengthening precipitate is metastable at elevated temperature, the low-carbon Grade 1 material is the correct choice above roughly 650 °C, and the welding procedure and cooling rate must be controlled to avoid continuous carbide networks at the grain boundaries. This is the same class of problem as sensitisation in austenitic stainless steel, and it is the reason a 625 fabrication intended for hot service should not be ordered to the standard carbon grade simply because the price is lower. For low-temperature service, seawater duty and most chemical process applications, the standard grade is entirely satisfactory and is the grade we supply by default.
Dissimilar joints between these alloys and austenitic stainless steel, low-alloy steel or other nickel alloys are common and generally straightforward, with nickel-based fillers being the usual choice for a 625-to-stainless or 617-to-steel transition. The consideration that is most often overlooked is the mismatch in thermal expansion between the alloy and the steel side of the joint. Under thermal cycling, a constrained dissimilar joint develops stresses at the interface that a simple strength calculation will not reveal, and the standard mitigation is to place the transition in a region of lower stress and lower cycling, or to use an appropriate transition piece. Where the joint will see significant cycling, the design should account for the expansion mismatch explicitly rather than relying on the ductility of the weld metal.
Creep and Stress-Rupture Behaviour Above 800 °C
Above 800 °C the controlling property is no longer tensile strength but creep behaviour: the slow, time-dependent deformation that occurs under stress at high temperature, and the eventual rupture that terminates it. This is where Inconel 617 and Inconel 625 separate most decisively, and it is the single most common reason for a specification to be changed after the first design review.
Inconel 617 is one of the reference alloys for the 850-1,100 °C band. Its creep-rupture strength depends on two mechanisms that develop during service rather than during manufacture. The first is the precipitation of fine M23C6-type carbides on the grain boundaries, which resists grain-boundary sliding; the second is the slow formation of fine gamma-prime precipitates, helped by the aluminium and titanium in the chemistry. The cobalt addition contributes by lowering the stacking-fault energy of the matrix and by retarding recovery, which slows the deformation processes that lead to rupture. The practical consequence is that 617 gains strength during long exposure, so its performance in a 100,000-hour test is better than a short-term test would predict. For components designed for decades of service - reformer outlet manifolds, gas turbine combustion hardware, high-temperature heat exchangers and nuclear intermediate heat exchangers - that behaviour is the reason the alloy is specified.
Inconel 625 behaves differently. Its useful elevated-temperature strength comes initially from solid-solution strengthening by molybdenum and niobium and from the gamma-double-prime precipitates that form on ageing, but that precipitate is metastable: above roughly 650 °C, extended exposure transforms it towards the stable delta phase, and the accompanying change in precipitate morphology reduces long-term creep-rupture strength. The alloy therefore performs well in short-term elevated-temperature tests and less well in long-term service, and the divergence grows with temperature and time. This is not a defect in the material; it is the reason 625's domain is defined by corrosion resistance and fatigue life rather than by maximum creep strength, and it is why a designer who selects 625 for a 900 °C structural duty will usually regret it.
| Service condition | Inconel 617 | Inconel 625 | Basis |
|---|---|---|---|
| Long-term creep-rupture above 850 °C | preferred material | not recommended under sustained load | typical published guidance |
| Creep-rupture 700-850 °C under load | acceptable, verify allowables | marginal under sustained load, acceptable at low stress | ASME Code allowables |
| Short-term high-temperature strength (hours) | good | good | typical published data |
| Long-term microstructural stability above 650 °C | carbides and gamma-prime, beneficial | gamma-double-prime transforms towards delta, strength falls | published metallurgy |
| Thermal fatigue resistance | good, lower expansion than many superalloys | excellent, widely used in bellows and ducting | published practice |
| Low-cycle fatigue in oxidising gas | good | very good | published practice |
| Thermal cycling of constrained joints | design review required | design review required | engineering practice |
Table note: The rankings reflect typical published guidance on creep and stress-rupture behaviour and are not standard requirements. For any design above approximately 500 °C, the allowable stresses must be taken from the ASME Boiler and Pressure Vessel Code for the specific product form and specification, including the elevated-temperature allowable stress tables, and the design must be checked against the intended life and the actual atmosphere. Where a design falls within a Code case rather than the main tables, the applicable case governs and should be identified on the drawing.
Two practical rules follow. First, if the component must carry a sustained load at 900 °C for tens of thousands of hours, the material is 617 and the decision should be documented at the design stage, not revisited after the material is on site. Second, if the component sees a hot environment but the load is negligible - ducting, liners, shields, thermocouple protection, expansion bellows - the controlling requirement is oxidation and thermal fatigue resistance, both alloys can serve, and 625 is often the more economical answer because its thermal fatigue behaviour is excellent and it does not need 617's creep capability.
Oxidation, Carburisation and High-Temperature Corrosion
In a high-temperature atmosphere the material is attacked not by an electrolyte but by the gas itself, and the two alloys respond differently because their protective oxide scales are different. Inconel 617 forms a chromium-rich surface oxide supported by its aluminium content, and that scale is more protective and more resistant to spalling under thermal cycling than the oxide formed by most nickel-chromium alloys. Inconel 625 also forms a chromium oxide scale and performs well in oxidising conditions up to its practical ceiling, but it lacks the aluminium contribution and its resistance to repeated cycling through the oxide-spallation temperature range is correspondingly lower.
| Service environment | Inconel 617 | Inconel 625 | Basis |
|---|---|---|---|
| Oxidising, clean combustion gas, 900-1,100 °C | excellent | good up to ~950-980 °C | typical published guidance |
| Cyclic oxidation with spalling | excellent | good | typical published guidance |
| Carburising atmospheres, 850-1,050 °C | good, aluminium assists | limited at the upper range | typical published guidance |
| Nitriding atmospheres | fair, design review | fair, design review | typical published guidance |
| Sulphidising / reducing sulphur atmospheres | limited, review required | limited, review required | typical published guidance |
| Wet chlorine and chlorine-bearing process streams | not the selection | good in oxidising chloride service, review required | published corrosion data |
| Seawater and chloride pitting | not the selection | excellent | published corrosion data |
| Mixed and reducing acids | not the selection | good to excellent, depends on the acid | published corrosion data |
| Molten salts and hot corrosion | good, used in thermal storage and nitrate salt service | review required | typical published guidance |
Table note: The rankings summarise typical published guidance on high-temperature corrosion and aqueous corrosion behaviour. They are engineering guidance rather than standard requirements, and the actual performance depends on temperature, gas composition, sulphur and alkali content, cycling frequency and stress. For sulphur-bearing or strongly reducing atmospheres, or for any service where a protective scale may not form, the correct step is a corrosion study of the actual conditions rather than a datasheet comparison. For aqueous chloride service, testing to ASTM G48 or ASTM G28 on the actual grade and condition is the accepted way to establish a margin.
The practical division is clean. Where the atmosphere is hot and oxidising, or carburising, 617 is the better material and is the safer choice at the top of the temperature range. Where the aggressive species are chlorides, whether in a wet process stream or in seawater cooling, 625 is the better material and 617 is not a substitute. The two mechanisms are not symmetrical: a designer who selects 617 for a hot oxidising duty is making the conventional choice, while a designer who selects 617 for a chloride-bearing wet duty is making a mistake that 617's high-temperature reputation often conceals. Hot corrosion in the presence of sulphur and alkali deposits is a special case in both alloys and should not be extrapolated from simple oxidation data.
One further point is worth making for furnace and reformer operators. In carburising service, the failure mode is not general metal loss but the inward diffusion of carbon, which produces a hard, brittle case, a change in volume and eventual cracking under thermal cycling. Inconel 617's aluminium content is helpful because the alumina-rich component of its scale is a better diffusion barrier than a pure chromia scale, but no wrought nickel alloy is immune to heavy carburisation, and where carbon activity is high the correct answer is often a higher-nickel cast alloy or a protective coating rather than either of these two wrought grades. We ask for the gas analysis and the cycle before recommending a material for a carburising furnace, because the answer frequently changes once the carbon activity is known.
Selection Matrix: Which Alloy for Which Duty
The selection between 617 and 625 reduces to a small number of questions, and the matrix below maps the common duties to a recommendation. The logic is consistent throughout: 617 where creep and high-temperature oxidation govern, 625 where corrosion and thermal fatigue govern, and a review wherever the duty sits in the overlap.
| Application / duty | Recommended alloy | Why |
|---|---|---|
| Gas turbine combustor and transition duct components | 617 | creep strength and cyclic oxidation resistance above 900 °C |
| Industrial furnace radiant tubes and high-temperature muffles | 617 | oxidation resistance and long-term creep strength |
| Ammonia and hydrogen reformer outlet manifolds and pigtails | 617 | creep-rupture strength at 850-950 °C |
| High-temperature heat exchangers and intermediate heat exchangers | 617 | creep strength plus oxidation resistance at 850-1,000 °C |
| Thermocouple protection sheaths in oxidising furnaces | 617 | slow scaling, retains section through cycles |
| Expansion bellows and ducting in hot gas service | 625 | excellent thermal fatigue life and good oxidation resistance |
| Seawater piping, valves and pump components | 625 | chloride pitting and crevice resistance |
| Chloride-bearing chemical process equipment | 625 | pitting, crevice and general corrosion resistance |
| Flue-gas, scrubber and incineration ducting | 625 | corrosion resistance with thermal cycling tolerance |
| Bellows, flexible elements and thin-wall fatigue parts | 625 | fatigue and formability |
| Hot structural parts below ~700 °C in aggressive chemistry | 625 | corrosion resistance, adequate creep margin |
| Sustained-load parts above ~900 °C | 617 | 625 is not the correct selection for sustained load at this level |
| Any chloride-bearing wet duty at any temperature | 625, not 617 | 617 has no advantage and is not intended for it |
| Above ~1,100 °C in oxidising gas | neither, review higher alloys | consider a higher-aluminium or oxide-dispersion alloy |
Table note: The recommendations reflect the typical published properties and corrosion behaviour of the two grades and are engineering guidance rather than standard requirements. The final selection for safety-critical or high-value equipment must be based on the actual temperature, stress, atmosphere and cycling of the duty, on the applicable design code, and on the required service life. Where the duty sits inside the overlap band of roughly 700-900 °C, the decision should be made from the ASME Code allowable stress for the specific product form and the actual design life rather than from room-temperature data.
Three rules are worth stating as rules rather than as rows in a table. First, if the component carries a sustained load above roughly 900 °C, the answer is 617 and the decision should be made early, because changing material after the design is released is expensive. Second, if chlorides or seawater are present in any form, the answer is 625; 617 brings no corrosion advantage in aqueous chloride service and is the more costly material. Third, if the component is thin-walled, heavily cycled and carries little load, the controlling requirement is fatigue and oxidation, and 625 will usually be the economical answer even at temperatures where 617 is technically permissible.
Price Reference (2026, EXW Shanghai)
Both alloys are nickel-based materials whose prices track the nickel market, and the difference between them is driven mainly by 617's cobalt content. Cobalt is the more expensive of the two principal alloying additions by a wide margin and is also subject to its own supply-market volatility, so the spread between the two alloys widens and narrows with the cobalt price rather than with nickel alone. The figures below are reference ranges for planning purposes and are not a quotation.
| Product form | Inconel 617 | Inconel 625 | Note |
|---|---|---|---|
| Round bar | USD 55-90/kg | USD 32-55/kg | 617 carries a cobalt premium |
| Plate and sheet | USD 60-100/kg | USD 35-60/kg | thickness and width affect yield |
| Seamless tube and pipe | USD 70-120/kg | USD 42-70/kg | size and wall thickness dominate |
| Welded pipe | USD 60-100/kg | USD 38-62/kg | weld quality regime matters |
| Forgings | quote by drawing | quote by drawing | complexity and quantity dominate |
| Filler wire (ERNiCrCoMo-1 / ERNiCrMo-3) | quote by size | quote by size | priced separately from base metal |
Table note: Reference range only, 2026, EXW Shanghai, USD/kg. These figures float with the LME nickel price and, for 617, with the cobalt price, and they are not a quotation. Wide ranges are shown deliberately because price depends strongly on quantity, product form, size and the certification and testing regime required; a 617 tube order with full third-party inspection and corrosion testing sits at the top of the range, while a standard certificate of conformity bar order sits lower. Cobalt market volatility means the two alloys can move independently, and an enquiry should always be re-priced rather than extrapolated from a previous quotation.
The commercial conclusion is simple. Inconel 625 is the lower-cost material and is the correct default wherever its corrosion and fatigue advantages are what the duty requires. Inconel 617 should be specified only when its creep and oxidation capability is genuinely needed, because paying the cobalt premium for a duty that 625 would serve just as well is a straightforward waste of money. Where the duty genuinely requires 617, the correct way to control cost is to minimise the quantity of 617 in the design - using it only in the hot, highly stressed region and transitioning to a lower-cost alloy in the cooler sections - rather than to substitute a cheaper chemistry that will not survive.
Standard Index
| Standard | Title / scope | Covers | Form |
|---|---|---|---|
| ASTM B166 | Nickel-chromium-cobalt-molybdenum alloy (UNS N06617) rod, bar and wire | composition + mechanical | rod, bar, wire |
| ASTM B168 | Nickel-chromium-cobalt-molybdenum alloy (UNS N06617) plate, sheet and strip | composition + mechanical | plate, sheet, strip |
| ASTM B446 | Nickel-chromium-molybdenum-columbium alloy (UNS N06625) rod, bar and wire | composition + mechanical | rod, bar, wire |
| ASTM B443 | Nickel-chromium-molybdenum-columbium alloy (UNS N06625) plate, sheet and strip | composition + mechanical | plate, sheet, strip |
| ASTM B444 | Nickel-chromium-molybdenum-columbium alloy (UNS N06625) pipe and tube | composition + mechanical | seamless pipe, tube |
| ASTM B704 / B705 | Welded UNS N06625 tube (B704) and welded pipe (B705) | composition + mechanical | welded tube, pipe |
| ASTM B564 | Nickel alloy forgings | composition + mechanical | forgings |
| ASME SB-166 / SB-168 / SB-446 / SB-443 / SB-444 / SB-564 | ASME Code adoption of the above | Code allowable basis | all forms |
| ASTM E8 / E8M | Tension testing of metallic materials | test method | - |
| ASTM E21 | Elevated-temperature tension testing of metallic materials | test method | - |
| ASTM E10 / E18 | Brinell and Rockwell hardness testing | test method | - |
| ASTM E112 | Determining average grain size | test method | - |
| ASTM E1476 | Standard guide for metals identification, sorting and examination (PMI) | test method | - |
| ASTM E572 | Analysis of stainless steel and nickel alloys by X-ray spectrometry | test method | - |
| ASTM G48 | Pitting and crevice corrosion resistance in ferric chloride | test method | - |
| ASTM G28 | Detecting susceptibility to intergranular corrosion | test method | - |
| AWS A5.14 | Nickel and nickel-alloy bare welding electrodes and rods | consumable selection | filler wire |
| GB/T 14992 | Classification and designation of deformation superalloys (Chinese system) | cross-system designation reference | - |
Table note: Standards are listed by number and scope; where an edition year is not quoted, the latest edition applies and the controlling edition is the one named on the purchase order. The Chinese designation system entry is shown for cross-system recognition only. GH3625 is the commonly used Chinese designation corresponding to the Inconel 625 type of alloy; Inconel 617 has no direct GB equivalent in common use. A GB designation must never be substituted line for line for the ASTM limits on an ASTM specification, and any conversion must be stated explicitly with the source system identified.
FAQ
Q1: What is the main difference between Inconel 617 and Inconel 625?
The main difference is what each alloy is optimised to resist. Inconel 617 is a nickel-chromium-cobalt-molybdenum alloy containing 10-15 % cobalt and 0.8-1.5 % aluminium, and those additions make it significantly stronger in creep at 850-1,100 °C and more resistant to high-temperature oxidation and carburisation. Inconel 625 is a nickel-chromium-molybdenum-niobium alloy containing 3.15-4.15 % niobium with essentially no cobalt, and its niobium plus molybdenum combination gives it outstanding resistance to chlorides, pitting, crevice corrosion and a wide range of acids, together with excellent fatigue performance. The two also differ in temperature behaviour: 617 gains strength during long exposure as carbides and gamma-prime precipitate, while 625's main strengthening precipitate is metastable above roughly 650 °C and long-term creep strength falls away in that band. In short, 617 is a high-temperature creep and oxidation alloy, and 625 is a corrosion and thermal-fatigue alloy that also has useful elevated-temperature strength. Neither is a general substitute for the other, and a specification that treats them as interchangeable will eventually produce a failure.
Q2: Which alloy is better for service above 900 °C?
Inconel 617 is the correct choice above roughly 900 °C whenever the component carries a sustained load. In that temperature band, creep and stress-rupture behaviour control the design, and 617's cobalt and aluminium content gives it a substantial advantage: the alloy develops fine M23C6 carbides and gamma-prime precipitates during service that resist grain-boundary sliding and slow the deformation that leads to rupture. Inconel 625, by contrast, relies on a gamma-double-prime precipitate that is metastable above about 650 °C and gradually transforms towards the stable delta phase, so its long-term rupture strength at 900 °C is not in the same class. The distinction applies to sustained load, not to every high-temperature duty. Where the component is thin, lightly loaded and cycled - ducting, liners, expansion bellows, thermocouple sheaths - the controlling requirements are oxidation resistance and thermal fatigue life, and 625 performs well in that role up to roughly 950-980 °C. For any design above approximately 500 °C, the allowable stress should be taken from the ASME Code tables for the specific product form rather than from datasheet values.
Q3: Can Inconel 617 and Inconel 625 be used interchangeably?
No, they are not interchangeable, and the practical danger is that they look and machine almost identically, so a substitution is easy to make and hard to detect by inspection. The two alloys share similar nickel, chromium and molybdenum levels, which means a chemistry-only verification that checks only those elements will not separate them. Separating them requires the cobalt and aluminium lines, which are present in 617 and substantially absent in 625, or the niobium-plus-tantalum line, which is present in 625 and absent in 617. Portable optical emission spectrometry to ASTM E1476 or ASTM E572 resolves both cases in seconds, which is why we recommend a PMI check on any delivery where both grades are stocked together. Substitution in the other direction is equally problematic: putting 617 into a chloride-bearing wet duty gains nothing, because 617 has no corrosion advantage in that environment, and it costs the cobalt premium. Substitution in either direction should be treated as an engineering change requiring review of the temperature, load, atmosphere and life, not as a procurement convenience.
Q4: Which welding filler metal should be used for each alloy?
Use a cobalt-bearing matching filler of the ERNiCrCoMo-1 type for Inconel 617 and ERNiCrMo-3 for Inconel 625, in both cases selected under AWS A5.14 and, for aerospace work, under the applicable AMS filler-metal specification. The filler choice matters more for 617 than for most alloys: 617's creep and oxidation performance depends on cobalt and aluminium, so a cobalt-free nickel-chromium-molybdenum filler produces a weld that passes visual and radiographic inspection while having markedly lower creep-rupture strength in exactly the temperature band the component was designed for. Damage of that kind appears as cracking in the weld metal or heat-affected zone only after extended service, by which time the fabrication has been accepted and often installed. For 625, the matching filler is widely available and the concern is different: heat input and cooling rate should be controlled so that continuous grain-boundary carbide networks do not form, particularly when the low-carbon Grade 1 material is being welded for service above roughly 650 °C. Neither alloy normally requires post-weld heat treatment, but both require sulphur and lead contamination to be excluded from marking materials, lubricants and shop dirt around the joint.
Q5: Why does Inconel 625 need a low-carbon Grade 1 above 650 °C?
Inconel 625 is supplied in two carbon grades, and the low-carbon Grade 1 material, capped at approximately 0.02 % carbon, exists because carbon behaviour at the grain boundaries controls ductility and toughness in elevated-temperature service. In the standard grade, carbon is permitted up to about 0.10 %, which is harmless for most corrosion, seawater and ambient-temperature duties and is the grade normally supplied. Above roughly 650 °C, however, the carbon in the standard grade can precipitate as continuous or semi-continuous carbides along the grain boundaries, particularly in the heat-affected zone of a weld or after slow cooling, and that network reduces ductility and promotes cracking under thermal cycling. Grade 1 avoids this by keeping so little carbon in solution that a continuous boundary network cannot form. The practical consequence is that a high-temperature 625 fabrication should always name Grade 1 explicitly on the drawing and the purchase order, because where the grade is not stated the standard grade is what the mill will supply. The same principle applies in reverse to buying: a Grade 1 quote is not an expensive alternative to a standard grade quote, it is a different material requirement, and the two should not be compared as if they were the same line item.
Q6: How can I verify that I received Inconel 617 and not a substitute?
Verify by element, not by appearance and not by the certificate alone, because Inconel 617 and Inconel 625 are visually indistinguishable in bar, plate and tube form and are similar in their nickel, chromium and molybdenum content. The decisive lines are cobalt and aluminium, which are present in 617 at 10.0-15.0 % and 0.8-1.5 % respectively and are essentially absent in 625, and niobium plus tantalum, which is present in 625 at 3.15-4.15 % and absent in 617. Portable optical emission spectrometry performed to ASTM E1476 or ASTM E572 will separate the two materials in the field in seconds, and laboratory analysis to ASTM E572 gives the documented result for a claim. Alongside the chemistry check, reconcile the heat number marked on the material with the heat number on the mill certificate, confirm that the certificate names the producing mill, and check that the specification and edition quoted on the certificate are the ones named on the purchase order. For high-temperature service, also confirm that the certificate reports the annealing cycle, because a 617 or 625 component that was not correctly annealed will not deliver its design properties even though its chemistry is correct. Our technical knowledge centre sets out the reconciliation steps we apply to incoming material.
Q7: Is Inconel 625 suitable for seawater and chloride service?
Yes, and chloride service is where Inconel 625 is at its best. The molybdenum and niobium in the alloy, combined with its chromium content, give it excellent resistance to chloride pitting and crevice corrosion, and it is widely used in seawater piping, valve trim, pump components and offshore hardware where stainless steels pit and crevice-corrode. It also performs well in chloride-bearing chemical process streams and in oxidising chloride environments where 316L and even the super-austenitic stainless steels have insufficient margin. Inconel 617, by contrast, has no advantage in aqueous chloride service and should not be selected for it: 617 is a high-temperature alloy and its value lies in creep strength and oxidation resistance, not in resistance to wet chloride attack. Where the duty is both hot and chloride-bearing, the decision needs a proper review rather than a datasheet comparison, because the failure mechanisms interact. For any order where chloride pitting resistance is a design requirement, the acceptance criteria should be written into the purchase order and verified by test, typically to ASTM G48, on the actual grade and heat treatment condition rather than on a generic material certificate. Our other corrosion and materials selection articles set out how those test requirements are written and witnessed.
Q8: Which alloy should I choose for a gas turbine combustor or a reformer outlet manifold?
Both applications are classic Inconel 617 duties, and the reasoning is the same in each case. Gas turbine combustor liners, transition ducts and similar hot-section components operate in oxidising combustion gas at temperatures where creep and cyclic oxidation together control the design, and 617's cobalt-plus-aluminium chemistry gives it both the creep-rupture strength and the spallation-resistant oxide scale that the duty requires. Reformer outlet manifolds and pigtails in hydrogen and ammonia plants operate at roughly 850-950 °C under internal pressure in a strongly carburising and sometimes oxidising atmosphere, and here again 617 is the established material because it retains strength through long exposures and resists carbon ingress better than a chromia-only scale does. Inconel 625 is not the right selection for either duty at the top of its temperature range, because under sustained load above roughly 700 °C its long-term rupture strength is not comparable. Where a hot-gas component is ducting, a liner in a lower-temperature zone, an expansion joint or a bellows with little sustained load, 625 is often the better and more economical choice on the strength of its thermal fatigue behaviour.
Q9: What heat treatment is required after welding these alloys?
Neither Inconel 617 nor Inconel 625 normally requires a post-weld heat treatment, which is one of the practical advantages both alloys hold over the precipitation-hardening nickel alloys. The welds are used in the as-welded condition, and the engineering effort therefore goes into the welding procedure rather than into a furnace cycle: qualified procedures, controlled heat input and interpass temperature, clean joint preparation, and the correct matching filler for each alloy. Two disciplines are essential rather than optional. First, sulphur and lead must be excluded from marking crayons, lubricants, cutting fluids and general shop contamination around the joint, because both elements embrittle these alloys at elevated temperature and a metallurgically sound weld can crack in service because of surface contamination. Second, when Inconel 625 Grade 1 is welded for service above roughly 650 °C, the procedure and the cooling rate must be controlled to avoid forming a continuous carbide network at the grain boundaries. Where a component has already seen high-temperature service, repair welding of 617 requires a procedure that accounts for the aged microstructure, and the original certificate values should not be assumed to describe the material being repaired.
Q10: How do Inconel 617 and Inconel 625 compare in price in 2026?
Inconel 617 is appreciably more expensive per kilogram than Inconel 625, and the reason is cobalt. In 2026 EXW Shanghai reference terms, 617 round bar typically falls in the range of roughly USD 55-90/kg while 625 round bar falls in the range of roughly USD 32-55/kg, with tubular products at higher levels in both cases because size, wall thickness and testing requirements dominate the tube price. Cobalt is the more costly of the two principal alloying additions and it has its own supply-market volatility, so the spread between the two alloys moves with the cobalt price as well as with nickel, and the relationship is not stable over long periods. These figures are reference ranges only and are not a quotation; the wide bands reflect the fact that quantity, product form, size and the certification regime all move the price materially. The commercial conclusion is that 625 should be the default wherever its corrosion and fatigue advantages are what the duty needs, and 617 should be specified only where creep strength or high-temperature oxidation resistance is genuinely required. Where 617 is required, containing cost is best done by limiting the 617 to the hot, highly stressed region of the design rather than by substituting a cheaper chemistry.
Q11: What alloys should be considered above 1,100 °C?
Above roughly 1,100 °C in an oxidising atmosphere, the wrought nickel-chromium alloys reach the limit of their useful life and the selection moves to materials with a higher protective-oxide content or to a different class of material altogether. Inconel 617 remains usable at the very top of its range for lightly loaded components because of its aluminium-supported oxide scale, but sustained-load service at that temperature is normally the domain of oxide-dispersion-strengthened alloys, of higher-aluminium nickel alloys, or of ceramic and refractory systems, and such applications are usually reviewed by the equipment designer rather than specified from a general material list. Where the temperature is below 1,100 °C but the atmosphere is carburising or contains sulphur, the answer often changes away from the wrought alloys completely, because carbon ingress and sulphidation are not solved by more chromium alone; cast heat-resistant alloys, coatings or a design change to reduce metal temperature may all be better answers. Our Incoloy 800H and 800HT grades and the nickel-chromium iron alloys cover the 700-1,000 °C band, and for lower-temperature aggressive chemistry the nickel-chromium-molybdenum family is the appropriate route. The honest answer above 1,100 °C is that the duty should be reviewed rather than specified from a catalogue.
Q12: What information should I provide to get a correct material recommendation?
To make a defensible recommendation between these two alloys, we need five pieces of information: the maximum metal temperature in continuous service and any short-term excursions, the stress the component must carry (internal pressure, weight, thermal stress or a combination), the chemical composition of the environment including whether chlorides are present, the number and severity of thermal cycles expected, and the required service life. Those five answers resolve the great majority of cases immediately, because they establish whether creep or corrosion is the controlling mechanism. It also helps to know the product form and size, the governing standard and edition, the certification regime (for example an EN 10204 3.1 inspection document, third-party inspection, or a specific corrosion test), and whether the application has an existing approved material from which a change would require re-approval. Where the answer is not clear from the process data, we will say so and recommend a corrosion test on the actual stream rather than issue a recommendation with an artificial margin. Shanghai Hangbo Alloy Group Co., Ltd. supplies both alloys across bar, plate, sheet, tube, pipe and forgings with full traceability, and we will review your process data before quoting so that the grade, the product standard and the condition are matched to the duty. Our technical team can be reached through the contact page.
Conclusion and Selection Rules
The selection rules for these two alloys are short and they resolve almost every real case. Use Inconel 617 when the component carries a sustained load above roughly 850-900 °C, when the atmosphere is hot and oxidising or carburising, and when the required life is measured in tens of thousands of hours or more. Use Inconel 625 when chlorides, seawater or mixed acids are present, when thermal fatigue governs a thin-walled or heavily cycled component, and for elevated-temperature duties below roughly 650-700 °C where there is no sustained high load to carry. Where the two overlap, the deciding question is simple: is the component limited by what it must carry at temperature, or by what the environment does to it? Creep answers 617; corrosion answers 625.
Two of the rules are constraints rather than preferences and should be treated as such. Inconel 617 is not the correct selection for a chloride-bearing wet duty at any temperature, because it brings no corrosion advantage there and costs the cobalt premium. Inconel 625 is not the correct selection for sustained load above roughly 900 °C, because its long-term rupture strength in that band does not support it. Everything else in this comparison is a trade between the cobalt premium and the creep margin, and where the margin is genuinely uncertain the right next step is a design check against the ASME allowable stresses for the specific product form rather than another datasheet comparison.
Shanghai Hangbo Alloy Group Co., Ltd. supplies Inconel 617 and Inconel 625 as bar, plate, sheet, seamless tube, welded pipe and forgings, with mill test certification, PMI to ASTM E1476 or E572 where required, and third-party inspection by SGS, BV or TUV on request. We hold material in both chemistries and can supply tested and certified lots against a specific product standard and edition. Send your operating temperature, load, atmosphere and required life through our contact page and we will return a material recommendation with the governing standard citations and a costed quotation. For related selection questions, see our other technical articles comparing high-temperature and corrosion-resistant alloys.
Contact & Complete Product Range
Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier
Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com
All Grades
Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA
Product Forms: Bars(6-500mm) | Tubes(OD 6-610mm) | Plates(0.1-100mm) | Forgings | Welded Pipes | Fittings | Wire(0.05-10mm) | Strip(0.02-4.0mm)
SGS/BV/TUV. www.nickel-alloy.com
