Inconel X-750 vs Inconel 718: Age-Hardening Alloy Choice
Date: 2026年9月26日 Categories: News Views: 372
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 X-750 or Inconel 718?
Inconel 718 is the default choice for weldable structural parts, large forgings, downhole and aerospace components because its gamma-double-prime hardening and slow precipitation kinetics make it far more weldable and stronger in the aged condition. Inconel X-750 is chosen for high-temperature springs, seals, retaining rings and cryogenic fasteners, where its gamma-prime hardening resists stress relaxation better and its oxidation resistance holds to higher temperatures.
Key Takeaways
- 718 is the stronger and more weldable alloy, and it is not close on weldability. Gamma-double-prime hardening from niobium plus tantalum precipitates slowly enough that 718 can be welded and repaired in the aged or solution-treated condition, while X-750's faster gamma-prime kinetics make it prone to strain-age cracking in the heat-affected zone.
- X-750 holds its spring force at temperature better. Its gamma-prime precipitate is more resistant to over-ageing in the 540-650 °C band, which is why high-temperature springs, retaining rings and seal elements are traditionally specified in X-750 rather than 718.
- X-750 resists oxidation to a higher temperature despite having less chromium. Molybdenum in 718 is detrimental to oxide-scale stability, so X-750 is the better choice for sustained exposure to oxidising gas in the upper part of the range, typically quoted to about 815 °C continuous and 980 °C intermittent.
- 718 dominates high-strength sour-service and downhole fasteners. The aged alloy is the recognised material for NACE MR0175/ISO 15156 high-strength bolting and for wellhead and downhole components, and it is available in far larger forging sizes than X-750.
- Both are used for cryogenic service. X-750 is a traditional cryogenic fastener and spring material down to about -253 °C, and 718 is used in cryogenic valve and propulsion components, because neither alloy has a ductile-to-brittle transition of the kind that limits ferritic steels.
- The ageing condition must be stated on the purchase order. These alloys can be supplied solution treated, solution treated and aged, or in several different ageing conditions that give materially different strength and ductility, and an order that does not state the condition leaves the mill to choose.
What Are Inconel X-750 and Inconel 718?
Inconel X-750 is a precipitation-hardenable nickel-chromium alloy, designated UNS N07750 and W.Nr 2.4669, with approximately 70 % minimum nickel, 14.0-17.0 % chromium, 5.0-9.0 % iron, 2.25-2.75 % titanium, 0.40-1.00 % aluminium and 0.70-1.20 % niobium plus tantalum. It was developed as a high-strength, oxidation-resistant material for springs, fasteners and other highly stressed components that must operate at temperature for long periods, and it is supplied as bar, wire, plate, sheet and strip under the ASTM and AMS specifications that govern precipitation-hardening nickel alloys. Our Inconel alloy supplier page lists the forms and certifications we hold for the age-hardenable grades.
Inconel 718 is also a precipitation-hardenable nickel-chromium alloy, designated UNS N07718 and W.Nr 2.4668, but its chemistry and its precipitation behaviour are organised around different elements: approximately 50.0-55.0 % nickel, 17.0-21.0 % chromium, 4.75-5.50 % niobium plus tantalum, 2.80-3.30 % molybdenum, 0.65-1.15 % titanium and 0.20-0.80 % aluminium, with iron forming the balance. The niobium is the decisive addition. It forms the gamma-double-prime phase that gives 718 its high strength, and because that phase precipitates slowly, 718 tolerates welding and can be aged after welding without the cracking that plagues the gamma-prime alloys. That single metallurgical fact is why 718 became the dominant superalloy of the aerospace and oil and gas industries and why its production volume exceeds that of every other precipitation-hardening nickel alloy.
Both alloys are age-hardenable, both are used in the aged condition for high strength, and both are specified in similar product forms and to the same family of standards, which is the source of most of the confusion between them. They are nevertheless different materials with different precipitation mechanisms, different weldability, different oxidation behaviour and different strengthening ceilings, and a specification that treats them as alternatives on a quote sheet is a specification that has not been thought through. This article examines their chemistry, mechanical properties, ageing cycles, weldability, stress-relaxation behaviour and availability, and closes with explicit selection rules and a decision matrix.
It is worth stating the metallurgical relationship plainly at the outset, because every recommendation that follows depends on it. X-750 is hardened by gamma-prime, an ordered Ni3(Al,Ti) phase that forms relatively quickly and that gives high strength at the cost of welding difficulty and a sensitivity to over-ageing above roughly 650 °C. 718 is hardened predominantly by gamma-double-prime, a metastable Ni3Nb phase that forms slowly, can be controlled by a two-step ageing cycle, and gives both high strength and excellent weldability, with the trade-off that the precipitate is metastable and transforms towards delta phase if held above about 650 °C for long periods. If you hold those two facts, the selection rules at the end of this article follow automatically, and so does the reason why 718 is now the more widely used of the two despite X-750 being the older and in some respects the more temperature-tolerant alloy.
Chemical Composition: Gamma-Prime versus Gamma-Double-Prime Chemistry
The composition table below sets out the two chemistries with the governing standard named for each. The line that identifies each alloy in a dispute is niobium plus tantalum, which is 4.75-5.50 % in 718 and 0.70-1.20 % in X-750, and the molybdenum line, which is present in 718 at 2.80-3.30 % and is not an intentional addition in X-750.
| Element (wt %) | Inconel X-750 (N07750) | Inconel 718 (N07718) | Per standard | Function in the alloy |
|---|---|---|---|---|
| Nickel | 70.0 min | 50.0-55.0 | ASTM B637 / AMS 5667 / AMS 5662 | matrix |
| Chromium | 14.0-17.0 | 17.0-21.0 | ASTM B637 / AMS 5667 / AMS 5662 | oxidation and corrosion resistance |
| Iron | 5.0-9.0 | balance | ASTM B637 / AMS 5667 / AMS 5662 | matrix component |
| Niobium + Tantalum | 0.70-1.20 | 4.75-5.50 | ASTM B637 / AMS 5667 / AMS 5662 | gamma-double-prime former in 718 |
| Molybdenum | not specified | 2.80-3.30 | ASTM B637 / AMS 5662 | solid-solution strengthening |
| Titanium | 2.25-2.75 | 0.65-1.15 | ASTM B637 / AMS 5667 / AMS 5662 | gamma-prime former in X-750 |
| Aluminium | 0.40-1.00 | 0.20-0.80 | ASTM B637 / AMS 5667 / AMS 5662 | gamma-prime former and oxidation resistance |
| Cobalt | 1.0 max | 1.0 max | ASTM B637 / AMS 5667 / AMS 5662 | residual |
| Carbon | 0.08 max | 0.08 max | ASTM B637 / AMS 5667 / AMS 5662 | carbide formation |
| Manganese | 1.0 max | 0.35 max | ASTM B637 / AMS 5667 / AMS 5662 | deoxidation residual |
| Silicon | 0.50 max | 0.35 max | ASTM B637 / AMS 5667 / AMS 5662 | deoxidation residual |
| Sulphur | 0.010 max | 0.015 max | ASTM B637 / AMS 5667 / AMS 5662 | residual, tightly controlled |
| Boron | not specified | 0.006 max for some product forms | ASTM B637 / AMS 5662 | grain-boundary strengthening |
Table note: Values are the standard composition limits of the governing specifications (ASTM B637 for bars, forgings and forging stock, ASTM B670 for 718 plate, sheet and strip, AMS 5662, AMS 5663, AMS 5667, AMS 5542 and AMS 5596/5597 for the aerospace forms), latest editions. Composition limits vary slightly between product forms and between the ASTM and AMS documents, so the controlling edition is the one named on the purchase order. The niobium-plus-tantalum line and the molybdenum line are the two chemistries that separate the alloys conclusively; nickel, chromium and titanium alone are not sufficient to identify which material was supplied. X-750 is not normally supplied with the very low sulphur levels that some aerospace 718 specifications require, and where low sulphur or a controlled boron addition is specified, the applicable AMS document must be named explicitly rather than assumed.
Two consequences follow from the table. First, the higher chromium in 718 does not translate into better high-temperature oxidation performance, because the molybdenum addition that gives 718 its strength is detrimental to the stability of the protective oxide scale. That apparent contradiction is one of the most frequently misunderstood points in alloy selection, and it explains why X-750 is quoted to higher oxidation-limited service temperatures than 718 despite carrying less chromium. Second, the much higher nickel content of X-750 makes it the more expensive material on a nickel-content basis, while 718's lower nickel and higher iron content make it cheaper to produce and, in practice, available in a far wider range of sizes and in much greater quantity. Availability, rather than price per kilogram, is often the deciding factor for large forgings.
The cross-system reference deserves the usual caution. In the Chinese designation system, GH4145 is the commonly used designation corresponding to the Inconel X-750 type of alloy and GH4169 corresponds to the Inconel 718 type, and both are widely recognised commercially. They are not ASTM equivalents: their composition and property limits are written in their own standard system and must not be substituted line for line for the ASTM or AMS limits in the tables above. 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, and the delivered material must be verified against the specification actually named on the purchase order.
Mechanical Properties in the Aged Condition
In the aged condition, 718 is the stronger of the two alloys at every temperature up to about 650 °C, and its yield strength advantage is substantial rather than marginal. X-750 compensates with a higher practical ceiling for oxidation-limited service and with better retention of spring force, and its ductility in the aged condition is comparable. The table below gives typical published values with the standard basis stated; acceptance minima for a specific order come from the governing specification, not from this table.
| Property (typical, aged) | Inconel X-750 | Inconel 718 | Basis |
|---|---|---|---|
| Tensile strength, 20 °C | ~1,200-1,300 MPa | ~1,250-1,400 MPa | typical, not a standard minimum |
| 0.2 % yield strength, 20 °C | ~800-850 MPa | ~1,030-1,150 MPa | typical, not a standard minimum |
| Elongation, 20 °C | ~15-20 % | ~15-20 % | typical, not a standard minimum |
| Hardness, aged | ~32-40 HRC | ~36-44 HRC | typical, not a standard minimum |
| Tensile strength, 650 °C | ~900-1,000 MPa | ~950-1,100 MPa | typical, not a standard minimum |
| Stress-rupture capability | good to ~700 °C | good to ~650-700 °C | typical published guidance |
| Oxidation-limited service | ~815 °C continuous, ~980 °C intermittent | ~650-700 °C continuous under load | typical published guidance |
| Cryogenic service | traditional fastener and spring material to ~-253 °C | widely used in cryogenic valve and propulsion parts | published practice |
| Magnetic behaviour | weakly magnetic | weakly magnetic | published data |
Table note: The values shown are typical published properties in the aged condition and are explicitly not standard minima. Acceptance minima for a specific product form are fixed by the governing specification named on the purchase order - ASTM B637 or ASTM B670 for the base alloys and the applicable AMS document for aerospace forms - and the required condition (solution treated, or solution treated and aged) must be stated on that order. Room-temperature tensile testing follows ASTM E8/E8M, elevated-temperature testing ASTM E21, hardness testing ASTM E10 or E18, and grain size determination ASTM E112. The service temperatures shown are typical published guidance rather than standard limits; the permissible temperature depends on stress, atmosphere, cycling and required life, and for pressure-retaining design the ASME Code allowable stresses for the specific product form apply.
The practical reading of the table is that 718's advantage is concentrated in yield strength and in the room-temperature to 650 °C band, while X-750's advantage is concentrated in stress-relaxation behaviour and in the upper part of the temperature range for lightly loaded parts. A designer who needs the highest possible strength in a bolted joint or a structural fitting should be looking at 718. A designer who needs a spring or a seal element to hold its load at 600 °C for years should be looking at X-750, because that is a relaxation problem and not a tensile-strength problem. Confusing the two is a common error: a 718 spring can have a higher initial load than the X-750 spring it replaces and still lose that load faster in service, because the controlling property is the resistance of the precipitate to coarsening at temperature.
Two further property differences matter in practice. First, X-750 is notch-sensitive in some ageing conditions and is normally selected with the ageing cycle matched to the application; a part aged for maximum hardness may suffer an unacceptable reduction in notch-rupture life, which is why the governing specification for critical parts often constrains the ageing cycle rather than permitting the producer to choose it. Second, both alloys work harden readily, so cold-drawn wire and cold-rolled strip are supplied at higher strength and lower ductility than the annealed product, and a spring specification that calls for a temper or a cold-worked condition is specifying a different product from an annealed bar order. Where a component requires both strength and formability, that conflict must be resolved explicitly in the specification.
Heat Treatment: Solution Treatment and Ageing Cycles
Both alloys require a two-stage thermal treatment: a high-temperature solution treatment to dissolve the precipitates and homogenise the structure, followed by a controlled ageing cycle that precipitates the strengthening phase. The cycles are not interchangeable between the alloys, and within each alloy there is a choice of ageing cycles that trades strength against ductility and stress-rupture life.
| Parameter | Inconel X-750 | Inconel 718 | Basis |
|---|---|---|---|
| Solution treatment (typical) | ~1,090-1,150 °C, rapid cool | ~940-1,010 °C, rapid cool | typical commercial practice |
| Ageing cycle, maximum strength (typical) | ~730 °C, hold, air cool | not the standard cycle for 718 | typical commercial practice |
| Ageing cycle, balanced properties (typical) | ~730 °C then ~620 °C, two steps | ~720 °C for 8 h, furnace cool to ~620 °C for 8 h | AMS 5662 / AMS 5667 |
| Alternative ageing cycle (typical) | ~885 °C for 24 h then ~705 °C for 20 h | ~760 °C for 10 h then ~650 °C for 8 h | AMS 5662 / AMS 5667 |
| Cooling during ageing | controlled, per specification | controlled furnace cool between steps | AMS 5662 |
| Effect of cycle choice | trades hardness against notch-rupture life | trades tensile strength against stress-rupture life | published metallurgy |
| Condition supplied if not stated | producer's standard cycle | solution treated or aged per order | order requirement |
| Post-weld ageing | required after welding | normally performed after welding | fabrication practice |
Table note: The cycles shown are typical commercial practice for the two alloys; the governing specification on the purchase order - AMS 5662, AMS 5663, AMS 5667 or the corresponding ASTM document - fixes the actual requirement, and the certified heat treatment record for the delivered lot is the document that proves compliance. Where an ageing cycle is not stated on the order, the producer will supply the condition it normally uses, which may differ in strength and notch-rupture life from the condition the design assumed. Furnace qualification and temperature uniformity for these cycles are governed by the applicable heat-treatment specification (AMS 2750 for aerospace work). Solution treatment temperature is not a range to be adjusted for convenience: it controls grain size, which in turn controls creep and rupture properties.
The reason the ageing cycle is a design decision rather than a shop detail is that these alloys respond to temperature with a combination of precipitation and coarsening. In X-750, a single-step age near 730 °C produces the highest hardness and tensile strength, while the two-step cycles run at lower secondary temperatures produce slightly lower strength with better stress-rupture and notch-rupture behaviour, and the 885 °C plus 705 °C cycle gives the best creep resistance at the cost of some tensile strength. In 718, the standard two-step cycle of about 720 °C followed by about 620 °C produces the balance of yield strength, ductility and rupture life that the aerospace specifications were written around, and deviating from it without a test programme is not advisable. The practical consequence for procurement is that a drawing which says only "Inconel 718, aged" or "Inconel X-750, heat treated" does not define the delivered property set, and the correct wording names the governing specification and, where the part is critical, the ageing cycle.
One further point is worth making about dimensional control. Both alloys change dimension slightly during ageing - the precipitation reaction and the relief of residual stress both produce small movements - so finish machining should be planned around the heat treatment sequence rather than after it, and a part that is finished to final tolerance before ageing may need rework afterwards. Where the drawing is dimensionally critical, the correct approach is to agree the process route, including the point at which ageing occurs, before the material is issued to the machine shop.
Weldability, Strain-Age Cracking and Repair
The single most important practical difference between these two alloys is weldability, and it is the reason 718 became the dominant precipitation-hardening superalloy in the world. Inconel 718 can be welded in the solution-treated condition, and it can be repaired and re-aged, because its gamma-double-prime precipitate forms slowly enough that the weld and heat-affected zone do not harden faster than they can accommodate the stresses of cooling. Inconel X-750 hardens quickly by gamma-prime precipitation and is therefore susceptible to strain-age cracking: if the heat-affected zone begins to strengthen while welding or post-weld cooling stresses are still relaxing, the material cracks before it can be inspected.
| Fabrication parameter | Inconel X-750 | Inconel 718 | Basis |
|---|---|---|---|
| Weldability in the aged condition | poor, not recommended | limited, procedures exist | published practice |
| Weldability in the solution-treated condition | possible with qualified procedure | good | published practice |
| Strain-age cracking risk | high in thick sections and restrained joints | low | published metallurgy |
| Filler metal | matching nickel-chromium filler per specification | matching nickel-chromium-iron filler per specification | AWS A5.14 |
| Pre-weld condition | solution treated, per procedure | solution treated, per procedure | fabrication practice |
| Post-weld heat treatment | ageing required, cycle per specification | ageing normally required after welding | fabrication practice |
| Repair welding | high risk, requires approved procedure and re-ageing | routine, with approved procedure | published practice |
| Machining in the aged condition | good, high tool wear | good, high tool wear, controlled cutting data | published practice |
| Distortion risk during ageing | moderate, plan process route | moderate, plan process route | fabrication practice |
Table note: The table summarises typical published fabrication practice and is not a substitute for a qualified welding procedure specification. Filler metals are selected to match the base material under AWS A5.14 and, for aerospace work, under the applicable AMS filler-metal specification. Weld procedure qualification, heat input control, interpass temperature limits and post-weld ageing requirements are established by the applicable code or by the purchaser's specification, and welding of these alloys without a qualified procedure is a common source of rejectable work.
The consequence for design is that 718 is the alloy of choice whenever the component must be fabricated by welding, repaired in service or built up by additive processes. Large welded structures, fabricated casings, welded impellers, downhole tool bodies and additively manufactured parts are all natural 718 applications precisely because the alloy tolerates the thermal history that welding imposes. X-750, by contrast, is best thought of as a wrought or machined product: springs, rings, fasteners, seals and small machined parts where welding is avoided or where a fully qualified, tightly controlled procedure has been developed and proven. Attempting to substitute X-750 for 718 in a welded fabrication to save cost, or because material is available, usually ends in a rejected or cracked fabrication.
Filler metal selection follows the same logic as the base alloy. Both alloys are welded with matching nickel-based fillers selected under AWS A5.14 and the applicable AMS document, and both require cleanliness discipline at the joint: sulphur, lead and other low-melting-point contaminants embrittle nickel alloys at elevated temperature, so marking crayons, lubricants, cutting fluids and shop dirt must be excluded from the joint and the surrounding area. For 718, the more common practical issue is heat input control and the sequencing of the post-weld ageing treatment; for X-750, the more common practical issue is whether welding should be performed at all. Where a customer's design requires welding on an X-750 component, we ask for the drawing and the qualified procedure before quoting rather than after, because the answer sometimes is that the component should be redesigned in 718.
High-Temperature Springs, Fasteners and Stress Relaxation
Springs and bolted joints fail at temperature by relaxation, not by tensile overload, and this is the domain where X-750 has retained its position against 718 despite being the older alloy. A spring specified in terms of load at room temperature can lose a substantial part of that load after long exposure at 550-650 °C, and the rate of loss depends on the stability of the strengthening precipitate, the initial stress level, the operating temperature and the wire or strip condition.
| Application | Preferred alloy | Why |
|---|---|---|
| High-temperature compression and extension springs | X-750 | better stress-relaxation resistance in the 540-650 °C band |
| Retaining rings, snap rings and seal elements | X-750 | relaxation resistance plus available cold-worked tempers |
| Gas turbine seal and shroud components | X-750 | high-temperature relaxation plus oxidation resistance |
| Cryogenic springs and fasteners | X-750 | traditional material, no ductile-to-brittle transition |
| High-strength bolting for pressure and process service | 718 | highest aged yield strength, widely qualified |
| Sour-service bolting and wellhead components | 718 | recognised material under NACE MR0175 / ISO 15156 |
| Downhole tool bodies, mandrels and adapters | 718 | strength plus weldability for fabrication and repair |
| Large forged discs, shafts and structural fittings | 718 | availability in large forging sizes |
| Aerospace structural and engine hardware | both, per specification | 718 for structural and welded parts, X-750 for springs and seals |
| Fasteners requiring the least relaxation at 600 °C | X-750 | gamma-prime resistance to over-ageing |
Table note: The recommendations reflect the typical published relaxation, strength and fabrication behaviour of the two alloys and are engineering guidance rather than standard requirements. For bolted joints where relaxation is critical, the design should be based on published relaxation data for the specific alloy, condition, initial stress and temperature, and confirmed by a joint test where the consequence of load loss is significant. For sour-service applications, the material and hardness limits are set by NACE MR0175 / ISO 15156, and compliance must be verified against the current edition of that standard rather than assumed from a generic material datasheet.
Two practical points govern most spring and joint failures in these alloys. The first is that relaxation is strongly dependent on the initial stress level: a spring or bolt stressed closer to its yield strength relaxes faster and further than one operating at a lower stress fraction, so reducing the working stress is often a more effective remedy than changing material. The second is that surface condition and residual stress matter: shot peening, controlled cold work and a properly specified ageing cycle all influence relaxation performance, and a change of supplier that alters any of them can change the spring rate loss over the life of the component even though the material specification and the chemistry are nominally identical. Where a customer has a qualified spring or fastener that has been in service for years, the safest course is to reproduce the drawing, the condition and the surface treatment exactly, and to treat any proposed change in processing route as an engineering change requiring requalification.
Selection Matrix: Which Alloy for Which Duty?
The selection between X-750 and 718 reduces to a short sequence of questions, and the matrix below maps common duties to a recommendation. The logic is consistent throughout: 718 where strength, weldability and availability govern; X-750 where relaxation resistance or oxidation resistance at the top of the temperature range governs.
| Duty / requirement | Recommended alloy | Note |
|---|---|---|
| Welded fabrication or repair requirement | 718 | X-750 is not a practical welded fabrication material |
| Highest aged yield strength required | 718 | substantial advantage over X-750 |
| Spring or bolt must hold load at 550-650 °C | X-750 | better relaxation resistance |
| Sustained exposure to oxidising gas above 700 °C | X-750 | molybdenum in 718 limits oxide-scale stability |
| Large forging or thick section required | 718 | much wider availability in heavy sections |
| Sour service to NACE MR0175 / ISO 15156 | 718 | recognised and widely qualified material |
| Cryogenic spring or fastener | X-750 | traditional selection, proven at low temperature |
| Low cost and short lead time are dominant | 718 | larger production volume, better availability |
| Component must be machined from bar, no welding | either, decide on relaxation or strength | 718 if strength governs, X-750 if relaxation governs |
| Highly restrained joint subject to thermal cycling | 718 | X-750 carries strain-age cracking risk |
Table note: The recommendations reflect the typical published properties, fabrication behaviour and market availability of the two alloys and are engineering guidance rather than standard requirements. Final selection for safety-critical or high-value equipment must consider the actual stress, temperature, atmosphere, cycling and required life, and must satisfy the applicable design code and any application-specific standard such as NACE MR0175 / ISO 15156 for sour service. Where a component in one alloy has failed, the correct response is to establish the failure mechanism before substituting the other alloy, because relaxation, fatigue, oxidation and overload produce different remedies.
Three rules are worth stating as rules rather than as rows in a table. First, if the component is to be welded, the answer is 718 unless there is a specific reason that X-750 is required and a qualified procedure exists. Second, if the component is a spring, seal or retaining ring that must hold load at temperature, the answer is normally X-750, because that is a relaxation problem and 718 does not solve it better. Third, if the requirement is simply high strength at room temperature or moderate temperature in a machined part, 718 is usually both cheaper and more readily available, and there is no reason to specify X-750.
Price Reference (2026, EXW Shanghai)
Both alloys are nickel-based materials, and X-750 contains substantially more nickel than 718 - approximately 70 % minimum against 50-55 % - which gives X-750 the higher base-metal cost. That relationship is partly offset by 718's niobium content and by the very much larger production volume in which 718 is made, so the realised price difference depends on the product form and the size required.
| Product form | Inconel X-750 | Inconel 718 | Note |
|---|---|---|---|
| Round bar | USD 38-60/kg | USD 30-50/kg | availability favours 718 |
| Plate and sheet | USD 42-70/kg | USD 35-58/kg | thickness and width affect yield |
| Wire and spring wire | USD 45-80/kg | quote by size | diameter and temper dominate |
| Seamless tube and pipe | USD 55-95/kg | USD 45-80/kg | size and wall thickness dominate |
| Large forgings | quote by drawing | quote by drawing | 718 available in much larger sections |
| Strip and cold-rolled product | USD 45-75/kg | USD 38-62/kg | temper and tolerance regime matter |
Table note: Reference range only, 2026, EXW Shanghai, USD/kg. These figures float with the LME nickel price and are not a quotation. The bands are wide because price depends strongly on quantity, product form, size, temper and the certification and testing regime required; a qualified aerospace order with full traceability and third-party release sits at the top of a range, while a standard commercial certificate order sits lower. Availability, rather than price per kilogram, is frequently the deciding factor for X-750 in non-standard sizes, and for large 718 forgings the lead time rather than the unit price usually dominates the commercial decision.
The commercial conclusion is straightforward. Inconel 718 is the lower-cost and far more available material in almost every product form, and it should be the default selection wherever its strength, weldability and corrosion performance meet the duty. Inconel X-750 should be specified when its specific advantages are genuinely required - relaxation resistance in a spring or joint, oxidation resistance at the top of the temperature range, or a proven cryogenic application - because paying the higher nickel cost for a duty that 718 would serve is a straightforward waste. Where the component is a spring or a fastener with a long qualified history in X-750, the safe course is to reproduce the existing specification exactly rather than to re-engineer a working part.
Standard Index
| Standard | Title / scope | Covers | Form |
|---|---|---|---|
| ASTM B637 | Precipitation-hardening and cold-worked nickel alloy bars, forgings and forging stock for moderate or high temperature service | composition + mechanical + heat treatment | bar, forging, forging stock |
| ASTM B670 | Precipitation-hardening nickel alloy (UNS N07718) plate, sheet and strip for high-temperature service | composition + mechanical | plate, sheet, strip |
| AMS 5662 | Nickel alloy (UNS N07718) bars, forgings and rings, solution heat treated and precipitation hardened | mechanical + heat treatment | bar, forging, ring |
| AMS 5663 | Nickel alloy (UNS N07718) bars, forgings and rings, solution heat treated and precipitation hardened (variant requirements) | mechanical + heat treatment | bar, forging, ring |
| AMS 5667 | Nickel alloy (UNS N07750) bars, forgings and rings, solution heat treated and precipitation hardened | mechanical + heat treatment | bar, forging, ring |
| AMS 5542 / AMS 5596 / AMS 5597 | Nickel alloy flat product, solution heat treated and precipitation hardened | mechanical + heat treatment | sheet, plate, strip |
| AMS 2750 | Pyrometry - furnace class and temperature uniformity requirements | heat treatment control | - |
| 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 / E572 | Metals identification by PMI and by X-ray spectrometry | test method | - |
| AWS A5.14 | Nickel and nickel-alloy bare welding electrodes and rods | consumable selection | filler wire |
| NACE MR0175 / ISO 15156 | Materials for use in H2S-containing environments in oil and gas production | material and hardness limits | all forms |
| 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 AMS document numbers are listed for the aerospace family of requirements and the exact document applicable to a given product form and condition must be confirmed against the order. Entries from the Chinese designation system are shown for cross-system recognition only: GH4145 corresponds to the Inconel X-750 type of alloy and GH4169 to the Inconel 718 type, but neither designation is an ASTM or AMS equivalent.
FAQ
Q1: What is the difference between Inconel X-750 and Inconel 718?
The fundamental difference is the precipitate that gives each alloy its strength. Inconel X-750 is hardened by gamma-prime, an ordered nickel-aluminium-titanium phase that forms relatively quickly from its 2.25-2.75 % titanium and 0.40-1.00 % aluminium content. Inconel 718 is hardened predominantly by gamma-double-prime, a nickel-niobium phase formed from its 4.75-5.50 % niobium plus tantalum content, with molybdenum providing additional solid-solution strengthening. That metallurgical difference explains everything practical about the two alloys. Because gamma-double-prime precipitates slowly, 718 can be welded, repaired and re-aged without cracking, and it achieves the higher yield strength in the aged condition. Because gamma-prime precipitates quickly and resists coarsening in the 540-650 °C band, X-750 makes a better spring and a better high-temperature fastener, and it resists stress relaxation longer. X-750 also carries more nickel, around 70 % minimum against 50-55 % in 718, making it the more expensive base material.
Q2: Which alloy is better for high-temperature springs?
Inconel X-750 is the traditional and generally better choice for springs that must hold load at elevated temperature. A spring does not fail by tensile overload, it fails by stress relaxation - the gradual loss of load as the material deforms slowly under sustained stress at temperature - and relaxation resistance depends on how stable the strengthening precipitate is under prolonged exposure. X-750's gamma-prime precipitate resists coarsening better than 718's gamma-double-prime phase in the 540-650 °C band, so an X-750 spring retains more of its initial load over the same period. This is why gas turbine seals, retaining rings, seal elements and high-temperature compression springs have historically been specified in X-750. That said, relaxation is also strongly influenced by the initial stress level, the wire or strip temper, the surface condition and the ageing cycle, so a 718 spring operating at a lower stress fraction can outperform an X-750 spring operating near its limit. Our other articles on high-temperature alloy behaviour cover how relaxation data should be used in design.
Q3: Why is Inconel 718 easier to weld than X-750?
Inconel 718 is easier to weld because its gamma-double-prime precipitate forms slowly, which gives the heat-affected zone time to accommodate welding stresses before it hardens. Inconel X-750 hardens much more quickly by gamma-prime precipitation from its titanium and aluminium content, so a welded joint can begin to strengthen while residual stresses from welding are still relaxing, producing strain-age cracking in the heat-affected zone. This form of cracking is particularly dangerous because it can occur after the weld has cooled and sometimes after inspection, and it is most severe in thick sections and highly restrained joints. The practical consequence is that 718 is the standard choice for welded fabrications, for repair work and for additively manufactured parts, while X-750 is generally treated as a wrought or machined material for springs, rings and fasteners. Where welding on X-750 is unavoidable, it must be done in the solution-treated condition under a fully qualified procedure with controlled heat input, followed by the specified ageing treatment.
Q4: Can Inconel X-750 be used for sour gas service fasteners?
Inconel 718 is the recognised high-strength fastener material for sour service under NACE MR0175 / ISO 15156, and it is the alloy most commonly specified for wellhead, valve and downhole bolting where both high strength and resistance to sulphide stress cracking are required. X-750 is also used in sour service applications in some specifications, but it is less commonly qualified for high-strength bolting than 718 and its acceptability depends on the specific hardness, condition and application limits of the current edition of the standard. The controlling factors in sour service are the material's hardness and yield strength together with the severity of the environment expressed as H2S partial pressure, and the standard sets limits on all of them. The practical guidance is straightforward: for a new sour-service design, specify 718 aged to the hardness limits of the applicable standard, confirm compliance against the current edition, and do not assume that because an alloy performs well in a general corrosion test it will be acceptable in a sour environment.
Q5: Are these alloys suitable for cryogenic service?
Yes, both alloys are used at very low temperatures, and neither exhibits the ductile-to-brittle transition that limits ferritic steels. Inconel X-750 is a long-established cryogenic material for fasteners and springs and is used at temperatures down to about -253 °C in liquid hydrogen service, which is well below the range in which most structural metals become brittle. Inconel 718 is likewise used in cryogenic valve components, propulsion hardware and similar applications, and its combination of strength and toughness at low temperature is well documented. The selection between them at cryogenic temperature follows the same logic as at room temperature: 718 where the highest strength and weldability are required, X-750 where a spring or fastener with proven cryogenic qualification already exists. Because both alloys remain austenitic and tough at low temperature, the design concerns shift to thermal contraction, differential expansion between dissimilar materials in the assembly, and the effect of the low temperature on any non-metallic components. Our technical knowledge centre covers the low-temperature material selection principles we apply in enquiry review.
Q6: Which alloy has better oxidation resistance at high temperature?
Inconel X-750 has the better oxidation resistance in sustained high-temperature exposure, and this is one of the most counter-intuitive facts in alloy selection because 718 contains more chromium - 17.0-21.0 % against 14.0-17.0 % in X-750. The reason is molybdenum. Inconel 718 contains 2.80-3.30 % molybdenum for solid-solution strengthening, and molybdenum is detrimental to the stability of the protective chromium oxide scale, because it forms volatile oxides that disrupt the scale and accelerate attack. Inconel X-750 contains no intentional molybdenum addition and carries more aluminium, which supports a more protective and more spallation-resistant scale. The practical result is that X-750 is quoted for service in oxidising gas to roughly 815 °C continuous and about 980 °C intermittent as typical published guidance, while 718 under sustained load and continuous exposure is normally limited to about 650-700 °C. Chromium content alone is therefore a poor predictor of oxidation performance, and the full chemistry must be considered.
Q7: What ageing condition should I specify on the purchase order?
Specify the governing specification and the ageing cycle, not merely the words "aged" or "heat treated". Both alloys are available in the solution-treated condition and in several aged conditions that produce materially different strength, ductility and notch-rupture properties, and an order that does not state the condition leaves the producer to supply its standard cycle, which may not match the properties the design assumed. For Inconel 718, the conventional aerospace cycle is a two-step treatment of roughly 720 °C followed by roughly 620 °C, as fixed by AMS 5662 or AMS 5663, and for Inconel X-750 there is a choice between a single-step age near 730 °C for maximum strength and two-step or higher-temperature cycles that trade some strength for improved rupture and notch-rupture behaviour. The correct wording names the standard and, where the part is critical, the cycle. It is also worth stating whether ageing is to be performed before or after final machining, because both alloys move slightly during ageing.
Q8: How do the two alloys compare in strength at 650 °C?
Inconel 718 retains the higher strength at 650 °C, as it does at room temperature, with typical published aged tensile strength in the region of 950-1,100 MPa against roughly 900-1,000 MPa for X-750, and a corresponding advantage in yield strength. The gap narrows as temperature rises, and above roughly 700 °C the comparison becomes more complicated because long-term exposure changes the microstructure of both alloys: 718's gamma-double-prime precipitate is metastable and gradually transforms towards delta phase, while X-750's gamma-prime coarsens. Under sustained load, 718's useful range for stress-rupture service is generally quoted to about 650-700 °C and X-750's to about 700 °C, with both figures being typical published guidance rather than standard limits. The practical implication is that neither alloy should be selected for sustained-load service far above 700 °C, and a duty that requires load carrying above that temperature should be reviewed against a different material family rather than resolved by choosing between these two.
Q9: Is there a cheaper alternative if Inconel 718 is not needed?
Yes, and the right alternative depends on which property is actually controlling. Where the requirement is high strength at ambient or moderate temperature with no significant corrosion load, the precipitation-hardening stainless steels such as 17-4PH provide high strength at a fraction of the cost and are far easier to machine, and they are the correct choice for many fasteners, shafts and valve parts that are currently over-specified in nickel alloy. Where moderate corrosion resistance with high strength is required, the super-austenitic and duplex stainless families offer useful combinations. Where the requirement is elevated-temperature strength above about 500 °C, however, the stainless alternatives fall away quickly and a nickel-based alloy is genuinely necessary, so the substitution must be assessed against the actual temperature and not against the room-temperature strength alone. Our precipitation-hardening and duplex stainless range covers the grades we supply for those substitution cases, and we will say plainly when a nickel alloy is not required.
Q10: How do I verify which alloy I received?
Verify by chemistry, because the two alloys are visually and dimensionally indistinguishable and because their nickel, chromium and titanium contents overlap enough to be inconclusive. The identifying lines are niobium plus tantalum, which is 4.75-5.50 % in 718 and only 0.70-1.20 % in X-750, and molybdenum, which is present at 2.80-3.30 % in 718 and is not an intentional addition in X-750. Portable optical emission spectrometry to ASTM E1476 or ASTM E572 will separate the two materials in the field, and laboratory analysis to ASTM E572 provides the documented result for a claim. Alongside the chemistry, confirm the condition: the certificate must state whether the material is solution treated or solution treated and aged, and the ageing cycle it records, because a correctly identified alloy in the wrong condition will not deliver its design properties. Reconcile the heat number marked on the material with the certificate, confirm the producing mill is named, and check that the specification and edition quoted match the purchase order. Our Inconel product range page lists the forms and certifications we hold for both grades.
Q11: Which alloy is used for aero-engine and gas turbine components?
Both are used extensively, and the split follows the weldability and relaxation logic rather than any difference in specification status. Inconel 718 dominates the welded and structural side of the engine and airframe: casings, fabricated assemblies, discs, shafts, brackets, fasteners and a large proportion of additively manufactured aerospace hardware are made in 718, because the alloy can be welded, repaired and re-aged reliably at the sizes involved. Inconel X-750 is used where its specific advantages are needed: high-temperature springs, seal rings, retaining rings, shroud and seal elements, and bolting where relaxation resistance matters. Gas turbine hot-section components that must operate above the range of both alloys are generally made in other families entirely, such as the gamma-prime strengthened alloys for turbine blades and the cobalt-bearing solid-solution alloys for combustion hardware. The practical point for procurement is that both alloys are usually specified by an AMS document rather than by a generic trade name, and the drawing should always name the applicable AMS specification and condition.
Q12: What does the GB designation GH4145 or GH4169 mean on a drawing?
GH4145 is the commonly used Chinese designation for the alloy corresponding to the Inconel X-750 type and GH4169 corresponds to the Inconel 718 type. They are widely used commercially and their chemistries are close to the western alloys, which is why they are frequently shown as cross-references on drawings and certificates. They are not ASTM or AMS equivalents, however, and the distinction matters because the GB designation is written against a different standard system with its own composition limits, property minima, testing requirements and inspection documents. A heat produced to GH4169 is not automatically compliant with AMS 5662 or ASTM B637, and a purchase order that names both without saying which governs creates an ambiguity that will eventually surface at inspection. Where a drawing originates in China and will be manufactured to a western specification, or the reverse, the conversion should be stated explicitly with the source system identified in its own column, and the delivered material should be verified against the specification actually named on the order.
Conclusion and Selection Rules
The selection rules for these two alloys are short and they resolve almost every real case. Use Inconel 718 when the component has to be welded, repaired or produced by additive manufacturing; when the highest aged yield strength is required; when the part is a large forging or a thick section; when sour-service qualification under NACE MR0175 / ISO 15156 is needed; and when cost and availability are significant. Use Inconel X-750 when the component is a spring, seal, retaining ring or fastener that must hold load at 550-650 °C; when sustained exposure to oxidising gas above roughly 700 °C is involved; when a proven cryogenic spring or fastener design already exists in the alloy; and when the part is machined or formed rather than welded.
Two of the rules are constraints rather than preferences. Inconel X-750 is not a practical welded fabrication material, and specifying it for a welded assembly will eventually produce strain-age cracking in the heat-affected zone unless a fully qualified procedure exists and is followed. Inconel 718 is not the correct selection for sustained-load service far above about 700 °C, because its metastable gamma-double-prime precipitate transforms towards delta phase and its rupture strength falls away. Everything else is a trade of cost and availability against relaxation and oxidation performance.
Shanghai Hangbo Alloy Group Co., Ltd. supplies Inconel X-750 and Inconel 718 as bar, wire, plate, sheet, strip, tube and forgings, in the solution-treated and aged conditions, with mill test certification, PMI to ASTM E1476 or E572 where required, and third-party inspection by SGS, BV or TUV on request. We supply springs and fastener stock in X-750 and structural, welded and sour-service grades in 718, and we will review your drawing, condition requirement and application before quoting so that the alloy and the ageing cycle are matched to the duty. Send your requirement through our contact page and we will return a material recommendation with the governing specification citations and a costed quotation. For closely related selection questions, see our other technical comparison articles.
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Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier
Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com
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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
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