Inconel 718 vs Nimonic 263: Alloy Selection Guide

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 718 vs Nimonic 263

Inconel 718 wins where maximum strength from room temperature to about 650 C is the deciding requirement, and Nimonic 263 wins for welded sheet fabrications in long-term service above 700 C. Choose 718 for casings, forgings and bolting; choose 263 for combustor cans, liners and exhaust ducting.

Key Takeaways

• The strengthening mechanism decides the application. Inconel 718 is strengthened mainly by gamma double prime (Ni3Nb) with molybdenum in solution, while Nimonic 263 is strengthened by gamma prime plus cobalt and molybdenum in solution; that single metallurgical difference explains almost every practical selection decision between them.
• Inconel 718 is the strength champion, not the temperature champion. Its yield strength at room temperature is the highest of any readily weldable superalloy, but above roughly 650 C the metastable gamma double prime transforms toward delta phase and the alloy loses the advantage that justified its selection.
• Nimonic 263 is the long-soak, weld-fabrication champion. Its cobalt-bearing chemistry is unusually resistant to sigma and other topologically close-packed phases, so it holds properties through thousands of hours at 700-850 C, exactly the regime of combustor cans, liners and exhaust ducting.
• For sheet metal work, 263 is the easier alloy. It tolerates forming and welding with far less risk of strain-age cracking than 718, and it usually does not need a full post-weld re-solution treatment to restore ductility in a large welded assembly.
• For heavy sections, forgings and bolting, 718 is the better economics. It is the most widely produced precipitation-hardening nickel alloy in the world, available in every product form to ASTM B637, ASTM B670 and AMS 5662 / 5663, at a materially lower cost than a 20 % cobalt alloy.
• Neither alloy is universally better. The correct choice is a function of service temperature, soak time, section thickness, weldability and budget; the selection table later in this article states a winner for each condition and the exceptions that override it.

What Inconel 718 and Nimonic 263 Actually Are

Inconel 718 and Nimonic 263 are both precipitation-hardenable nickel-base superalloys, and both are bought for hot structural components rather than for corrosion resistance, but they were designed against different problems and they behave very differently in service. Inconel 718, UNS N07718, is a nickel-chromium-iron alloy hardened by niobium and molybdenum; Nimonic 263, UNS N07263, is a nickel-chromium-cobalt-molybdenum alloy hardened by titanium and aluminium with cobalt playing a supporting role. Both are supplied as bar, forgings, plate, sheet and strip, and both can be welded, which is why they are so often compared for the same gas turbine and exhaust components.

The difference in intent is worth setting out because it frames everything that follows. Inconel 718 was developed as a high-strength alloy that could be welded and would resist strain-age cracking during post-weld heat treatment, at a time when the gamma-prime-hardened alloys of the day cracked badly when welded in thick sections. Its designers accepted a modest temperature ceiling, roughly 650 C for long-term service, in exchange for exceptional strength and weldability. Nimonic 263 was developed a few years later for a different duty: gas turbine combustor components and ducting that would soak for thousands of hours at 700-850 C, be fabricated from thin sheet, and be welded into large assemblies. Its designers traded some peak strength for outstanding long-term metallurgical stability, and added cobalt specifically to suppress the brittle intermetallic phases that degrade the gamma-prime alloys in that temperature band.

Element (wt %, typical)	Inconel 718 (UNS N07718)	Nimonic 263 (UNS N07263)	Function	Standard source
Nickel	balance	balance	matrix	ASTM B637 / ASTM B670
Chromium	17.0-21.0	19.0-21.0	oxidation and hot corrosion resistance	ASTM B637 / ASTM B670
Iron	balance of the remainder (typically 17-19)	max 0.7 (residual)	matrix in 718 only	ASTM B637 / ASTM B670
Cobalt	max 1.0	19.0-21.0	raises solvus, stabilises gamma prime	ASTM B637 / ASTM B670
Molybdenum	2.8-3.3	5.6-6.1	solid-solution strengthener	ASTM B637 / ASTM B670
Niobium + tantalum	4.75-5.50	not specified	forms gamma double prime in 718	ASTM B637 / ASTM B670
Titanium	0.65-1.15	1.9-2.4	forms gamma prime	ASTM B637 / ASTM B670
Aluminium	0.20-0.80	0.3-0.6	forms gamma prime	ASTM B637 / ASTM B670
Carbon	max 0.08	0.04-0.08	carbide former	ASTM B637 / ASTM B670
Copper	max 0.30	max 0.2	residual	ASTM B637 / ASTM B670

Table note: The ranges above are typical published composition limits for the two grades and are given to show the metallurgical contrast rather than to replace the purchase specification; the controlling chemistry is always the one named on the order, normally ASTM B637 for bar and forgings, ASTM B670 for plate, sheet and strip, with ASME SB637 as the identical ASME designation. The decisive entries are the niobium-plus-tantalum addition of roughly 5 % in Inconel 718, which has no counterpart in Nimonic 263, and the cobalt addition of roughly 20 % in Nimonic 263, which is capped near 1 % in Inconel 718. Those two lines of the table are the whole reason the two alloys behave differently in a hot structure.

The commercial picture follows directly from the chemistry. Inconel 718 contains no cobalt and only about 9 % of combined chromium plus molybdenum alloying, so it can be melted and processed economically at very large tonnage and is available from every mill in the world in every form from 6 mm bar to 100 mm plate. Nimonic 263 contains roughly 20 % cobalt, a metal whose price is volatile and often several times that of nickel, so 263 is inherently more expensive per kilogram at equal processing cost and is produced in a narrower range of forms, concentrated on sheet, strip, plate and bar for combustion-section fabrication. Our Inconel alloy range and Nimonic alloy range illustrate that split, and the practical consequence for a buyer is that 718 should be the default unless 263 is genuinely required by the service temperature.

Gamma Double Prime vs Gamma Prime: How Each Alloy Gets Its Strength

The mechanical behaviour of both alloys comes from coherent precipitates that strain the nickel matrix and block dislocation motion, but the precipitate is different in each case and that difference governs the temperature ceiling. Inconel 718 is strengthened predominantly by gamma double prime, a body-centred-tetragonal Ni3Nb phase that forms on ageing, with a smaller contribution from gamma prime and from niobium-rich carbides at grain boundaries. Nimonic 263 is strengthened by gamma prime, the ordered face-centred-cubic Ni3(Al,Ti) phase found in most high-temperature nickel alloys, stabilised and refined by the 20 % cobalt in the matrix. Gamma prime is thermodynamically stable to high temperature; gamma double prime is metastable and that metastability is the key to the whole comparison.

Gamma double prime produces more strengthening per unit volume than gamma prime because its tetragonal distortion of the matrix is larger, which is why Inconel 718 reaches yield strengths around 1100-1200 MPa in the fully aged condition, among the highest of any weldable superalloy. The price of that strength is thermal instability. Above roughly 650 C, gamma double prime progressively transforms into delta phase, an orthorhombic Ni3Nb precipitate that is incoherent with the matrix, forms preferentially on grain boundaries as platelets, and contributes far less strengthening while consuming the niobium that the gamma double prime needs. Long-term exposure therefore produces a double penalty in 718: the hardening precipitate is depleted and the grain boundaries are decorated with a phase that promotes notch sensitivity and reduces creep ductility.

Gamma prime behaves differently. It is the equilibrium precipitate, and in Nimonic 263 the combination of about 2 % titanium with a matrix enriched in cobalt raises the gamma prime solvus and slows the coarsening that destroys strength over time. Cobalt does not itself form a precipitate; it partitions between the matrix and gamma prime, altering the lattice mismatch and the stacking-fault energy, and in doing so it lowers the driving force for the unwanted topologically close-packed phases, particularly sigma phase, that plague simpler nickel-chromium-molybdenum alloys during long soaks. The result is an alloy that starts with less peak strength than 718 but retains a much larger fraction of its strength after thousands of hours at 750-800 C.

Feature	Inconel 718 (gamma double prime)	Nimonic 263 (gamma prime)	Consequence
Main hardening phase	gamma double prime, Ni3Nb, tetragonal	gamma prime, Ni3(Al,Ti), ordered cubic	718 strengthens more; 263 strengthens more stably
Stability of phase	metastable above about 650 C	stable to well above 800 C	sets the service temperature ceiling
Deleterious transformation	gamma double prime to delta phase	slow gamma prime coarsening	718 embrittles; 263 loses strength gradually
Role of cobalt	capped near 1 %, no role	about 20 %, raises solvus, suppresses TCP phases	263 more stable but more costly
Role of molybdenum	about 3 %, solid solution	about 6 %, solid solution	both resist creep, 263 more so at temperature
Typical aged yield strength	about 1100-1200 MPa	about 700-800 MPa	718 wins on strength at any temperature below its ceiling
Governing standards	AMS 5662, AMS 5663, AMS 5596	ASTM B637, ASTM B670	both fully standardised
Test methods for verification	ASTM E8, ASTM E21, ASTM E139	ASTM E8, ASTM E21, ASTM E139	identical qualification route

Table note: The table compares the two strengthening systems and the standards and test methods that verify them, and the property figures are typical published values in the fully heat-treated condition rather than guaranteed minima; the mechanical minima applicable to a delivery are those of the governing standard, for example AMS 5662 and AMS 5663 for Inconel 718 bar and forgings, AMS 5596 for 718 sheet, strip and plate, and ASTM B637 or ASTM B670 for the two alloys in bar, forging, plate, sheet and strip form. Tension testing is normally to ASTM E8 at room temperature and ASTM E21 at elevated temperature, and creep and stress-rupture testing, where specified, is to ASTM E139. The single sentence to carry away is that 718 buys strength with a metastable phase and pays for it above about 650 C, whereas 263 buys stability with an equilibrium phase and pays for it in cost and in peak strength.

The practical reading of that table is straightforward. If a component spends its life below roughly 650 C and the design is driven by strength, stiffness or bolt preload, the gamma double prime route gives more load capacity per kilogram and per dollar. If a component soaks above 700 C for thousands of hours, the gamma prime route with cobalt protection gives a structure that stays sound, because the failure mode being avoided is not a loss of tensile strength but the slow precipitation of delta phase (in 718) or sigma phase (in the simpler nickel-chromium-molybdenum alloys) that turns a ductile, forgiving structure into a notch-sensitive one.

Mechanical Properties and Temperature Capability

Inconel 718 delivers the higher strength at every temperature below its ceiling, but its usable temperature is capped by the phase transformation described above, whereas Nimonic 263 gives up peak strength in exchange for a service ceiling roughly 150-200 C higher. In round numbers, aged Inconel 718 reaches a room-temperature yield strength of about 1100-1200 MPa and a tensile strength of about 1350-1450 MPa, and it retains useful strength to about 650 C. Aged Nimonic 263 reaches a room-temperature yield strength of about 700-800 MPa with a tensile strength of about 950-1050 MPa, and it is specified for long-term service up to roughly 800-850 C, where 718 would already be degrading.

Those figures need two qualifications. The first is that they are typical values for material in the standard solution-annealed and aged condition, not guaranteed minima; a buyer should take the guaranteed figures from the governing standard or the mill certificate and treat every number in this section as indicative. The second is that the decisive property at temperature is rarely tensile strength at all. For a combustor can or a transition duct, the design is limited by creep and stress-rupture life, by thermal fatigue, and by oxidation and hot corrosion, so the useful comparison is the stress that produces rupture in a given time at a given temperature, and the time that a structure retains ductility during a long soak. In those comparisons the gap between the alloys closes and reverses.

Property (typical, aged condition)	Inconel 718	Nimonic 263	Basis
Room-temperature yield strength	about 1100-1200 MPa	about 700-800 MPa	ASTM E8
Room-temperature tensile strength	about 1350-1450 MPa	about 950-1050 MPa	ASTM E8
Room-temperature elongation	about 12-20 %	about 20-30 %	ASTM E8
Hardness, Rockwell C	about 36-44 HRC	about 24-32 HRC	ASTM E18
Long-term service ceiling	about 650 C	about 800-850 C	engineering practice
Short-term excursion ceiling	about 700 C	about 900 C	engineering practice
Creep and rupture behaviour	strong to 650 C, degrades above	stable to 800 C and beyond	ASTM E139
Thermal expansion, 20-800 C	about 16-17 x 10-6 /K	about 15-17 x 10-6 /K	typical data
Density	about 8.19 g/cm3	about 8.36 g/cm3	typical data
Magnetic behaviour	weakly ferromagnetic when aged	essentially non-magnetic	typical data

Table note: Every strength, ductility and physical figure in the table is a typical value for the fully heat-treated condition and is provided for comparison only; guaranteed minima come from the controlling standard, and the elevated-temperature and rupture behaviour should be taken from ASTM E21 and ASTM E139 testing on the actual heat rather than from a generic table. Two entries deserve elaboration. The hardness difference matters because a 718 component in the 40 HRC range is at or near the hardness limit that sour-service specifications impose for some wet environments, while 263 sits comfortably below it; conversely, the higher hardness of 718 is what makes it a good bolting material. The magnetic entry matters because aged 718 is slightly ferromagnetic, which is occasionally prohibited in instrumentation or magnetically sensitive applications where 263 is preferred.

The most common specification error in this pair is to select Inconel 718 for a component whose true duty is well above its ceiling and to justify the choice with a short-term tensile figure. A short-term test at 700 C or 750 C can look convincing, because 718 does not lose all of its strength instantly; the damage is cumulative and appears as delta-phase precipitation during thousands of hours of soak, and it is detected not by a tensile test but by a rupture or ductility test after long exposure. Where the service temperature is genuinely in question, the safe and conventional approach is to choose 263 above roughly 700 C and reserve 718 for the lower-temperature, high-strength duty.

Heat Treatment of Inconel 718 and Nimonic 263

Both alloys are strengthened by a solution treatment followed by an ageing treatment, but the schedules differ because the precipitates differ, and getting the schedule wrong is the most common reason a delivery misses its mechanical properties. Inconel 718 is normally solution annealed in the range of about 954-982 C, then aged in two steps: hold at about 720 C for eight hours and cool in the furnace to about 620 C, hold for a further eight hours, and air cool. Nimonic 263 is normally solution treated higher, at about 1150 C, to dissolve the carbides and homogenise the gamma prime, then aged at about 800 C for eight hours. The higher solution temperature of 263 is not interchangeable with the 718 schedule, and the two-step age of 718 is not a substitute for the single-step age of 263.

The reason for the two-step age in Inconel 718 is the metastable nature of gamma double prime. A single low-temperature age would precipitate gamma double prime quickly but leave the grain boundaries under-decorated and the matrix prone to the delta transformation; the two-step schedule precipitates gamma double prime in the matrix while allowing a controlled amount of gamma prime and carbides to form, which is what produces the classic combination of very high strength with usable ductility. Ageing 718 above about 870 C deliberately overages it and produces an "oil patch" microstructure of delta phase, which has specific uses in applications that need lower hardness and better notch behaviour but is not the standard structural condition. In Nimonic 263 the age is designed to grow a stable gamma prime distribution without reaching the temperature at which carbides coarsen excessively.

Treatment	Inconel 718	Nimonic 263	Notes and standards
Solution anneal	about 954-982 C, hold and air or water quench	about 1150 C, hold and air cool	per AMS 5662 / AMS 5663 for 718 bar and AMS 5596 for 718 sheet
Ageing, first step	about 720 C for 8 h, furnace cool	about 800 C for 8 h, air cool	single-step age for 263
Ageing, second step	about 620 C for 8 h, air cool	not normally required	characteristic two-step age of 718
Resulting phase	gamma double prime with minor gamma prime	gamma prime with MC and M23C6 carbides	engineering metallurgy
Alternative condition	delta-processed or overaged at about 870-900 C	single solution plus age	applied only where specified
Post-weld heat treatment	often required in thick sections	usually not required for thin sheet	see welding section
Verification	ASTM E8, ASTM E21, ASTM E139, ASTM E18	ASTM E8, ASTM E21, ASTM E139, ASTM E18	testing after final heat treatment
Heat treatment documentation	EN 10204 3.1 certificate	EN 10204 3.1 certificate	on request

Table note: The schedules above are the conventional industrials treatments and are given as typical practice; the controlling schedule is the one specified on the order or referenced by the governing material standard, and for Inconel 718 bar and forgings that is normally AMS 5662 or AMS 5663, with AMS 5596 covering 718 sheet, strip and plate. Any deviation such as an overage or a delta-processed condition must be specified explicitly and agreed before manufacture, because the mechanical property requirements of the standard are tied to the specified heat treatment. Heat treatment should be documented on an EN 10204 3.1 inspection certificate, and the hardness, tensile and where applicable rupture results should be reported against the same final treatment that the parts received.

One borderline point worth flagging is that the two-step age of 718 creates a hardness in the region of 36-44 HRC that is a virtue for bolting and a problem for any application that imposes a hardness ceiling, such as certain sour-service specifications. The remedy in those cases is either a different alloy or an overaged condition with lower hardness and correspondingly lower strength; the choice should be made deliberately at the specification stage rather than discovered during final inspection.

Welding and Sheet Formability

Inconel 718 is weldable and Nimonic 263 is exceptionally weldable, and the difference becomes decisive the moment a component is a thin-sheet fabrication rather than a machined or forged part. Inconel 718 can be joined by gas tungsten arc, electron beam, laser and resistance welding, but in heavy sections it requires attention to interpass temperature and to filler selection, and it is prone to strain-age cracking when a welded assembly is post-weld aged: the ageing treatment that hardens the alloy also shrinks the weld and the heat-affected zone, and if the surrounding material cannot accommodate that contraction, cracks form. Nimonic 263 was designed to tolerate exactly this duty, with a chemistry that resists strain-age cracking and permits large welded sheet assemblies to be aged with far less risk. For Inconel 718 the recommended filler is the matching ERNiFeCr-2 type, and the choice between the matching filler and a niobium-free or a 625-type filler changes the weld's strength, its ductility and its response to post-weld heat treatment.

Formability follows the same pattern. Inconel 718 in the solution-annealed condition can be cold formed, but it work hardens quickly and it needs to be formed before ageing, because forming in the aged condition is impractical; the alloy's high strength is a liability when a press brake has to bend it. Nimonic 263 in sheet form is easier to form and easier to weld, which is why it holds its place in combustion-section sheet work even though its strength is lower, and why a change from 263 to 718 in a thin-walled fabricated liner is not a like-for-like substitution.

Welding and forming topic	Inconel 718	Nimonic 263	Comment
General weldability	good, widely welded	excellent, designed for it	AMS 5662 / ASTM B637 base metal
Matching filler metal	ERNiFeCr-2 type	matching 263 filler	filler must be specified
Strain-age cracking risk	significant in heavy sections	low	key difference for fabrications
Post-weld heat treatment	often required for thick sections	usually not required for thin sheet	drives fabrication cost
Weld ductility after ageing	reduced if schedule poorly controlled	retained	affects design allowables
Sheet formability	work hardens quickly, form before ageing	better formability in sheet	AMS 5596 covers 718 sheet
Thin-wall fabrication	workable with care	preferred	combustor liners, ducts
Thick-section fabrication	preferred for forgings and machined parts	less usual	casings, bolted joints

Table note: The table summarises fabrication behaviour against the base-metal standards, and it is a practical comparison rather than a welding procedure; the applicable welding specification, the filler metal, the interpass temperature and any post-weld heat treatment must be fixed by the fabricator's qualified welding procedure and, where the component is safety related, by the governing code. The single most useful statement for a buyer is that the choice between the two alloys is often made by the fabricator rather than by the designer: a large thin-sheet combustor assembly that must be welded and then aged is far more robust in Nimonic 263, and the designer who specifies 718 for that duty is relying on a weld procedure that is harder to qualify and to repeat.

The practical consequence is a rule of thumb used throughout the gas turbine industry. Where the component is thin, welded and hot, 263 is the natural choice and 718 is a challenge; where the component is thick, forged or machined and strength-driven, 718 is the natural choice and 263 is not strong enough. This is why the two alloys appear in different zones of the same engine rather than competing for the same drawing.

Long-Term Thermal Stability and Sigma Phase

Nimonic 263's decisive advantage is metallurgical stability during long soaks at temperature, and the phase it is designed to avoid is sigma. Sigma phase is a hard, brittle, topologically close-packed intermetallic that precipitates in nickel-chromium-molybdenum and nickel-chromium-cobalt systems during prolonged exposure in the roughly 600-900 C band. It forms on grain boundaries first, then as a Widmanstatten network, and it consumes the chromium and molybdenum that provide corrosion resistance and solid-solution strength. The consequences are a loss of toughness, a rise in notch sensitivity, an increased risk of brittle fracture during a thermal transient, and, if enough sigma has formed, a measurable reduction in ductility and in corrosion resistance. Inconel 718 does not suffer the classic sigma problem because the niobium addition shifts the precipitation toward delta phase instead, but the delta phase does the same damage in a different crystallographic guise: it forms on grain boundaries, it removes niobium from the gamma double prime, and it embrittles the grain boundaries during long exposure above roughly 650 C.

Long-soak behaviour	Inconel 718	Nimonic 263	Practical effect
Main unwanted phase	delta phase, orthorhombic Ni3Nb	sigma phase, but slow to form	both must be controlled by composition and time
Temperature band of concern	above about 650 C	above about 700-750 C	sets the service ceiling
Onset time at 700 C	significant in thousands of hours	very slow	263 is the stable option
Effect on grain boundaries	platelets, notch sensitivity	limited precipitation	design factor for 718
Effect on corrosion resistance	chromium largely retained	chromium depleted only if sigma forms	263 tolerates hot corrosion well
Effect on ductility	reduced after long soak	well retained	burst and fatigue margins
Design remedy	respect the 650 C ceiling	respect the creep limit above 800 C	both are honest design choices
Reference for stability data	ASTM E139 long-term rupture data	ASTM E139 long-term rupture data	test the actual heat where it matters

Table note: The table describes the precipitation behaviour that limits the two alloys and the test method used to quantify it, and it is written as engineering explanation rather than as a standard; the long-term stability of a specific heat is best established by stress-rupture testing to ASTM E139 on material that has been exposed to the intended condition, because precipitation kinetics depend on composition within the allowed range, on grain size and on the exact temperature history. The most valuable single point is that the temperature ceiling of Inconel 718 is a metallurgical constraint, not a conservative design rule, and that the ceiling of Nimonic 263 is set by creep and oxidation rather than by phase instability.

For a designer the message is that both alloys have a defensible temperature limit and that neither should be pushed beyond it on the strength of short-term data. Where a component must run for tens of thousands of hours at 750 C, the choice should be 263, and the design should then be sized on creep and rupture rather than on tensile strength; where the same component runs at 600 C for the same time, 718 will retain its structure and its higher strength throughout, and it will be the cheaper and stronger answer. This is the same conclusion the industry has reached in practice, and it is why the combustion section of a gas turbine is largely 263-type gamma-prime material while the casings, discs and bolting are largely 718-type gamma-double-prime material.

Application Selection: Which Alloy Wins Where

Nimonic 263 should be chosen for hot sheet fabrications in long-term service above roughly 700 C, and Inconel 718 should be chosen for high-strength structural components from room temperature to about 650 C; the exceptions are stated in the table below. This is the selection rule in one sentence, and the rest of this section explains the conditions under which each alloy wins and the cases that override the simple rule. The pattern is consistent across gas turbine, aerospace and industrial furnace practice: strength and cost favour 718, temperature and fabrication favour 263.

Application or condition	Recommended alloy	Why it wins	Exception or caution
Gas turbine casing, machined or forged	Inconel 718	high strength to 650 C, lower cost, every form available	use 263 if the casing soaks above 700 C
Compressor discs, shafts and bolting	Inconel 718	highest yield strength and relaxation resistance	hardness limit may apply in sour service
Combustor can and liner, thin sheet	Nimonic 263	stable gamma prime, welds and forms well	718 only if temperature stays below 650 C
Transition duct and exhaust ducting	Nimonic 263	long-soak stability, ductility retained	718 acceptable for lower-temperature ducts
Thin-wall welded fabrication, large assembly	Nimonic 263	low strain-age cracking risk, no full re-solution needed	718 requires a qualified weld procedure
High-strength fastener or bolting	Inconel 718	about 1100-1200 MPa yield, low relaxation	check hardness limit for sour service
Service above 700 C for thousands of hours	Nimonic 263	resists sigma and retains ductility	never use 718 in this regime
Service below 650 C, strength-driven	Inconel 718	more capacity per kilogram and per dollar	263 is over-specified and more costly
Cryogenic or instrumentation duty	Nimonic 263 or 718 per project	263 is non-magnetic, 718 weakly magnetic	magnetism may prohibit 718
Cost-driven, non-critical duty	Inconel 718	most widely produced PH nickel alloy	confirm the temperature ceiling

Table note: The table states a recommended alloy for each condition on the basis of established metallurgical behaviour and gas turbine practice, and it is engineering guidance rather than a specification; the final choice must satisfy the design code, the material specification and any customer or airworthiness requirement. The two rows to read with most care are the long-soak row and the thin-wall fabrication row, because they are the conditions in which Inconel 718 is genuinely the wrong answer rather than merely the more expensive one. Where a project's temperature is marginal, the conservative choice is the lower-temperature alloy only when the strength case is overwhelming; otherwise 263 is preferred above 700 C.

The exceptions deserve one more sentence each. Inconel 718 is the right answer even above 650 C only when the exposure is short and the strength requirement dominates, for example a short-life static structure in a test rig. Nimonic 263 is the right answer below 650 C only when a property other than strength decides the case, such as the need for a non-magnetic part, a very high ductility weldment, or continuity with an adjacent 263 structure to avoid a dissimilar-metal joint. Otherwise the rule holds, and a buyer who applies it will rarely be wrong. For related material choices in aggressive service, our Hastelloy alloy range covers the corrosion-dominant grades, and our high-temperature alloy news tracks the market and specification changes that affect these grades.

Grade	Product form	2026, EXW Shanghai, USD/kg	Reference basis
Inconel 718, bar and forging stock	bar 20-200 mm, forged blank	35-55	reference range only
Inconel 718, plate	plate 10-100 mm	40-62	reference range only
Inconel 718, sheet and strip	sheet 0.5-4.0 mm	45-70	reference range only
Nimonic 263, bar and forging stock	bar 20-150 mm	55-85	reference range only
Nimonic 263, plate	plate 6-60 mm	60-90	reference range only
Nimonic 263, sheet and strip	sheet 0.5-4.0 mm	65-95	reference range only

Table note: The prices above are a 2026, EXW Shanghai, USD/kg reference range only, given to show the relative cost position of the two alloys and the way form and size affect the price; they are not a quotation, they exclude freight, duty, testing and inspection unless agreed, and they move with the nickel, cobalt and molybdenum markets. The single reliable conclusion is the ranking: Nimonic 263 carries a premium over Inconel 718 that reflects its roughly 20 % cobalt content, its narrower production base and its more demanding heat treatment, so the temperature advantage of 263 has to justify a materially higher material cost. A firm price should always be requested against a specific size, condition, quantity and specification.

Standard Equivalents Across Five Systems

Every major industrial system specifies an equivalent grade for each alloy, and a buyer working across US, Chinese, Japanese, European and Russian documents should be able to move between the designations without ambiguity. Inconel 718 appears in the US system as ASTM B637 and ASME SB637 for bar and forgings, ASTM B670 for plate, sheet and strip, and the aerospace material specifications AMS 5662, AMS 5663 and AMS 5596; in the Chinese GB system as GB/T 14992 cast and wrought superalloy grade GH4169; in the JIS system within JIS G 4902 heat-resisting alloy sheet and plate; in the DIN and EN system as material number W.Nr 2.4668; and in the GOST system as grade KhN73MBTYu (ХН73МБТЮ). Nimonic 263 appears as ASTM B637 and ASME SB637 for bar and forgings, ASTM B670 for plate, sheet and strip; as GB/T 14992 grade GH4263; within JIS G 4902 type sheet and plate; as W.Nr 2.4650 (NiCr20Co20Mo) in the DIN and EN system; and as the corresponding GOST heat-resisting nickel grade for the same duty.

System	Inconel 718 (UNS N07718)	Nimonic 263 (UNS N07263)	Typical product standards
US (ASTM / ASME / AMS)	ASTM B637, ASME SB637, AMS 5662, AMS 5663, AMS 5596, AMS 5872, ASTM B670	ASTM B637, ASME SB637, ASTM B670	bar, forging, plate, sheet, strip
GB (China)	GB/T 14992, grade GH4169	GB/T 14992, grade GH4263	wrought superalloy bar, sheet, plate
JIS (Japan)	JIS G 4902, NCF 718 type	JIS G 4902 type sheet and plate	heat-resisting alloy sheet, plate, strip
DIN / EN (Europe)	W.Nr 2.4668, NiCr19Fe19Nb5Mo3	W.Nr 2.4650, NiCr20Co20Mo	bar, forging, sheet, plate
GOST (Russia and CIS)	GOST 5632, KhN73MBTYu (ХН73МБТЮ)	GOST 5632, nickel-cobalt heat-resisting grade	bar, forging, sheet

Table note: The table maps the designations used in the five main standards systems so that a buyer can reconcile a drawing, a purchase order and a mill certificate written in different languages; the designations are given by grade name and standard number and are not an assertion that every product form is covered by every listed standard. Where a project requires a formal equivalence statement, the mapping should be confirmed against the current edition of each standard and against the actual product form, because coverage differs by form and edition, and a specification should always be written in one governing system with the others quoted for information. Inspection documents are normally issued to EN 10204, with type 3.1 as the common requirement for these grades.

Two cautions follow from the table. First, equivalence of designation is not identity of requirement: GH4169 and W.Nr 2.4668 both describe an alloy broadly equivalent to Inconel 718, but the permitted chemistry ranges, the mechanical minima and the heat treatment requirements differ slightly between systems, so a certificate to one system does not automatically satisfy a purchase order written to another. Second, some designations cover more than one condition, and the condition must be named alongside the grade; "GH4169, solution treated and aged" and "GH4169, solution treated" are different deliveries with different properties. A buyer who states the system, the grade, the form and the condition has closed every gap that matters.

Standard Index

Standard	Title / scope	Covers	Form
ASTM B637	Precipitation-hardening and other nickel alloys for bar, forgings and forging stock	composition, mechanical properties, heat treatment	bar, forging, forging stock
ASME SB637	ASME designation identical in technical content to ASTM B637	composition, mechanical properties	bar, forging
ASTM B670	Precipitation-hardening nickel alloy plate, sheet and strip	composition, mechanical properties	plate, sheet, strip
AMS 5662	Nickel alloy bars and forgings, solution heat treated and aged	aerospace grade requirements	bar, forging
AMS 5663	Nickel alloy bars and forgings, solution heat treated, aged and descaled	aerospace grade requirements	bar, forging
AMS 5596	Nickel alloy sheet, strip and plate, solution heat treated and aged	aerospace sheet and plate	sheet, strip, plate
AMS 5872	Nickel alloy wire and bar for welding and brazing	filler metal	wire, bar
ASTM E8	Tension testing of metallic materials at room temperature	test method	all forms
ASTM E21	Elevated-temperature tension testing of metallic materials	test method	all forms
ASTM E139	Conducting creep, creep-rupture and stress-rupture tests	test method	all forms
ASTM E112	Determining average grain size	test method	all forms
EN 10204	Metallic products, types of inspection documents	certification	all forms
GB/T 14992	Classification and designation of wrought superalloys	grade designations, GH4169, GH4263	bar, sheet, plate
JIS G 4902	Heat-resisting alloy sheet, plate and strip for corrosion and heat resistance	sheet and plate grades	sheet, plate, strip
GOST 5632	High-alloy steels and corrosion-resistant, heat-resistant and creep-resistant alloys	grade designations	bar, sheet, forging

Table note: The index lists the standards most often named for these two alloys and states what each one covers so that a purchase order can be written against the right document; the grade designations and their scope are given as normally applied and the controlling text remains the current published edition of each standard. Where aerospace release is required, the AMS documents govern; for industrial bar and forging supply, ASTM B637 and ASME SB637 are the common route; for plate, sheet and strip, ASTM B670 applies; and the test methods ASTM E8, ASTM E21, ASTM E139 and ASTM E112 are the ones normally invoked on the certificate.

FAQ

Q1: Which alloy is stronger, Inconel 718 or Nimonic 263?

Inconel 718 is significantly stronger, and it is stronger at every temperature up to its ceiling. In the standard solution-treated and aged condition, 718 typically reaches a room-temperature yield strength of about 1100-1200 MPa and a tensile strength of about 1350-1450 MPa, whereas aged Nimonic 263 typically reaches about 700-800 MPa yield and about 950-1050 MPa tensile. The gap is roughly 40 % in yield strength, which is a large difference in engineering terms and it is the reason 718 is used for bolting, casings, discs and shafts where load capacity per unit section matters. The strength of 718 comes from gamma double prime, a metastable Ni3Nb precipitate that strains the matrix more than gamma prime does, so it produces more strengthening per unit volume. The caveat is that this advantage is not permanent. Above roughly 650 C the gamma double prime transforms toward delta phase, and after long exposure the strength advantage narrows and the ductility falls. Nimonic 263 gives up peak strength to gain stability, so it is the weaker alloy at low temperature and the more reliable one above 700 C.

Q2: Which alloy should I choose for a gas turbine casing?

Choose Inconel 718 for a gas turbine casing unless the casing's own metal temperature soaks above about 700 C, in which case Nimonic 263 is the safer answer. Casings are usually heavy, often forged or machined, and they are loaded mechanically as well as thermally, so the high yield strength of 718 is directly useful and the lower cost is a further advantage, because 718 contains no cobalt and is produced in every form by every mill. Casings also usually run cooler than the combustion section, so their metal temperature often sits below the 650 C ceiling that 718 tolerates for long-term service. Where a casing does see sustained temperatures above 700 C, for example an exhaust casing or a casing adjacent to the combustor, the decision changes: the long-term stability of 263 becomes the governing property, and the reduced strength can be designed around by increasing section where weight permits. Many engines use both alloys in different casings for exactly this reason, so the material should be chosen casing by casing against its own duty cycle rather than applied uniformly across the engine.

Q3: Can Inconel 718 be used above 700 C?

Inconel 718 is not recommended for long-term service above about 650 C, and it should not be used above 700 C in a permanent structure. The limitation is metallurgical rather than a matter of conservative practice. The gamma double prime precipitate that gives 718 its strength is metastable, and above roughly 650 C it progressively transforms into delta phase, an orthorhombic Ni3Nb precipitate that forms on grain boundaries as platelets, contributes little strengthening, and consumes the niobium that the hardening phase requires. The practical results after thousands of hours are a loss of strength, reduced ductility, increased notch sensitivity and a greater risk of cracking during a thermal transient. A short-term tensile test at 700 C can look acceptable because the transformation takes time, so the wrong conclusion is easily reached from short data. Where the duty genuinely exceeds 650 C, the correct choices are Nimonic 263, which is stable to about 800-850 C, or another gamma-prime alloy such as Waspaloy or a higher-strength nickel alloy, rather than 718 pressed beyond its ceiling.

Q4: Is Nimonic 263 easier to weld than Inconel 718?

Yes, Nimonic 263 is markedly easier to weld, especially in thin sheet and in large welded assemblies, and this is one of the main reasons it survives in combustion-section work. Inconel 718 can be welded successfully by gas tungsten arc, electron beam, laser and resistance methods, and it has excellent filler-metal support through the ERNiFeCr-2 type, but it carries a real risk of strain-age cracking: when a welded assembly is subsequently aged, the weld and heat-affected zone contract as they harden, and if the surrounding material cannot accommodate that contraction, cracks form. The risk grows with section thickness and with restraint. Nimonic 263 was designed with a chemistry that resists strain-age cracking, so a large thin-sheet assembly can be welded and aged with much less risk and usually without a full post-weld re-solution treatment. For a fabricator, that translates into a simpler qualified weld procedure, fewer repairs and lower cost. Neither alloy is difficult in absolute terms, but 263 is the more forgiving of the two in fabrication.

Q5: Does Inconel 718 need post-weld heat treatment?

Inconel 718 often needs post-weld heat treatment when the section is thick or the component is highly restrained, and the treatment must be planned at the specification stage rather than discovered during fabrication. The purpose is twofold: to relieve the residual stresses introduced by welding, and to restore the aged condition in the weld and heat-affected zone so that the joint achieves the properties of the base metal. The conventional route is a solution treatment followed by the two-step age at about 720 C and about 620 C, though for some components a direct ageing treatment is used instead. The difficulty is that ageing hardens and shrinks the weld while the surrounding cold material resists, which is the strain-age cracking mechanism, so the procedure, the interpass temperature, the restraint and the filler metal must all be controlled together. In thin-sheet Inconel 718 fabrications the need is reduced, but it does not disappear. Nimonic 263 usually escapes this requirement for thin sheet, which is a material advantage in fabrication cost as well as in risk.

Q6: What is the difference between gamma double prime and gamma prime?

Both are coherent precipitates that strengthen a nickel matrix by blocking dislocation motion, but they are different phases with different stability, and that difference governs the whole comparison. Gamma prime is the ordered face-centred-cubic Ni3(Al,Ti) phase found in most high-temperature nickel alloys and is the equilibrium strengthening phase of Nimonic 263; it is thermodynamically stable to high temperature and it strengthens by creating a small lattice mismatch with the matrix and by resisting dislocation cutting. Gamma double prime is a body-centred-tetragonal Ni3Nb phase, the main strengthener of Inconel 718, which produces a larger misfit and therefore more strengthening per unit volume, but which is metastable. Above roughly 650 C it transforms toward delta phase, an incoherent orthorhombic Ni3Nb precipitate that decorates grain boundaries and destroys ductility. In one sentence, gamma prime trades some peak strength for stability at temperature, and gamma double prime trades stability for peak strength. Cobalt in the 263 matrix raises the gamma prime solvus and suppresses brittle topologically close-packed phases.

Q7: What is sigma phase and why does it matter?

Sigma phase is a hard, brittle intermetallic with a topologically close-packed crystal structure that precipitates in nickel-chromium-molybdenum and nickel-chromium-cobalt alloys during prolonged exposure at roughly 600-900 C. It matters because it forms preferentially on grain boundaries, where it consumes chromium and molybdenum that the alloy needs for corrosion resistance and solid-solution strength, and because it converts a ductile, forgiving microstructure into a notch-sensitive one. A component that has formed significant sigma can lose toughness, crack during a thermal transient or a thermal shock, and show reduced corrosion resistance in the affected zones, and once formed the phase cannot be removed except by a full re-solution treatment. Inconel 718 does not suffer the classic sigma problem because its niobium content diverts precipitation toward delta phase, but delta phase causes the same class of damage above about 650 C, so both alloys have a stability limit. Nimonic 263 is engineered with about 20 % cobalt specifically to slow the formation of sigma and other topologically close-packed phases, which is why it holds up in long-soak combustion service.

Q8: What are the GB equivalents GH4169 and GH4263?

GH4169 is the Chinese designation under GB/T 14992 for the wrought superalloy broadly equivalent to Inconel 718, and GH4263 is the corresponding designation for the grade broadly equivalent to Nimonic 263. The GH prefix identifies a wrought high-temperature alloy in the Chinese classification, and the numeric suffix identifies the grade; GH4169 is one of the most widely produced superalloys in China and is used for discs, casings, fasteners and structural parts in gas turbine and aerospace work, while GH4263 covers the cobalt-bearing gamma-prime grade used for combustor and ducting components. A buyer working between a Chinese drawing and a US purchase order should treat the designations as equivalents for engineering purposes but confirm the detail, because the permitted chemistry ranges, the mechanical property minima and the heat treatment requirements can differ slightly between the GB and ASTM or AMS systems. The recommended approach is to write the purchase order in one governing system, quote the equivalent for information, and ask the mill to certify against the system that the design code requires.

Q9: What are the DIN and EN equivalents, W.Nr 2.4668 and 2.4650?

In the German DIN and European EN system, Inconel 718 carries the material number W.Nr 2.4668 and the designation NiCr19Fe19Nb5Mo3, and Nimonic 263 carries the material number W.Nr 2.4650 and the designation NiCr20Co20Mo. The material number is the compact identifier used on European drawings and certificates, and the alphanumeric designation states the nominal composition in a shorthand that is easy to read once the pattern is understood: NiCr19Fe19Nb5Mo3 means a nickel-base alloy with about 19 % chromium, 19 % iron, 5 % niobium and 3 % molybdenum, while NiCr20Co20Mo means a nickel-base alloy with about 20 % chromium, 20 % cobalt and molybdenum but essentially no iron. The two designations show the fundamental difference between the alloys at a glance, because one is a nickel-chromium-iron alloy with niobium and the other is a nickel-chromium-cobalt alloy without iron. European orders for these grades normally specify the material number together with the relevant EN or national standard and the product form, and inspection documents are issued to EN 10204, typically type 3.1.

Q10: Which alloy should I use for thin-sheet exhaust ducting?

Nimonic 263 is normally the better choice for thin-sheet exhaust ducting, and Inconel 718 should be used only where the duct temperature stays comfortably below 650 C. Exhaust ducting is usually a thin-walled, welded fabrication exposed to a hot gas stream, often at temperatures in the 650-800 C range, and it is loaded thermally rather than mechanically. In that duty the properties that matter are long-term metallurgical stability, weldability and retained ductility rather than tensile strength, and those are exactly the properties in which 263 excels. Its gamma prime structure resists the precipitation of sigma and other brittle phases, so the duct retains its toughness after thousands of hours, and it welds and forms more easily than 718, which reduces the risk of strain-age cracking during fabrication and post-weld ageing. Inconel 718 remains acceptable for cooler ducts or where a strength requirement dominates, but above about 700 C it degrades and the reason for choosing it, its strength, is no longer available. Design the duct on creep and thermal fatigue and select 263 for the hot sections of the run.

Q11: How are Inconel 718 and Nimonic 263 heat treated?

Both alloys are solution treated and then aged, but the schedules differ. Inconel 718 is conventionally solution annealed at about 954-982 C and then aged in two steps: hold at about 720 C for eight hours, cool in the furnace to about 620 C, hold for a further eight hours and air cool. The two-step age is designed around the metastable gamma double prime: it precipitates the hardening phase in the matrix while allowing a controlled amount of gamma prime and carbides to form, giving very high strength with usable ductility. Nimonic 263 is solution treated higher, at about 1150 C, to dissolve carbides and homogenise the structure, and then aged at about 800 C for eight hours in a single step to develop a stable gamma prime distribution. The schedules are not interchangeable, and an overaged condition of 718, produced by ageing at about 870-900 C, is used only where lower hardness and better notch behaviour are deliberately required. Heat treatment should always follow the governing standard and be documented on an EN 10204 3.1 certificate with the test results for the final condition.

Q12: How does Hangbo Alloy supply Inconel 718 and Nimonic 263?

Hangbo Alloy, trading as Shanghai Hangbo Alloy Group Co., Ltd., supplies both Inconel 718 and Nimonic 263 in bar, forging stock, plate, sheet and strip from its Shanghai mill, with the grades certified to ASTM B637, ASME SB637 and ASTM B670 as standard and to AMS 5662, AMS 5663 and AMS 5596 where aerospace release is required. Bar is available from about 6 mm to 500 mm, plate from 0.1 mm to 100 mm, tube and pipe, forgings, welded pipe, fittings, wire and strip alongside, so that a casing, a combustor can and its fasteners can be sourced from one supply chain with consistent certification. Each order is supplied with the heat treatment schedule, the mechanical test results, the chemical analysis and a positive material identification report, documented to EN 10204 on request, and third-party inspection by SGS, BV or TUV can be arranged before shipment. Buyers can review the Inconel alloy range, the Nimonic alloy range and the current nickel alloy news and buying guides, or reach the contact page to send a drawing, a specification and a quantity for a firm price.

Conclusion: Choose 718 for Strength Below 650 C and 263 for Stability Above 700 C

The choice between Inconel 718 and Nimonic 263 reduces to one question: is the component limited by strength or by temperature? Below about 650 C, Inconel 718 is stronger, cheaper, available in every form and supported by the widest range of standards, and it is the correct default for casings, forgings, discs, shafts and bolting. Above about 700 C, and especially where the component is a thin welded sheet fabrication that must survive thousands of hours of soak, Nimonic 263 is the correct answer, because its stable gamma prime structure and its cobalt-protected matrix resist the brittle phases that degrade 718 in that regime. The exception cases are honest and few: 718 remains usable above its nominal ceiling only for short exposures, and 263 remains sensible below 650 C only where a property other than strength, such as non-magnetic behaviour or weldment ductility, decides the case.

Make the decision early, at the specification stage, and write the purchase order in one governing system with an explicit grade, form, condition and heat treatment. Then send the specification to a mill that can certify it. Buyers can request a firm quotation for Inconel 718 or Nimonic 263 bar, forging stock, plate, sheet or strip, with chemistry, mechanical properties and positive material identification documented to EN 10204, by contacting our sales team with the drawing, the service conditions and the quantity required.

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

Inconel: 600, 601, 617, 625, 690, 718, 725, X-750 | Nimonic: 75, 80A, 90, 105, 115, 263 | Incoloy: 800, 800H, 825, 925, 028, 330 | Monel: 400, K-500 | Hastelloy: C-276, C-22, C-2000, B-2, B-3, X, G-30 | Nickel: 200, 201 | Pure Nickel: Ni 200/201 | Titanium: Grade 1-5, Ti-6Al-4V | Stainless Steel: 304/L, 316/L, 317L, 321, 347, 904L, 2205, 2507 | Alloy 20 | Cu-Ni: 90/10, 70/30 | Waspaloy | Rene 41 | Alloy 188 | L605 | Haynes 25 | UNS-specified equivalents to ASTM, ASME, AMS, GB/T 14992, JIS G 4902, DIN/EN (W.Nr) and GOST 5632 grades.

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All material supplied with mill test certificates to EN 10204 3.1, heat treatment records, mechanical and chemical test reports and positive material identification. Third-party inspection and certification by SGS, BV or TUV on request. ISO 9001:2015 certified. www.nickel-alloy.com