Inconel 718 vs A286 for High-Temperature Fasteners

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 A286 - Which Should I Specify?

Inconel 718 is the stronger alloy: in the aged condition it provides roughly 1275 MPa UTS against 896 MPa for A286, and it retains useful strength to about 700 C while A286 is limited to roughly 650 C. A286 is iron-based and cheaper. Specify 718 when strength and temperature govern; specify A286 when they do not.

Key Takeaways

• Inconel 718 (UNS N07718) is the higher-strength system. In the solution-treated and aged condition it delivers about 1275 MPa minimum tensile and 1034 MPa minimum yield per AMS 5662, against roughly 896 MPa and 586 MPa for A286 per AMS 5737.
• A286 (UNS S66286) is iron-based and materially cheaper. Its 25 % nickel level and iron matrix make it the economical choice for the large volume of fasteners that never exceed 650 C.
• The temperature ceilings differ by roughly 50 C for long-term bolting. A286 is the practical limit to about 650 C; 718 extends useful stress-rupture life to about 700 C because of its gamma-double-prime strengthened nickel matrix.
• Bolting standards are not interchangeable. A286 bolting is ordered to ASTM A453 Grade 660 while 718 bar and forgings are ordered to AMS 5662 and ASTM B637; the two call up different test temperatures and different stress-rupture regimes.
• Hardening is a controlled, mandatory step for both alloys. A286 uses a solution treatment plus a single age; 718 uses a solution treatment plus a two-step age. Neither alloy performs to specification without it.
• Magnetic behaviour and corrosion margin favour 718. 718 is essentially non-magnetic and is more tolerant of chlorides, which decides instrument and marine-adjacent joints.

What Are Inconel 718 and A286?

Inconel 718 (UNS N07718, Werkstoff 2.4668) is a precipitation-hardenable nickel-chromium-iron superalloy, and it is the most widely used superalloy in the world. It is specified for turbine discs, compressor blades and vanes, engine casings, high-pressure fasteners, downhole tools and cryogenic hardware. Its hardening does not come from carbon or from cold work; it comes from a controlled two-step aging treatment that precipitates gamma-double-prime (gamma'') phase in a nickel-rich austenitic matrix. That precipitation reaction is the origin of its unusual combination of strength, fatigue resistance and creep resistance up to roughly 700 C, which no iron-based alloy of comparable cost can reproduce. Our Inconel alloy supplier page documents the bar, forging stock, plate and fastener material we hold and certify against AMS and ASTM purchase specifications.

A286 (UNS S66286, Werkstoff 1.4980) is an iron-based austenitic superalloy, and that single word "iron" explains almost everything about its commercial position. Its matrix is iron, with a nominal 25 % nickel and 15 % chromium, and it hardens by precipitation of gamma-prime (gamma') phase from titanium and aluminium additions. Because it contains roughly half the nickel of 718 and no deliberate niobium, A286 costs considerably less per kilogram and is markedly lighter per unit volume. Its density, around 7.9 g/cm3 against roughly 8.2 g/cm3 for 718, is a real advantage on aerospace fasteners where every gram of a large bolt circle matters.

Both alloys are used for the same class of parts, which is exactly why the selection is contested. A286 is the default aerospace bolting alloy up to about 650 C, and 718 takes over where preload, temperature, corrosion duty or a non-magnetic requirement exceeds what A286 can sustain. The decision in practice rests on three variables: design temperature, required stress-rupture life, and cost per finished part. This article works through each of them using the governing standard numbers, and it ends with an unambiguous recommendation for the common cases.

There is one more structural difference worth stating at the outset. A286 and 718 are not grades of one family; they belong to two different metallurgical families that happen to overlap in application. A286 is related to the austenitic stainless steels and the iron-nickel superalloys, while 718 is a genuine nickel-base superalloy of the same family as Waspaloy and Inconel X-750. That family difference, not any single element, is the reason the two alloys diverge above 650 C and the reason their bolting specifications are written differently.

Chemical Composition: Iron-Based A286 versus Nickel-Based 718

The composition difference between the two alloys is the root cause of every performance difference discussed later in this article. A286 is iron-based with about 25 % nickel, 15 % chromium and small but critical additions of titanium, aluminium, molybdenum, vanadium and boron. Inconel 718 is nickel-based with 50-55 % nickel, 17-21 % chromium, and the niobium-plus-tantalum and molybdenum additions that create the gamma-double-prime strengthening phase. The table below lists the standard composition limits from the governing product specifications.

Element (wt %)	A286 / UNS S66286	Inconel 718 / UNS N07718	Per standard
Nickel	24.00-27.00	50.00-55.00 (balance incl. Co)	ASTM A638 / AMS 5662
Chromium	13.50-16.00	17.00-21.00	ASTM A638 / AMS 5662
Iron	balance	balance	ASTM A638 / AMS 5662
Molybdenum	1.00-1.50	2.80-3.30	ASTM A638 / AMS 5662
Niobium + Tantalum	—	5.00-5.50 (AMS 5662)	AMS 5662 / ASTM B637
Titanium	1.90-2.35	0.65-1.15	ASTM A638 / AMS 5662
Aluminium	0.35 max	0.20-0.80	ASTM A638 / AMS 5662
Vanadium	0.10-0.50	—	ASTM A638
Carbon	0.08 max	0.08 max	ASTM A638 / AMS 5662
Manganese	2.00 max	0.35 max	ASTM A638 / AMS 5662
Silicon	1.00 max	0.35 max	ASTM A638 / AMS 5662
Boron	0.001-0.010	0.006 max	ASTM A638 / AMS 5662
Cobalt	—	1.00 max	AMS 5662
Copper	—	0.30 max	AMS 5662
Sulphur	0.025 max	0.015 max	ASTM A638 / AMS 5662

Table note: Values are standard composition limits taken from ASTM A638 Grade 660 and AMS 5737 for A286, and from AMS 5662 and ASTM B637 for Inconel 718 (latest editions). Niobium plus tantalum is specified as 5.00-5.50 in AMS 5662; ASTM B637 lists a marginally wider range, which is a genuine difference between the two standards rather than a supplier variation. Do not transfer a limit from one standard to the other without checking the edition on the purchase order.

The most consequential single line in that table is nickel. A286 carries 24-27 % nickel while 718 carries 50-55 %, and nickel is the dominant cost driver in both alloys. Everything else being equal, that roughly doubles the raw material cost of 718 before any processing is considered. The second most consequential line is niobium plus tantalum. Niobium is what allows 718 to form gamma-double-prime, the phase responsible for its high strength, and no amount of titanium and aluminium in an iron matrix can reproduce that phase. This is the metallurgical reason why A286 and 718 are not simply a cheap and an expensive version of the same material.

The titanium and aluminium balance in A286 is delicate and worth understanding for procurement. A286 hardens by gamma-prime precipitation, so the ratio of titanium to aluminium and the absolute titanium level both control the age response. A heat with titanium at the low end of the range and aluminium at the high end will respond differently from one at the opposite end, and this is why A286 purchase specifications are written with both a minimum and a maximum on titanium rather than a simple maximum. When we certify A286 bar we report titanium and aluminium as separate line items on the certificate precisely because a customer who is qualifying a forging or a bolted joint needs to know the age response, not just that the heat is "within specification."

The boron and carbon controls point in opposite directions between the two alloys and are easy to confuse. A286 carries a deliberately controlled boron addition, typically 0.001-0.010 %, which improves creep resistance at grain boundaries. Inconel 718 caps boron at 0.006 % to protect weldability and hot ductility. Carbon is capped at 0.08 % in both, but the reasons differ: in A286 a high carbon level risks chromium carbide precipitation at grain boundaries during aging, while in 718 carbon is held low to preserve the creep and fatigue behaviour of the gamma-double-prime matrix. A PMI verification that reports nickel, chromium, molybdenum and niobium will separate the two alloys unambiguously, because no iron-based alloy carries 5 % niobium.

Mechanical Properties at Room and Elevated Temperature

The strength gap between the two alloys is large and, at room temperature, easy to quantify from the governing specifications. In the solution-treated and aged condition, Inconel 718 meets a minimum ultimate tensile strength of about 1275 MPa (185 ksi) and a minimum 0.2 % offset yield strength of about 1034 MPa (150 ksi) with a minimum elongation of 12 % under AMS 5662. A286 in the solution-treated and aged condition meets a minimum tensile strength of about 896 MPa (130 ksi) and a minimum yield of about 586 MPa (85 ksi) under AMS 5737. The table below sets out the figures together with the elevated-temperature behaviour.

Alloy, condition	Temperature	UTS	0.2 % yield	Elongation	Hardness	Per standard
Inconel 718, solution treated + aged	20 C	1275 MPa min (185 ksi)	1034 MPa min (150 ksi)	12 % min	~36-40 HRC (typical)	AMS 5662
A286, solution treated + aged	20 C	896 MPa min (130 ksi)	586 MPa min (85 ksi)	15 % min (typical)	~24-32 HRC (typical)	AMS 5737
A286, Grade 660 bar	20 C	896 MPa min (130 ksi)	586 MPa min (85 ksi)	15 % min (typical)	—	ASTM A638
Inconel 718, aged	650 C	~1000 MPa (typical)	~860 MPa (typical)	—	—	typical, not a standard minimum
A286, aged	650 C	~650 MPa (typical)	~450 MPa (typical)	—	—	typical, not a standard minimum

Table note: Room-temperature room-temperature figures are standard minima from the cited AMS specifications and are confirmed by the tensile test methods ASTM E8 (room temperature) and ASTM E21 (elevated temperature); hardness is measured to ASTM E18. The elevated-temperature rows are typical published values at 650 C and are explicitly not standard minima, because the governing specifications do not fix room-temperature-strength-equivalent tensile minima at that temperature. Always confirm elevated-temperature design values against the stress-rupture requirements of the applicable specification, not against a tensile table.

Two points deserve emphasis for anyone reading this table for a fastener design. First, the ratio between the two alloys at room temperature is roughly 1.4 to 1.75 depending on whether tensile or yield is compared, and that ratio is large enough to change bolt sizing rather than merely to change a safety factor. Second, and more important, tensile strength is not the property that governs a bolt in high-temperature service. A bolt holds preload, and its failure mode above about 500 C is stress rupture and relaxation, not tensile overload. That is why the elevated-temperature rows are marked as typical and why the next section deals with stress rupture directly.

The hardness difference follows the strength difference and is a useful shop-floor check. A286 in the aged condition typically reads in the 24-32 HRC band while aged 718 typically reads 36-40 HRC, so a hardness check separates them immediately. However, hardness alone does not confirm that either alloy has been correctly aged; a low reading tells you the age is wrong, but a correct reading does not prove the heat treatment is right. For that, tensile testing per ASTM E8 and, for qualification, elevated-temperature testing per ASTM E21 remain the reference methods. We report both room-temperature tensile and hardness on every A286 and 718 heat we ship, and we retain the test coupons so a customer qualification can be repeated if a downstream audit requires it.

A final mechanical-property nuance that catches designers is the anisotropy and grain-flow effect in fasteners. A rolled thread has a continuous grain flow that follows the thread root, whereas a cut thread interrupts it. The fatigue strength of a rolled-thread 718 bolt can be substantially higher than that of the same bolt with cut threads, and this is a manufacturing variable rather than an alloy variable. We discuss the practical consequences in the machining and thread-rolling section below, because the choice of alloy and the choice of thread process should be made together.

Stress-Rupture and Creep: The Practical Temperature Ceiling

For high-temperature bolting, stress-rupture life rather than tensile strength defines the usable temperature, and this is where Inconel 718 and A286 separate decisively. A286 develops useful stress-rupture life to roughly 650 C; Inconel 718 retains useful stress-rupture life to roughly 700 C and in short-term service to considerably higher. The difference comes from the strengthening phase: A286 is hardened by gamma-prime in an iron matrix, while 718 is hardened by gamma-double-prime in a nickel matrix, and the nickel matrix with niobium additions resists coarsening and creep deformation more effectively at the top of the range.

Property (typical published data)	A286	Inconel 718	Basis
Practical long-term bolting ceiling	~650 C	~700 C	typical, published service guidance
100 h stress-rupture stress at 650 C	~330 MPa (typical)	~690 MPa (typical)	typical, not a standard minimum
100 h stress-rupture stress at 700 C	declines steeply (typical)	~450-500 MPa (typical)	typical, not a standard minimum
Strengthening phase	gamma-prime	gamma-double-prime	published metallurgy
Oxidation-limited short-term ceiling	~800 C (typical)	~980 C (typical)	typical, published guidance

Table note: All figures in this table are typical published values and are not standard minima; the governing ASTM and AMS specifications control stress-rupture acceptance through class-based requirements rather than through a single published stress. Stress-rupture testing is carried out under ASTM E139. Use these numbers to frame a comparison, then confirm the actual acceptance requirement against the class or grade named on the purchase order.

The practical consequence for a bolted joint is worth spelling out. A flange bolt at 650 C does not fail by tearing; it fails by relaxing. Under sustained load, the material creeps, the bolt elongates, preload drops, and the joint leaks. A286 at 650 C is operating near the point where its creep rate becomes a design constraint, so A286 bolting at that temperature is normally specified with a generous bolt stress margin and with an ASTM A453 Grade 660 class selected for the required stress-rupture life. Inconel 718 at 650 C still has substantial creep margin, which is why 718 is the standard choice for the highest-pressure and highest-temperature flanged joints in gas turbines and high-temperature process plant.

Above roughly 700 C the economics and the metallurgy both change. A286 is no longer competitive for long-term service because its creep rate rises steeply, and 718 itself begins to lose the gamma-double-prime phase as it coarsens and eventually transforms, so even 718 is normally replaced by a solid-solution or gamma-prime strengthened alloy such as Waspaloy, Inconel X-750 or a Nimonic grade for sustained service much above 700 C. That is a separate selection question, and our nickel alloy technical knowledge center covers the higher-temperature nickel grades and their ageing behaviour. The takeaway for this comparison is simple: within the 20-700 C band the choice between A286 and 718 is a cost and strength trade, and above about 700 C neither is the right answer for long-term service.

There is also a thermal-fatigue and thermal-expansion dimension that matters for turbine hardware. A286 and 718 have different coefficients of thermal expansion, and the difference is enough to matter in a bolted assembly that spans a wide temperature range or that joins dissimilar materials. A286, being iron-based and closer to the austenitic stainless steels, has a higher thermal expansion coefficient than the nickel-base 718. Where a high-temperature bolt clamps a component with a very different expansion coefficient, the resulting differential movement must be accounted for in the preload and the joint design, and the alloy choice interacts with that calculation. This is one more reason why the alloy decision should be made from the joint design rather than from a material datasheet alone.

Heat Treatment: Solution Treat and Age Cycles Compared

Both alloys are supplied in a solution-treated condition and both must be aged to develop their properties, but the cycles are different and must not be confused. A286 is solution treated at approximately 900 C (1650 F) and rapidly quenched, then age hardened in a single hold of about 16 hours in the 705-760 C (1300-1400 F) range followed by air cooling. Inconel 718 is solution treated in the 940-1010 C (1725-1850 F) range and rapidly quenched, then given a two-step age: a first hold at about 720 C (1325 F) for 8 hours, a controlled cool to about 620 C (1150 F), a second 8-hour hold, and air cooling. The two-step age is what precipitates the gamma-double-prime phase in a controlled way and gives 718 its combination of strength and toughness.

Step	A286	Inconel 718	Per standard / note
Solution treatment	~900 C (1650 F), rapid quench	~940-1010 C (1725-1850 F), rapid quench	typical commercial cycles
Ageing	Single age, ~705-760 C for ~16 h, air cool	Two-step age, ~720 C for 8 h + ~620 C for 8 h, air cool	typical commercial cycles
Ageing mechanism	gamma-prime precipitation	gamma-double-prime precipitation	published metallurgy
Post-weld heat treatment	Solution treat + age, or age only depending on weld procedure	Solution treat + age, or direct age per procedure	in-house practice, confirm against specification
Dimensional stability	Low distortion on ageing	Low distortion on ageing	published behaviour

Table note: The solution and ageing temperatures shown are typical commercial cycles for the two alloy systems; the exact cycle, ramp rates, hold times and cooling rates are fixed by the governing AMS or ASTM specification and by the customer's process specification. Always confirm the cycle against the standard and the drawing before heat treating, because a mis-aged 718 or A286 part will pass a hardness check and still fail a stress-rupture test.

The consequence of getting the cycle wrong is asymmetric between the two alloys and worth understanding for quality control. An under-aged A286 part will typically show low strength and low hardness, which a hardness check will catch. An over-aged or mis-stepped 718 part is more dangerous, because it can pass a room-temperature tensile test while its elevated-temperature stress-rupture life is badly degraded by coarsening or by an incomplete gamma-double-prime precipitation. For critical 718 hardware the only reliable acceptance test is the stress-rupture test required by the applicable class or grade, and we retain samples for that test on request.

One practical heat-treatment rule applies to both alloys and is a frequent source of field failures: the material should be aged after all forming and machining operations that would otherwise expose it to a temperature above its ageing temperature. If a 718 part is machined and then locally heated, welded or straightened without a subsequent full ageing treatment, the affected zone will not carry the specified properties even though the rest of the part is correct. Where welding is unavoidable, the part is normally solution treated and aged again after welding, or a matching-ageable filler is used with a post-weld age and the resulting properties are verified by test. A286 behaves in the same way: any operation that disturbs the aged condition must be followed by a re-age.

Bolting and Fastener Specifications You Must Cite

The bolting standards for the two alloys are different documents with different acceptance philosophies, and quoting the wrong one is one of the most common specification errors we see. A286 bolting is ordered to ASTM A453 Grade 660, which is written specifically for high-temperature bolting materials with expansion coefficients comparable to the austenitic stainless steels, and it is divided into Classes A, B, C and D that differ in the required minimum stress-rupture performance and in the test temperature at which that performance is demonstrated. Higher classes impose a more demanding stress-rupture requirement, which is how the standard lets a designer match bolting to a specific joint duty rather than to a single strength level.

Inconel 718 bar and forging stock is ordered to AMS 5662, which fixes chemistry, the solution-and-age heat treatment and the mechanical property minima for the aged condition, and to ASTM B637, which covers the same alloy in bar, forging and forging-stock form for general and pressure-service use. For pressure-vessel and pressure-piping applications the same requirements are adopted through ASME SB-637 in Section II of the ASME Boiler and Pressure Vessel Code, which is where a designer should look for allowable stress values rather than in the raw ASTM document. Bar for bolting stock is also supplied against the AMS 5662 condition, and the finished fastener is then qualified by test.

Standard	Alloy	Applies to	Acceptance basis
ASTM A453 Grade 660	A286 bolting	High-temperature bolting material	Class-based stress-rupture, Classes A/B/C/D
AMS 5737	A286 bar	Solution treated and aged bar	Tensile minima, aged condition
ASTM A638 Grade 660	A286 bar and forging stock	Bar, forging stock	Composition + mechanical
ASTM A453	A286 bolting	Bolting for high-temperature service	Class-based requirement
AMS 5662	Inconel 718	Bar and forging stock	Tensile minima, solution + aged
ASTM B637	Inconel 718	Bar, forging, forging stock	Composition + mechanical
ASME SB-637	Inconel 718	Pressure-retaining components	ASME Code allowable stress basis
ASTM E8 / ASTM E21	Both	Tensile testing at room / elevated temperature	Test method
ASTM E18	Both	Rockwell hardness	Test method
ASTM E139	Both	Stress-rupture testing	Test method

Table note: Standard titles and scopes are summarised from the published documents (latest editions). The class structure of ASTM A453 Grade 660 assigns progressively more demanding stress-rupture requirements to the higher classes; always specify the class required for the joint, because a lower class supplied against a higher-class drawing is a nonconformance even though the chemistry is correct.

Two procurement habits protect against the most expensive bolting mistakes. The first is to place the class or grade on the purchase order together with the alloy name, because "A286 bolts" and "A286 per ASTM A453 Grade 660 Class C" are not the same order, and neither is "Inconel 718 bolts" against "Inconel 718 per AMS 5662." The second is to require that the stress-rupture test results for the actual class travel with the certificate. A chemistry certificate and a room-temperature tensile certificate are necessary but not sufficient for high-temperature bolting, and a supplier who cannot produce the class-specific stress-rupture data has not demonstrated compliance with the bolting standard.

For customers who are working through a first article with a new joint, we routinely supply the material certificate, the room-temperature tensile and hardness results, and the elevated-temperature test data for the class specified, and we will hold the balance of the heat until the customer's qualification test is complete. That is a slower way to buy the first batch and a considerably faster way to buy the second one. Our high-temperature fastener alloy selection guide walks through the class-selection logic for several common joint duties.

Forging, Machining and Thread Rolling

Both alloys are forgeable, machinable and desirable in rolled-thread form, but the process windows differ and the difference has a direct cost consequence. Inconel 718 is forged in a broad temperature window and is regarded as one of the more forgeable nickel superalloys; it can be produced as near-net-shape forgings and as large rolled bar, which is a major reason it displaced earlier, less forgeable superalloys. A286 forges readily in the 950-1150 C range and can be produced as bar, wire and forging stock, and its iron base makes it somewhat more tolerant of process variation than 718 in the forge shop.

Process step	A286	Inconel 718	Practical note
Forging window	~950-1150 C	broad, generally lower than A286	confirm against the forging procedure
Machinability	Moderate, gummy, work hardens	Lower, strongly work hardens	carbide tooling, positive feed, avoid dwelling
Best machining condition	Solution treated, cut then age	Solution treated, cut then age	in-house practice, not a standard requirement
Thread forming	Roll in solution-treated condition, then age	Roll in solution-treated condition, then age	rolled threads preferred for fatigue
Preferred thread form	Rolled	Rolled	manufacturing choice
Post-weld treatment	Re-solution + age, or age per procedure	Re-solution + age, or age per procedure	confirm against weld procedure

Table note: The temperature windows and process notes are typical shop practice and, except where a standard explicitly controls the process, are in-house recommendations rather than standard requirements. Forging temperatures and any subsequent heat treatment that affects final properties must be confirmed against the governing AMS or ASTM specification and the customer's process specification.

Thread rolling is the single most important manufacturing decision for a high-temperature fastener, and it applies equally to both alloys. A rolled thread is produced by cold deformation of the thread form, which leaves the grain flow continuous around the thread root and leaves a favourable compressive residual stress at the surface. A cut thread interrupts the grain flow and leaves a sharper root notch. In fatigue and in thermal cycling, which is the actual duty of most turbine and high-temperature flange bolting, the rolled thread substantially outperforms the cut thread. The recommended sequence for both alloys is to roll the thread while the material is in the solution-treated condition and then age it, so that the ageing treatment relieves the forming stresses without destroying the beneficial grain flow; rolling a fully aged bolt is possible but harder on the tooling and offers less benefit.

Machining the two alloys requires different expectations. Inconel 718 work hardens strongly, generates a great deal of heat at the cutting edge and demands rigid setups, positive tool geometry and cutting speeds well below those used on stainless steel. A286 machines more easily than 718 in comparative terms but tends to be gummy and to smear, so good chip control and sharp edges matter. Both alloys are best machined in the solution-treated condition and then aged, which improves machinability and, for a dimensionally critical part, allows the final grind or finish cut to be made after ageing once the small residual distortion has occurred.

Galling is a further practical consideration in fastener manufacture and in assembly. Both alloys are prone to galling when running a like-on-like thread, and high-temperature bolting is frequently assembled with a solid-film lubricant or an anti-seize compound rated for the service temperature. The lubricant choice must be checked against the temperature; a molybdenum-disulphide compound that performs well at 400 C may not be acceptable at 650 C, and some lubricants leave residues that attack the base metal or interfere with subsequent inspection. This is a small detail that causes a disproportionate number of assembly-stage and field problems.

Cost, Supply and Magnetic Permeability

A286 is the lower-cost alloy and Inconel 718 the higher-cost alloy, and the gap is driven almost entirely by nickel content and by the niobium addition. A286 contains roughly 25 % nickel in an iron matrix, while 718 contains roughly 52 % nickel plus several percent niobium and tantalum. Nickel and niobium are both substantially more expensive than iron, so the raw-material cost difference is structural and does not disappear in a strong or a weak market. The table below gives reference ranges for the two alloys in common product forms, followed by the magnetic behaviour that often decides instrument and sensor applications.

Form	A286	Inconel 718	Comment
Round bar	USD 25-40/kg	USD 45-75/kg	Nickel and niobium drive the 718 premium
Wire / fastener stock	USD 30-45/kg	USD 55-85/kg	Drawing and coating add cost
Plate	USD 28-45/kg	USD 50-80/kg	Width and thickness add spread
Forging / flange	Quote by drawing	Quote by drawing	Yield and NDT drive cost
Relative magnetic permeability	~1.01-1.05 (typical)	<1.005 (typical)	measured at low field, typical values

Table note: Prices are a reference range only and float with the LME nickel price and with the niobium and ferro-niobium markets; the ranges quoted here are 2026, EXW Shanghai, USD/kg and are not a quotation. Magnetic permeability figures are typical low-field values; where permeability is a hard requirement, ask for a measured value on the specific heat rather than relying on a typical figure.

The cost comparison has three layers that a purchasing decision should separate. The first is the raw-material cost per kilogram, where A286 has a clear and permanent advantage. The second is the finished-part cost, where the alloy choice changes the amount of material, the machining time and the scrap rate; because 718 is stronger, a 718 bolt can sometimes be smaller than the equivalent A286 bolt, which partially offsets the higher unit price. The third is the total installed cost, where the cost of a failed joint, a rework or an unplanned outage usually dwarfs the difference in material price. For a low-temperature joint the third layer is irrelevant and A286 wins on layer one; for a 680 C turbine flange the third layer dominates and 718 wins.

Supply behaviour also differs in a way that matters for project planning. Inconel 718 is a globally traded, multi-source superalloy available as bar, billet, forging stock, plate and wire from many producers, so lead times are relatively predictable even when the market is tight. A286 is also widely produced but the range of stocked mill product forms is narrower, and large forging stock or heavy bar can require a mill rolling schedule. We hold both alloys in the common bar and fastener-stock sizes and we can hold melt-specific inventory against a customer's programme, which is the practical way to remove lead-time risk from a fastening schedule.

Magnetic permeability frequently makes the decision for instrument, sensor and laboratory applications regardless of cost. Inconel 718 is essentially non-magnetic, with a relative permeability that stays very close to unity, which makes it acceptable in magnetic-sensitive assemblies where a weak ferromagnetic response would disturb a measurement or a magnetic circuit. A286 is more strongly influenced by its iron matrix and shows a measurably higher relative permeability at low field. Where a design specifies a maximum permeability, 718 is normally the safe choice and A286 must be tested on the actual heat before it is released. This is the clearest case in which a small metallurgical difference decides the material despite the cost penalty.

Decision Matrix: Which Alloy for Which Service?

The decision between these two alloys resolves to a small number of governing questions, and the matrix below maps the common duties to a recommendation. The logic is consistent throughout: choose A286 when the temperature, strength and corrosion requirements are within its capability and cost or weight matters, and choose 718 when any one of strength at 600-700 C, creep resistance at temperature, or corrosion and magnetic margin governs.

Application / duty	Recommended alloy	Why
Aerospace fasteners to ~650 C	A286	Adequate stress-rupture life, lower cost and density
Industrial gas-turbine bolting to ~650 C	A286	Proven, economical, ASTM A453 Grade 660 class-selectable
High-pressure turbine flange bolting ~650-700 C	Inconel 718	Creep and stress-rupture margin at the top of the band
Turbine discs, blades and engine hardware	Inconel 718	Strength, fatigue and creep resistance
Downhole and sour-service high-strength fasteners	Inconel 718	Strength plus tolerance of chlorides and H2S-bearing service
Non-magnetic instrument and sensor hardware	Inconel 718	Permeability stays close to unity
High-volume commercial fasteners below 550 C	A286	Lowest finished cost at adequate strength
Weight-critical, large bolt circles	A286	Lower density than 718
Cryogenic high-strength joints	A286	Strength with good toughness in the aged condition
Joints requiring maximum preload at temperature	Inconel 718	Higher yield and lower relaxation rate

Table note: The recommendations reflect the standard mechanical and thermal capabilities of the two alloys and typical published service guidance; they are engineering guidance, not standard requirements. For any safety-critical joint the final selection must be based on the applicable design code, the joint load, the temperature profile and the required stress-rupture life, with the bolting class or grade named on the purchase order.

The decision logic can be reduced to three questions asked in order. First, what is the maximum sustained metal temperature and the required life? If it is at or below about 650 C and the required stress-rupture life is achievable within an available A286 bolting class, A286 is normally the right and cheaper answer. Second, is the operating stress at temperature near the creep-limited regime for A286, for example a high-preload joint in the 600-680 C range where relaxation cannot be tolerated? If so, move to 718. Third, are there secondary requirements such as near-unity magnetic permeability, tolerance of chlorides, or a sour-service constraint? Any of these independently selects 718.

Where the two alloys are close, the tie-break is almost always cost, and the honest answer is that A286 is under-used rather than over-used. Many fastener applications specify 718 where a correctly classed A286 bolt would serve for the life of the plant, because 718 has become the default "safe" choice. A careful review of the actual joint temperature and stress-rupture requirement frequently converts a 718 line item to A286 and reduces both cost and weight with no loss of service life. This is the single most valuable piece of advice we give on this pairing, and it is the one most often ignored.

Standard Index

Standard	Title / scope	Covers	Form
ASTM A453/A453M	High-temperature bolting material with expansion comparable to austenitic stainless	Bolting acceptance, class-based	bolting
ASTM A638/A638M	Iron-nickel-chromium age-hardening alloy bar and forging stock	Composition + mechanical	bar, forging stock
AMS 5737	A286 bar, solution treated and aged	Mechanical minima	bar
AMS 5662	Inconel 718 bar and forging stock	Composition + mechanical + heat treatment	bar, forging stock
ASTM B637	Precipitation-hardening nickel alloy bar, forging and forging stock	Composition + mechanical	bar, forging
ASME SB-637	Same alloy for pressure-retaining components	Code allowable basis	bar, forging
ASTM E8/E8M	Tension testing of metallic materials	Test method	—
ASTM E21	Elevated-temperature tension testing	Test method	—
ASTM E18	Rockwell hardness of metallic materials	Test method	—
ASTM E139	Conducting creep, creep-rupture and stress-rupture tests	Test method	—
ASTM E1476	Standard guide for metals identification (PMI)	Test method	—
GB/T 14992 (China)	Classification and designation of superalloys	Cross-system designation reference only	—

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. GB/T 14992 is a Chinese designation-classification standard shown for cross-system reference only; the Chinese designations for these alloy families are not ASTM equivalents and their limits must never be substituted for the ASTM or AMS limits listed above.

FAQ

Q1: Is Inconel 718 stronger than A286?

Yes, by a wide and consistent margin. In the solution-treated and aged condition, Inconel 718 meets a minimum ultimate tensile strength of about 1275 MPa (185 ksi) and a minimum 0.2 % offset yield strength of about 1034 MPa (150 ksi) under AMS 5662. A286 in its solution-treated and aged condition meets a minimum tensile strength of about 896 MPa (130 ksi) and a minimum yield of about 586 MPa (85 ksi) under AMS 5737, and ASTM A638 Grade 660 for bar is aligned with those same minima. That is roughly a 42 % advantage in tensile strength and a 76 % advantage in yield strength for 718 at room temperature. The gap narrows somewhat at elevated temperature but does not close, because the two alloys harden by different mechanisms and the nickel matrix of 718 resists creep more effectively. For a design that is strength-limited, 718 is the stronger alloy; for a design that is governed by cost or weight and whose stresses are moderate, A286 is entirely adequate.

Q2: What is the maximum service temperature for A286 and for Inconel 718?

For long-term bolting and structural service, A286 is the practical limit to about 650 C and Inconel 718 to about 700 C. A286 begins to lose stress-rupture strength rapidly above roughly 650 C because its gamma-prime strengthened iron matrix creeps faster at that temperature, and its oxidation resistance also becomes a constraint well before 718's does. Inconel 718 retains useful stress-rupture life to about 700 C because its gamma-double-prime strengthened nickel matrix resists coarsening and creep more effectively in that band. Above about 700 C neither alloy is the right long-term answer, and selection moves to gamma-prime strengthened nickel alloys such as Waspaloy, Inconel X-750 or the Nimonic grades such as Nimonic 80A bar. For short-term excursions 718 tolerates much higher temperatures, with an oxidation-limited short-term ceiling on the order of 980 C, but that is not a basis for sustained bolting service.

Q3: Why is A286 cheaper than Inconel 718?

A286 is cheaper because its matrix is iron rather than nickel, and because it contains no deliberate niobium. A286 carries roughly 25 % nickel while Inconel 718 carries roughly 52 % nickel plus several percent of niobium and tantalum. Nickel and niobium are both far more expensive per kilogram than iron, and the difference is structural rather than a market fluctuation, so the A286 price advantage persists in strong and weak markets alike. In 2026 EXW Shanghai reference terms, the material price gap is on the order of 60-90 % for comparable bar and fastener stock. The finished-part gap can be smaller than the raw-material gap because 718's higher strength sometimes allows a smaller cross-section for the same load, and because machining and scrap costs differ. For high-volume fasteners that never exceed about 550-600 C, A286 is usually the more economical answer on a total installed cost basis.

Q4: Which alloy should I use for high-temperature bolting?

Use A286 where the sustained metal temperature is at or below about 650 C and the required stress-rupture life can be met by an available ASTM A453 Grade 660 class; use Inconel 718 where the temperature rises toward 700 C, where the preload is high enough that relaxation cannot be tolerated, or where chlorides, sour service or a non-magnetic requirement are present. The decisive property for a bolt is stress-rupture and relaxation life, not room-temperature tensile strength, so the selection should be made from the joint temperature and the required life and then checked against the class or grade on the drawing. A286 is the default aerospace bolting alloy below about 650 C and is frequently under-used; moving a joint from 718 to a correctly classed A286 bolt often reduces cost and weight with no loss of service life. Our high-temperature fastener alloy selection guide works through the class-selection logic for several common duties.

Q5: What is the difference between ASTM A453 Grade 660 and AMS 5737?

ASTM A453 Grade 660 is a bolting specification, while AMS 5737 is a bar specification. ASTM A453 Grade 660 is written specifically for high-temperature bolting materials whose expansion coefficients are comparable to the austenitic stainless steels, and it defines acceptance through classes A, B, C and D that differ in the required minimum stress-rupture performance and in the test temperature at which that performance is demonstrated. AMS 5737 covers A286 bar in the solution-treated and aged condition and fixes the chemistry, the heat treatment and the room-temperature mechanical property minima. A finished A286 bolt is therefore ordered against ASTM A453 Grade 660, while the bar from which it is made is typically ordered against AMS 5737 or ASTM A638 Grade 660. Quoting the wrong document is a common and expensive specification error, because a bolt made from compliant bar is not automatically a compliant bolt.

Q6: Can A286 and Inconel 718 be welded?

Both alloys can be welded, but the weld and heat-affected zone do not retain the aged condition, so any welded joint that must carry design load at temperature requires a post-weld solution treatment and ageing or a carefully qualified alternative procedure. Inconel 718 is welded with matching-ageable filler and the assembly is normally re-solution treated and aged after welding; direct ageing after welding is possible under some qualified procedures but changes the properties in and around the weld. A286 is welded with matching-ageable filler in the same way and normally requires a post-weld re-solution and age, or an age only, depending on the qualified procedure. Neither alloy needs a post-weld heat treatment for stress relief in the way that a low-alloy steel does, but both need the ageing treatment restored if the design depends on it. Welding should always be qualified by procedure on the actual material, and the properties of the welded joint verified by test.

Q7: What is the heat treatment cycle for A286 and for Inconel 718?

A286 is solution treated at approximately 900 C (1650 F) and rapidly quenched, then age hardened in a single hold of about 16 hours in the 705-760 C (1300-1400 F) range followed by air cooling. Inconel 718 is solution treated in the 940-1010 C (1725-1850 F) range, rapidly quenched, then given a two-step age: about 720 C (1325 F) for 8 hours, a controlled cool to about 620 C (1150 F), a second 8-hour hold, and air cooling. These are the typical commercial cycles and the exact figures, ramp rates and cooling rates are fixed by the governing AMS or ASTM specification and by the customer's process specification. Getting the cycle wrong is dangerous in different ways for the two alloys: an under-aged A286 part usually fails a hardness check, whereas a mis-aged 718 part can pass a room-temperature tensile test while its stress-rupture life is badly degraded.

Q8: Is Inconel 718 magnetic?

Inconel 718 is essentially non-magnetic, with a relative permeability that stays very close to unity, typically below about 1.005 at low field. That makes it acceptable in magnetic-sensitive instrument, sensor and laboratory assemblies where a ferromagnetic response would disturb a measurement or a magnetic circuit. A286 is more magnetic because its matrix is iron: its relative permeability at low field is typically in the 1.01-1.05 range, which is still low but measurably higher than 718. Where a drawing specifies a maximum permeability, 718 is usually the safe choice and A286 must be measured on the actual heat before release. Permeability also depends on the ageing condition and on any cold work, so a typical figure is a guide rather than a guarantee; ask for a measured value on the specific heat when permeability is a hard requirement.

Q9: Which alloy has better corrosion resistance?

Inconel 718 generally has the better high-temperature oxidation and hot-corrosion resistance, and it is more tolerant of chlorides and of sour service at high strength, because of its higher chromium, nickel and molybdenum levels and its nickel-rich matrix. A286 is not a corrosion alloy in the same sense: it behaves roughly like a high-nickel austenitic stainless steel at ambient temperature and is broadly acceptable in mild environments, but its lower chromium and iron matrix leave it less resistant to aggressive chlorides and to hot corrosion. Neither alloy is intended for strongly oxidising acid service, and neither substitutes for a purpose-designed corrosion alloy such as a Hastelloy or an Incoloy grade in severe chemical duty. Where corrosion rather than strength or temperature is the governing requirement, the selection should move to the appropriate corrosion-resistant family; our Hastelloy C-276 plate & bar range and the corrosion literature behind it are the relevant reference for aggressive chloride and acid service.

Q10: What are the UNS and Werkstoff numbers for these alloys?

Inconel 718 is UNS N07718 and Werkstoff 2.4668, and its Chinese cross-system designation is GH4169. A286 is UNS S66286 and Werkstoff 1.4980, and its iron-based Chinese cross-system designation is GH2132. The UNS number is the key identifier on a purchase order and should always be stated alongside the commercial name, because trade names such as A286 and Inconel 718 have several historical variants. The Werkstoff number is used mainly in European procurement and in the German and broader EU standards environment. The Chinese designations are useful for cross-system recognition, but they are not ASTM equivalents: the composition and property limits of the Chinese designations must never be substituted for the ASTM or AMS limits, and a cross-system comparison always requires its own column with the source system clearly identified.

Q11: Can A286 be substituted for Inconel 718?

A286 can be substituted for 718 only where the joint temperature, stress and corrosion requirements are within A286's capability, and never as a simple drop-in replacement on a drawing that specifies 718. The two alloys differ by roughly 40 % in tensile strength and by roughly 50 C in practical long-term temperature ceiling, so a substitution that ignores those differences will produce an under-strength or a creep-limited joint. In the other direction, substituting 718 for A286 is generally technically safe but commercially wasteful. Any substitution should be reviewed against the actual joint load, the maximum sustained temperature, the required stress-rupture life and any corrosion or magnetic requirement, and the bolting class or grade must be re-selected accordingly. Where a customer asks us to review a drawing, we apply exactly that check; a substitution that cannot be justified from the joint data is declined even when the customer has asked for it, because the failure mode would appear in service rather than at receiving inspection.

Q12: Which alloy is better for high-cycle fatigue in turbine hardware?

Inconel 718 is the better alloy for high-cycle fatigue in turbine hardware, because its higher strength, its finer control of inclusions and its resistance to crack initiation at temperature all favour fatigue life. Turbine discs, blades, compressor hardware and engine mountings are 718 applications for exactly this reason, and the alloy's fatigue behaviour is one of the properties that drove its adoption across the engine industry. A286 performs well in fatigue in the fastener and lower-stress hardware roles, and its use in bolting reflects a duty that is dominated by sustained load and thermal cycling rather than by the highest possible crack-initiation resistance. For a fatigue-critical rotating part the selection is 718 or a higher-temperature gamma-prime alloy, not A286. Fatigue performance is also governed by surface condition, so the manufacturing route, particularly thread rolling versus thread cutting and the surface finish of machined features, matters as much as the alloy choice.

Q13: How do I verify that I received the correct alloy?

Ask for the mill certificate and verify it by independent testing, because the two alloys look identical in the hand and a visual check proves nothing. Optical emission spectrometry or X-ray fluorescence portable metals identification will separate them at the first pass: a correct Inconel 718 reading shows roughly 50-55 % nickel with about 5 % niobium plus tantalum and 2.8-3.3 % molybdenum, while a correct A286 reading shows roughly 24-27 % nickel in an iron balance, with no niobium of significance and 1.0-1.5 % molybdenum. The niobium line is decisive, because no iron-based alloy of this family carries about 5 % niobium. Beyond chemistry, verify the condition by hardness and, for critical bolting, by the stress-rupture test required by the class or grade. Finally, reconcile the heat number on the certificate with the heat number stencilled or tagged on the material, and confirm the certificate is traceable to the mill.

Q14: How much do A286 and Inconel 718 cost?

In 2026 EXW Shanghai reference terms, A286 bar typically falls in the range of about USD 25-40/kg and Inconel 718 bar in the range of about USD 45-75/kg, with wire, fastener stock, plate and forgings priced above those ranges according to the amount of processing involved. These are reference ranges only and float with the LME nickel price and with the niobium and ferro-niobium markets, so they must never be treated as a quotation. The finished-part cost gap is usually narrower than the raw-material gap, because 718's higher strength can allow a smaller cross-section for the same load and because machining costs differ between the two alloys. The most important cost observation, however, is that the difference in material price is trivial compared with the cost of a failed high-temperature joint, so the selection should be made on engineering grounds first and then optimised for cost.

Conclusion and Recommendation

The recommendation for this pairing is unambiguous. Specify A286 when the sustained metal temperature is at or below about 650 C, the required stress-rupture life can be met by an available ASTM A453 Grade 660 class, the strength requirement is within roughly 896 MPa tensile and 586 MPa yield, and cost or weight matters. Specify Inconel 718 when the combination of strength at 600-700 C, creep resistance at temperature and corrosion resistance governs, when near-unity magnetic permeability is required, or when the joint is high-pressure and high-temperature enough that relaxation cannot be tolerated. The A286 choice is the economical and, in our experience, the under-used one; the 718 choice is the higher-assurance one.

Do not treat the two alloys as interchangeable. Quote the class or grade on the purchase order, confirm the heat treatment cycle against the governing standard, require the class-specific stress-rupture data for bolting, and verify the delivered material by chemistry and condition rather than by appearance. Shanghai Hangbo Alloy Group Co., Ltd. supplies both alloys with full traceability, EN 10204 3.1 certification and third-party inspection, and we will review a drawing or a joint calculation before quoting so that the alloy and the bolting class are matched to the duty. Send your drawing, temperature and load data through our contact page and we will return a material recommendation with the applicable standard citations.

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