​‌​​‌​

Inconel X750 / (2.4669) / UNS N07750 / AMS 5542 / AMS 5598

​‌​​‌​

Inconel X750, one of the most widely used of the nickel alloys, is a material that withstands even highly corrosive environments. Inconel Alloy X-750 can be hardened by ageing and is formed essentially from nickel and chromium.

Containing around 70% nickel and around 15% chromium, this nickel alloy shows considerable oxidation resistance up to 704 °C. It can also deliver very high mechanical strength up to that same temperature of 704 °C. Above this temperature Alloy X-750 loses much of its mechanical strength, although it retains a certain amount of it up to 982 °C. This nickel alloy also preserves its properties very well even at the cryogenic temperatures far below zero.​‌​​‌​

It has a very wide field of use. Alloy X-750 is used in gas turbine blades, gas turbine wheels, turbine bolts and other turbine fasteners. Another of its most common applications is in the thrust section components of rocket engines (the gas exit point, for example). Able to withstand high pressure, the material is also frequently used in high pressure vessels. It is further used in heat transfer fasteners, extrusion pusher tooling and many other parts requiring high mechanical strength. Springs and fasteners such as screws in this grade are frequently used in service environments operating below zero.

The Heat-Treatment Condition System — the Centrepiece of This Page​‌​​‌​

​‌​​‌​

​‌​​‌​

X-750 is not one material. “X-750” names a chemistry; the condition names the part. There is no standard condition, and buyers routinely order the wrong one. That is why this page starts with the condition system rather than with the standards.

AMS 5699 is ambiguous, and its default is good only to 700 °F. That number covers two different heat treatments, two different minimum tensile strengths and two different service windows. A purchase order saying only “AMS 5699” does not determine which you get — and the higher-strength default is the one that cannot go above about 700 °F. Specify the cycle and the service temperature on the order.​‌​​‌​

Why the cycles are what they are: 1149 °C dissolves γ′ and soluble carbides and removes dislocations — which is why the triple treatment gives creep strength, and why “once the material has been solution-treated, it should not be subjected to any cold work since it will generate new dislocations and thus impair rupture properties”. The 843 °C/24 h step precipitates grain-boundary M₂₃C₆ and leaves a γ′-denuded zone; the 704 °C/20 h age then fills that zone with γ′. Creep resistance stems from the uniform dispersion of intragranular γ′, whereas rupture properties relate more closely to the grain-boundary microstructure. Peak ageing hardness is reached at 704 °C/20 h.

Standards by Product Form — and Three Common Errors​‌​​‌​

Three errors, in order: (1) “ASTM B637 X-750 plate/sheet/tube” — B637’s title is “…Nickel Alloy Bars, Forgings, and Forging Stock…”; there is no flat product and no tube in it. (2) “ASTM B983 X-750 seamless tube” — B983’s alloy list is N07022, N07725, N07740, N09945, N09925, N07718, N10276 and N06985; N07750 is not in it. (3) “ASTM B670 X-750 plate” — B670 covers UNS N07718 only. Also, AMS 5667 cannot be used for sheet: it is bar, rod and forging stock only, and this is the most common form error precisely because AMS 5667 is the number buyers know. AMS 5669 is inferred from the mill’s “AMS 5667–5671” range wording but does not appear in independent listings — do not advertise it.

Composition: Ni (+Co) ≥70.0 % · Cr 14.0–17.0 % · Fe 5.0–9.0 % · Ti 2.25–2.75 % · Al 0.40–1.00 % · Nb (+Ta) 0.70–1.20 % · Mn ≤1.00 % · Si ≤0.50 % · Cu ≤0.50 % · C ≤0.08 % · Co ≤1.00 % · S ≤0.01 %. The hardening mechanism is γ′ — Ni₃(Al,Ti), plus grain-boundary M₂₃C₆ after the 843 °C stabilise. Two genuine divergences: Mn at 1.00 % max (originator/ASTM) against 0.30 % max (the tighter aerospace bar specifications); Ti at 2.25–2.75 % against 2.25–2.70 %. Always publish limits together with the standard they come from, never as “the chemistry of X-750”.​‌​​‌​

Mechanical Minimums — by Condition

The ductility collapse between 1000 and 1200 °F — the practical reason for the condition boundary. Typical values for equalized-and-aged bar: 1200 MPa tensile and 26.8 % elongation at 29 °C; 1076 MPa and 26.5 % at 538 °C; 1058 MPa and 19.0 % at 593 °C; 941 MPa and only 10.0 % elongation at 649 °C (reduction of area 17.7 %). That is why the 593 °C (1100 °F) line exists.

Cryogenic: triple-heat-treated bar gives 1196 MPa tensile at 21 °C, 1282 MPa at −76 °C, 1440 MPa at −196 °C and 1435 MPa at −253 °C; yield rises from 700 to 896 MPa while elongation falls from 25 % to 14.5 %. The notch tensile ratio (Kt = 6.3) is 0.70 at room temperature and 0.65 at −253 °C — so notch toughness does not collapse cryogenically.​‌​​‌​

Creep rates, S-N fatigue curves and spring relaxation percentages are not printed on this page — the originator publishes those data only as figures, and estimates read off graphs were deliberately discarded.

Welding, Machining and Forming​‌​​‌​

Welding — managing strain-age cracking

Processes: GTAW, plasma-arc, electron-beam, resistance and pressure-oxyacetylene welding. It is brazeable (select a braze alloy melting above the precipitation temperature, since ageing follows brazing). Resistance welding is done in the annealed or solution-treated condition.​‌​​‌​

The governing rule, verbatim from the originator: “It is possible to weld it when it is in the precipitation-treated condition, but neither the weld or the heat-affected zone should be subsequently precipitation-treated or exposed to service temperatures within the precipitation-hardening temperature range because of the danger of parent-metal cracking.” Before welding, the material should be in the annealed or solution-treated condition.

How to manage post-weld treatment: solution-treat before precipitation-treating a weldment. “Rate of heating of the weldment up to temperature must be fast and uniform” — the aim is to minimise time spent in the precipitation-hardening range, so heating must be fast through roughly 482–871 °C. The practical means is to charge the fabricated part into a pre-heated furnace. Optional pre-weld conditioning treatments: (1) 843 °C/16 h, air cool; or (2) 1066 °C/1 h, furnace-cool at 14–56 °C per hour to 649 °C, air cool. Interbead cleaning is mandatory: oxide films must be ground or abrasive-blasted off between passes.​‌​​‌​

Joint efficiency is about 100 % at room temperature and about 80 % at 704–816 °C; there is HAZ softening, and weld joints should be located where lower creep properties are acceptable. On 16 mm plate with Filler Metal 69: annealed and aged before welding and left as-welded, the transverse specimen gave 855 MPa / 600 MPa / 21 % at 27 °C; annealed before welding and then precipitation-treated after welding, 1203 MPa / 876 MPa / 16 %. So post-weld ageing recovers about 40 % more strength and pays for it in ductility.

Machining​‌​​‌​

Rough-machine before precipitation hardening; finish-machine after. Ageing relieves machining stresses — allow for warpage; aged material is then dimensionally stable. A slight permanent contraction occurs during the 704 °C/20 h age: 0.00044 in/in (hot-rolled), 0.00052 in/in (20 % cold-rolled), 0.00026 in/in (annealed) — which matters on finish-machined parts. The alloy work-hardens rapidly and is “more difficult to machine than most standard ferritic and martensitic alloys”. It machines most easily in the equalized (885 °C) condition. Use high machine power and slow cutting speeds; sharp, smooth-finished, rigid tooling; a continuous, smooth cutting action to avoid glazing and work-hardening; minimal backlash; rigid support of tool and workpiece; and avoid very light cuts and feeds (they ride on the work-hardened layer instead of cutting under it). Numeric cutting speeds by condition are not printed on this page — the mill’s separate machining publication and the industry machining guide could not be consulted.

Forming, coatings and pickling​‌​​‌​

Hot working 982–1204 °C; all heavy hot work above 1038 °C; forgings may be finished with light reduction in the 982–1038 °C band. Air-cool after heating; liquid quenching is not recommended (particularly for large sections and complex parts — it sets up stresses that can cause thermal cracking on subsequent heating). A specification requirement, not advice: approximately 20 % final reduction must be done below 1093 °C to meet the requirements of AMS 5667, 5670, 5671 and 5747; a forging finished entirely above 1093 °C may fail its own AMS acceptance. Cold forming: the strain-hardening rate is rapid and multi-stage reductions need sufficient intermediate anneals. Wire is process-annealed at 1038 °C, with about 40 % cold reduction between anneals preferred.

Two scrap generators: (1) Lead and copper lubricants must be removed before heat treatment. Lead is used for cold drawing, copper for cold heading and spring manufacture; residual lead causes “diseasing and cracking” (liquid-metal embrittlement) and residual copper dilutes the surface and loses properties. Removal is in a 15–20 % nitric acid bath. Where lead is prohibited — including nuclear applications — oxalates are used instead. (2) Nitric-hydrofluoric pickling causes intergranular attack, worst in the precipitation-hardened condition; keep the time short and the bath below 52 °C (125 °F). Fused-salt pretreatment is preferred for scale removal.​‌​​‌​

Springs must be supported on a snug-fitting arbor during the triple heat treatment, especially the 1149 °C step, to prevent sagging. The thin oxide formed in heat treatment is beneficial and should normally be left on — removing it mechanically lowers relaxation resistance, which is counter-intuitive and therefore routinely violated by finishing shops.

Service Limits and Physical Properties​‌​​‌​

Physical properties (triple heat treated): density 8.28 g/cm³ (one source gives 8.30; the 8.4 g/cm³ circulating on an aggregator site is wrong); melting range 1393–1427 °C; modulus of elasticity at 27 °C about 214 GPa (the same aggregator gives 190 GPa — the mill value is preferred); Poisson’s ratio 0.29; thermal conductivity about 12.0 W/m·K at 21 °C; specific heat about 431 J/kg·K; mean linear expansion about 12.6 × 10⁻⁶/°C over 21–93 °C; oxidised emissivity 0.895 at 316 °C and 0.925 at 1093 °C.​‌​​‌​

Frequently Asked Questions

Which heat-treatment condition should we order, and what goes wrong if we just order “X-750”?​‌​​‌​

There is no standard condition. “X-750” names a chemistry; the condition names the part. Order by service temperature. Below 593 °C (1100 °F) there are two choices: equalized plus aged (885 °C/24 h air cool, then 704 °C/20 h air cool — AMS 5667, 302–363 HB, 1138 MPa minimum tensile under 100 mm) and solution treated plus furnace-cooled age (982 °C/1 h, then 732 °C/8 h furnace-cooled to 621 °C, 18 h total — AMS 5670/5671/5747, 32–42 HRC, 1172 MPa minimum). Above 1100 °F you need the triple heat treatment: 1149 °C/2–4 h, 843 °C/24 h, 704 °C/20 h (AMS 5668), qualified against a 100-hour rupture life at 732 °C / 310 MPa. Springs are a world of their own: 649 °C/4 h Spring Temper for cryogenic to 371 °C; 732 °C/16 h No. 1 Temper for relaxation resistance to about 482 °C; triple-heat-treated Spring Temper for 482–649 °C. What goes wrong: order equalized-and-aged bar for a rotating part at 649 °C and you get material whose elongation has already fallen to about 10 %, with no grain-boundary carbide structure to carry rupture life. Order the triple heat treatment for a room-temperature bolt and you pay for a 1149 °C cycle that lowers your yield strength. And “equalized” is not “annealed”: equalized bar has never been solution treated, so you cannot re-age it into a different condition.

X-750, Inconel 718 or Waspaloy — when is each actually required?​‌​​‌​

Split them by temperature and by whether you have to weld. X-750 is the γ′ (Ni₃(Al,Ti)) alloy. Its strength story ends around 704 °C; the originator’s rating of oxidation resistance to 982 °C is not the same thing. It is the cheapest of the three, available in every mill form, effectively non-magnetic (permeability 1.0020–1.0035 at 70 °F), and outstanding for springs and bolting from cryogenic temperatures upward — tested to −253 °C with tensile strength rising to about 1435 MPa and elongation still 14.5 %. Inconel 718 is the one to pick when the fabrication involves welding. Its niobium-based hardening is deliberately sluggish: the originator states that “the alloy can be heated and cooled through the aging temperature range at normal speeds yet retain softness and ductility,” and calls its resistance to postweld cracking “outstanding”; service range −253 to 704 °C. X-750 has no such margin — age a welded X-750 assembly wrongly and the parent metal cracks. Waspaloy buys temperature: with 18–21 % Cr, 12–15 % Co, 3.5–5 % Mo and 2.75–3.25 % Ti it holds 1000-hour rupture strengths of 614 MPa at 649 °C, 290 MPa at 760 °C and 110 MPa at 871 °C, and is rated to 649 °C for critical rotating parts and 871 °C for less demanding ones. You pay for it in cost and in forging difficulty.

Our supplier quoted “ASTM B637 X-750 plate” or “ASTM B983 X-750 tube”. Is that real?​‌​​‌​

No, and both are worth catching before the order goes out. ASTM B637 is titled “Standard Specification for Precipitation-Hardening and Cold Worked Nickel Alloy Bars, Forgings, and Forging Stock for Moderate or High Temperature Service” (current revision B637-26), and its scope is “rod, bar, forgings, and forging stock”. There is no plate, sheet, strip or tube in it; a B637 certificate against flat product certifies nothing about that product. ASTM B983 does cover seamless nickel-alloy pipe and tube, but its alloy list is N07022, N07725, N07740, N09945, N09925, N07718, N10276 and N06985 — N07750 is not among them. X-750 seamless tubing is bought to AMS 5582, aged at 704 °C/20 h, with a 1069 MPa minimum tensile and a rupture requirement of 23 hours at 732 °C / 310 MPa. ASTM B670, sometimes offered for X-750 plate, covers UNS N07718 only. In fact there is no ASTM specification for X-750 in flat product at all: sheet, strip and plate are bought to AMS 5542 (704 °C/20 h age) or AMS 5598 (furnace-cooled age) — and note that AMS 5596 and 5597, which sit right beside 5598 in the series, are Inconel 718 sheet specifications, not X-750.

​‌​​‌​