Alloy X-750, 2.4669, UNS N07750, Inconel X-750

Alloy X-750, commonly referred to as Inconel® X-750, is a nickel-chromium alloy. It was specifically developed to achieve high creep resistance—resistance to deformation at elevated temperatures.

The titanium and aluminum content makes welding challenging, but since the Al+Ti level is below 6%, welding does not pose an immediate risk of strain-age cracking. Proper techniques and heat treatment are required. For more details, refer to the "Welding" section.

Enriching the X-750 alloy with elements such as cobalt, molybdenum, boron, zirconium, tungsten, and/or niobium transforms it into a superalloy capable of being used in gas turbine components.

Alloy X-750, 2.4669, UNS N07750, Inconel X-750

What is Alloy X-750, 2.4669, UNS N07750, Inconel X-750?

Alloy X-750: Properties and Applications

Properties:

  • High Strength: Excellent mechanical properties at temperatures up to 1300°F (700°C).
  • Corrosion Resistance: Good resistance to oxidation and corrosion.
  • Creep Resistance: Maintains performance under prolonged stress at high temperatures.
  • Precipitation-Hardenable: Strengthened by heat treatment (aging).

Applications:

  • Aerospace: Jet engine components, gas turbine parts.
  • Nuclear Industry: Reactor components, fuel assembly hardware.
  • Industrial: Fasteners, springs, high-temperature tooling.
  • Chemical Processing: Equipment exposed to corrosive environments.

A versatile nickel-chromium alloy, X-750 is ideal for high-stress, high-temperature applications.

Applications

Typical applications include gas turbine components (both aerospace and industrial turbines), high-temperature fasteners, springs, and rocket engines. With slightly modified heat treatments, Alloy X-750 is also used in nuclear reactor components, primarily in pressurized water reactors (PWRs) and boiling water reactors (BWRs). Additionally, it finds use in cryogenic applications.

Chemical Composition (%)

Alloy X-750, 2.4669, UNS N07750, Inconel® X-750 according to ASTM B637 and EN 10302

Standard Grade Ni: Cr: Fe: Ti: Al: Nb: Co: Mn: Cu: Si: C: S: P: Comments:
ASTM

Alloy X-750,N07750,Inconel X-750

 >70.0 14.0-17.0 5.0-9.0 2.25-2.75 0.4-1.0 0.7-1.2 <1.0 <1.0 <0.5 <0.5 <0.08 <0.01 - Nb:Nb+Ta
EN

2.4669 / NiCr15Fe7TiAl

 >70.0 14.0-17.0 5.0-9.0 2.25-2.75 0.4-1.0 0.7-1.2 <1.0 <1.0 <0.5 <0.5 <0.08 <0.015 <0.02 Nb:Nb+Ta
ISO

NW7750

 >70.0 14.0-17.0 5.0-9.0 2.2-2.8 0.4-1.0 0.7-1.2 <1.5 <1.0 <0.5 <0.5 <0.08 <0.015 - Nb:Nb+Ta; Ni:Ni+Co
GOST-R

EP601 / NiCr15Fe7TiAl

 Balance 14.0-17.0 5.0-9.0 2.0-2.6 1.10-1.35 0.7-1.2 - <0.5 - <0.5 0.03-0.10 <0.015 <0.015 Nb:Nb+Ta
JIS

NCF750

 >70.0 14.0-17.0 5.0-9.0 2.25-2.75 0.4-1.0 0.7-1.2 - <1.0 <0.5 <0.5 <0.08 <0.015 <0.03 Nb:Nb+Ta; Ni:Ni+Co

Material Properties

Mechanical properties of Inconel® X-750 bar at room temperature

  • Tensile strength: 1200 MPa
  • Yield strength: 815 MPa
  • Elongation: 27%

High-temperature (540°C) mechanical properties of Inconel® X-750 bar

  • Tensile strength: 1050 MPa
  • Yield strength: 725 MPa
  • Elongation: 26%

Rupture strength of Inconel X-750 bar at elevated temperatures

  • 650°C: 470 MPa
  • 870°C: 50 MPa

Corrosion Behavior

As a superalloy, X-750 exhibits high corrosion resistance in moderately aggressive environments. Below are the test results of its tolerance to some challenging conditions.

Molten carbonate – Corrosion data from 504-hour exposure to molten carbonate salt at 900°C demonstrate that Inconel® X-750 has sufficient tolerance to molten carbonates. The total corrosion depth measured was 0.27 mm.

High-temperature water – Inconel® X-750 is susceptible to stress corrosion cracking (SCC) in deaerated high-purity water at 300–350°C and to intergranular SCC in oxygenated high-temperature water applications. Non-optimized heat treatments further reduce the alloy’s resistance to high-temperature water (see the "Heat Treatment" section for more details).

Halides – X-750 is not resistant to hydrogen fluoride (HF) and hexafluorosilicic acid (H₂SiF₆) at temperatures above 50°C. Exposure to these agents can cause SCC.

X-750 is not tolerant to hydrogen fluoride (HF), which leads to stress corrosion cracking.

Physical Properties

  • Density: 8.28 g/cm³
  • Initial melting temperature: 1290 °C
  • Precipitation temperature: 955 °C

Heat Treatment

Soft Annealing

  • Temperature: 955–1155°C
  • Duration: 30–60 min (continuous annealing) or 60–180 min (batch annealing)
  • Cooling method: Air cooling

Solution Treatment Before Age Hardening
While nickel alloys typically do not require solution treatment before aging, Alloy X-750 benefits from it to enhance creep resistance, stress relaxation resistance, and rupture strength above 600°C—critical for high-temperature springs and turbine blades. For this purpose, Alloy X-750 can undergo solution treatment followed by air cooling before a dual (high- and low-temperature) aging cycle. Different temperatures are specified in AMS 5668 and AMS 5671.

However, a two-stage solution treatment does not provide optimal corrosion resistance in very hot, deaerated water (typical of boiling water reactor environments). For such applications, literature recommends a single-stage solution treatment (note: this may reduce corrosion resistance in oxygenated water).

Processing Performance

Weldability

This alloy falls under Group 43 in ISO 15608.

Welding consumables for Inconel® X-750:

  • Filler metal: ERNiFeCr-2

Best practices for welding Al- and Ti-containing alloys:

  • Use proper joint design, welding techniques, and filler metals to minimize residual and thermally induced stresses.
  • Keep heat input as low as possible during welding.
  • For heavy-section welds, apply multiple stress-relief anneals between welding passes.

Heat Treatment, Forming, and Machining

Aluminum or copper particles on the surface of Inconel® X-750 can rapidly alloy at high temperatures, degrading corrosion resistance and mechanical properties. All contaminants must be removed before heat treatment.

Hot-forming pressures for Alloy X-750:

  • 870°C: 335 MPa
  • 1040°C: 299 MPa
  • 1095°C: 265 MPa
  • 1150°C: 230 MPa

Forging
Below 980°C, this alloy is hard and difficult to work; attempting to forge it may cause cracking. Heavy forging should be performed at around 1040°C, and the metal should be reheated if cooled below this temperature. Some light reduction can be done between 980°C and 1040°C.

  • Forging range: 980–1205°C
  • Heavy forging range: 1040–1205°C
  • Cooling method: Air cooling

Variable cross-sections are particularly prone to hot cracking during cooling. For very large cross-sections, furnace cooling may be necessary.

Machining
Alloy X-750 belongs to Machinability Group D-2, meaning it is extremely difficult to machine. Rough machining should be performed in the solution-annealed condition, followed by finish machining after aging. Aging causes dimensional shrinkage of up to ~0.07%, which must be accounted for during rough machining.

For nickel-based alloys, power spinning is preferred over manual spinning. The practical limit for manual spinning of Alloy X-750 is a blank thickness of 0.94 mm, with a maximum blank hardness of 94 HRB.

Electropolishing

Electropolishing technique for Alloy X-750:

  • Electrolyte composition: 25 mL H₃PO₄, 25 mL HNO₃, 50 mL H₂O
  • Cathode: Platinum
  • Conditions: 17.8 A/cm² for 5–10 seconds

Alternative Names, Equivalents, and Other Designations

  • Alloy X-750
  • Alloy x750
  • N07750
  • AISI 688
  • 72Ni15.5Cr-0.95Cb
  • 2-5Ti
  • 0.70Al-7.0Fe
  • AMS 5698
  • ASTM B637 Grade 688
  • Pyromet Alloy X-750
  • NAS 750
  • AMS 5747
  • AMS 7246
  • AMS 5542
  • AMS 5583
  • AMS 5667
  • AMS 5671
  • AMS 5779
  • AMS 7246B
  • AMS AS7245
  • Inconel® X-750
  • Unitemp® 750
  • Nickelvac® X-750
  • J467
  • SAE J467
  • 2.4669
  • NW7750
  • NiCr15Fe7TiAl
  • NC750
  • EP601
  • Sanicro® 75XT
  • Sanicro® 75X1T