Inconel X-750: Spring & Fastener Superalloy

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Overview

Inconel X-750 (UNS N07750 / W.Nr. 2.4669) is a precipitation-hardenable nickel-chromium superalloy that derives its strength primarily from the gamma-prime (Ni₃(Al,Ti)) intermetallic phase. Developed as an evolution of the earlier Nimonic 80A alloy, Inconel X-750 was specifically designed to provide a combination of high strength, excellent spring performance, and outstanding relaxation resistance at elevated temperatures. It has become the industry standard material for hot springs, high-temperature fasteners, and structural components in gas turbines and jet engines.

The alloy's chemistry features carefully controlled additions of aluminum and titanium to form the gamma-prime strengthening phase during aging heat treatment. Unlike Inconel 718, which is strengthened primarily by gamma double-prime (Ni₃Nb), Inconel X-750 relies on the more thermally stable gamma-prime precipitate, which provides excellent long-term strength retention but at a lower absolute strength level compared to Inconel 718 at room temperature. This makes X-750 the preferred choice where sustained load-bearing capacity at 600-800°C is more important than peak room-temperature strength.

One of the most valuable characteristics of Inconel X-750 is its spring relaxation resistance. At temperatures where conventional spring steels lose 50-80% of their initial load, X-750 springs retain the majority of their preload, making it indispensable for turbine gland seals, compressor ring springs, and exhaust system clamps that must function reliably over thousands of thermal cycles. The alloy also exhibits excellent fatigue life in the aged condition, particularly when processed by the optimized double-aging treatment.

Processing typically involves vacuum induction melting (VIM) followed by vacuum arc remelting (VAR) or electroslag remelting (ESR). For spring wire production, the material is cold drawn to final diameter in the solution-annealed condition, then aged to develop the required spring properties. For bars and forgings, hot working in the 950-1100°C range is standard. Machining in the aged condition requires carbide tools with low cutting speeds, rigid setups, and ample coolant, similar to other nickel superalloys.

At Hangbo Alloy Group, Inconel X-750 is supplied in wire, round bars, sheets, and forgings per AMS 5670, AMS 5671, ASTM B637, and custom specifications. We serve aerospace engine manufacturers, power generation turbine builders, and industrial spring manufacturers worldwide with certified material and competitive delivery times.

Chemical Composition (ASTM B637 / AMS 5670)

The composition of Inconel X-750 is balanced to promote the formation of gamma-prime precipitates while maintaining good fabricability and oxidation resistance. Aluminum and titanium are the key strengthening elements; their ratio and total content directly affect the volume fraction and stability of the gamma-prime phase. Carbon is kept low to minimize carbide formation and protect ductility, while niobium provides additional strengthening and improves notch rupture life at elevated temperatures.

Element	Min %	Max %
Nickel + Cobalt (Ni+Co)	70.0	—
Chromium (Cr)	14.0	17.0
Iron (Fe)	5.0	9.0
Titanium (Ti)	2.25	2.75
Aluminum (Al)	0.40	1.00
Niobium + Tantalum (Nb+Ta)	0.70	1.20
Manganese (Mn)	—	1.00
Silicon (Si)	—	0.50
Sulfur (S)	—	0.010
Copper (Cu)	—	0.50
Carbon (C)	—	0.08
Cobalt (Co)	—	1.00
Phosphorus (P)	—	0.015
Boron (B)	—	0.006

Physical Properties

Inconel X-750 has a face-centered cubic (FCC) austenitic matrix that remains stable across its entire service temperature range. The gamma-prime precipitates formed during aging are coherent with the matrix and provide strength without significant dimensional change, which is critical for precision spring applications. The alloy's moderate thermal expansion coefficient and thermal conductivity are favorable for components experiencing thermal cycling.

Property	Value	Unit
Density	8.28	g/cm3
Melting Range	1393 - 1427	°C
Specific Heat (21°C)	431	J/kg·K
Thermal Conductivity (21°C)	11.7	W/m·K
Electrical Resistivity (21°C)	1.22	μΩ·m
Modulus of Elasticity (21°C)	214	GPa
Mean Coefficient of Thermal Expansion (21-93°C)	12.6	μm/m·°C
Curie Temperature	-135	°C

Mechanical Properties at Room Temperature

Mechanical properties of Inconel X-750 depend on the specific aging treatment applied. The standard single-age treatment (705°C for 24 hours) provides moderate strength with good ductility, suitable for general structural applications. The double-age treatment (705°C for 24 hours + 620°C for 8 hours) produces higher strength and is preferred for springs and fasteners. The values below are typical for bars and forgings in the fully aged condition per AMS 5671.

Property	Value
Tensile Strength	1170 MPa (170 ksi)
Yield Strength (0.2% offset)	795 MPa (115 ksi)
Elongation in 2 inches	15 - 25%
Reduction of Area	20 - 40%
Hardness	30 - 40 HRC
Charpy V-notch Impact (room temp)	30 - 50 J

Heat Treatment Conditions

Inconel X-750 is supplied in either solution-treated or solution-treated plus aged condition. The choice of heat treatment significantly affects both the absolute strength level and the relaxation resistance at elevated temperature. Three primary heat treatment schedules are used in practice, each optimized for different service requirements:

• Standard Single Age (AMS 5671): Solution anneal at 1040-1093°C, air cool; age at 705°C for 24 hours, air cool. Produces tensile strength of approximately 1080 MPa with good ductility. Suitable for general structural applications and moderate-temperature service.
• Double Age (Spring & Fastener Optimum): Solution anneal at 1040-1093°C, air cool; first age at 705°C for 24 hours, air cool; second age at 620°C for 8 hours, air cool. This treatment produces the highest yield strength (~795 MPa) and the best spring relaxation resistance. It is the preferred specification for hot springs and high-temperature fasteners.
• Equalized + Double Age (Turbine Discs): Equalizing treatment at 885°C for 24 hours, furnace cool to 705°C, hold 24 hours, air cool; then age at 620°C for 8 hours, air cool. Produces slightly lower strength but improved notch rupture life at 540-650°C. Used for turbine discs and shafts where creep rupture ductility is critical.

The equalizing step at 885°C serves to dissolve grain boundary carbide films that can form during prior processing, thereby improving stress rupture ductility. For spring wire, the cold-drawn material is typically given the double-age treatment after forming, which produces maximum spring properties without the need for prior solution treatment. Hangbo Alloy Group provides material in all standard heat treatment conditions with full certification and can perform custom heat treatments per customer specification.

High-Temperature Performance

Inconel X-750 is valued primarily for its ability to retain strength and resist stress relaxation at temperatures where most engineering alloys rapidly degrade. The gamma-prime precipitates remain stable up to approximately 750°C, providing a useful strength advantage over solid-solution alloys like Inconel 600 or 625. Above 800°C, the precipitates begin to coarsen significantly, and long-term strength drops to levels comparable to solid-solution alloys. However, short-term exposure to temperatures up to 870°C is permissible for applications with limited dwell times.

Temperature (°C)	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)
21 (Room)	1170	795	20
316	1060	720	22
427	985	655	22
538	895	570	24
649	690	440	26
704	530	360	28
760	380	270	32
816	260	190	38

Spring Relaxation Resistance

The defining performance advantage of Inconel X-750 over conventional spring steels (such as music wire or chrome-vanadium steel) is its dramatically superior relaxation resistance at elevated temperatures. In spring applications, relaxation refers to the loss of initial preload or deflection over time when the spring is held at temperature under load. This is a critical failure mode for turbine gland seal springs and exhaust clamps, where loss of clamping force can lead to gas leakage, vibration, or component separation.

Typical relaxation data for double-aged X-750 springs compared to chromium-vanadium steel:

• At 315°C: X-750 retains 95-98% of initial load vs. Cr-V steel retaining 70-80%.
• At 427°C: X-750 retains 90-95% of initial load vs. Cr-V steel retaining 40-60%.
• At 538°C: X-750 retains 80-90% of initial load vs. Cr-V steel retaining 20-30% (essentially unsuitable).
• At 650°C: X-750 retains 65-80% of initial load, making it the only practical metallic spring material for continuous service at this temperature without resorting to ceramic or refractory metal springs.

The double-age heat treatment consistently produces the best relaxation resistance because the secondary aging at 620°C stabilizes the precipitate structure against further transformation during service exposure. Springs manufactured from the single-aged condition tend to show slightly more relaxation because the precipitates undergo additional thermal stabilization during service, which is accompanied by a modest strength reduction.

Corrosion Resistance

Inconel X-750 possesses good corrosion resistance in a range of environments, although it is primarily selected for its mechanical and spring properties. Its 14-17% chromium content provides a protective oxide film that resists oxidation and many forms of atmospheric corrosion. In environments requiring maximum corrosion resistance, alloys such as Inconel 625 or Hastelloy C-276 are generally preferred.

Resistance to Specific Media:

• Oxidation Resistance: Excellent in air and combustion gases up to 800°C. The chromium oxide scale is adherent and protective, and the alloy performs well in cyclic oxidation conditions experienced in gas turbine hot sections.
• Chloride Stress Corrosion Cracking: Resistant to chloride-induced stress corrosion cracking at moderate temperatures, unlike austenitic stainless steels that are susceptible in hot chloride environments. However, it is not immune to pitting in concentrated chloride solutions.
• Sulfidation: Moderate resistance to sulfidation in combustion gases containing sulfur compounds. For severe sulfidation service, alloys with higher chromium content such as Haynes 230 or Inconel 625 are recommended.
• Hydrogen Embrittlement: Generally resistant to hydrogen embrittlement in the double-aged condition, which is important for fastener applications in hydrogen-containing environments. The alloy should not be used in the over-hardened condition (>40 HRC) in hydrogen service.

Applications

The combination of spring relaxation resistance, moderate-to-high strength, and thermal stability makes Inconel X-750 a preferred material for demanding high-temperature spring and fastener applications, as well as structural components requiring long-term dimensional stability under load at elevated temperature.

• Aerospace Gas Turbine Springs: Gland seal springs, compressor diaphragm springs, turbine ring springs, and thrust reverser springs. X-750 is the dominant spring material in jet engines worldwide because no other metallic spring alloy can match its relaxation resistance at 450-650°C.
• High-Temperature Fasteners: Bolts, studs, and nuts for gas turbine casings, exhaust manifolds, and heat exchanger flanges where preload must be maintained during thermal cycling. The alloy's strength retention and low relaxation make it ideal for bolted joints that cannot be re-torqued during service.
• Power Generation: Turbine disc retaining rings, steam turbine gland seals, boiler tube hangers, and hot-section structural supports in combined-cycle and advanced ultra-supercritical power plants.
• Nuclear Reactors: Control rod guide tube springs, fuel element spacer springs, and reactor internals springs where long-term reliability under radiation and temperature is required. X-750 has a long service history in both PWR and BWR reactor designs.
• Industrial Heat Treatment: Furnace door springs, conveyor chain springs, and heat treatment basket springs operating at 400-700°C in oxidizing atmospheres.
• Racing and Automotive: Exhaust system clamps and springs, turbocharger actuator springs, and high-temperature valve springs in competition engines.

Available Product Forms

Hangbo Alloy Group manufactures and supplies Inconel X-750 in a comprehensive range of product forms optimized for spring manufacturing, fastener production, and structural fabrication:

• Spring Wire: AMS 5670, cold-drawn to finished diameter, supplied in the solution-annealed condition for customer coiling and aging. Diameters 0.5 mm to 10 mm, precision tolerance, surface quality suitable for spring coiling.
• Round Bars: ASTM B637, AMS 5671, diameters 6 mm to 300 mm, hot-rolled, forged, or cold-drawn, solution treated or fully aged.
• Forgings: ASTM B637, custom open-die and closed-die forgings for turbine discs, rings, and structural components, heat treated per AMS specifications.
• Plates & Sheets: AMS 5598, AMS 5599, thickness 0.5 mm to 50 mm, for fabrications and structural panels.
• Seamless Tubes: AMS 5582, OD 6 mm to 120 mm, for instrumentation lines and structural tubing.
• Welding Wire: AWS A5.14 ERNiCrFe-8, for repair and fabrication of X-750 components.

Related Standards

Standard	Description
ASTM B637	Bar, Forging, and Forging Stock
AMS 5670	Wire, Spring, Solution Treated
AMS 5671	Wire, Spring, Spring Heat Treated
AMS 5598	Sheet, Strip, and Plate, Solution Treated
AMS 5599	Sheet, Strip, and Plate, Precipitation Heat Treated
AMS 5582	Seamless Tubing, Solution Treated
AMS 5667	Bar and Forging, Solution Treated
AMS 5668	Bar and Forging, Precipitation Heat Treated
AWS A5.14	Welding Wire (ERNiCrFe-8)
ASME SB-637	Boiler and Pressure Vessel Code

Contact Us for Inconel X-750

Hangbo Alloy Group maintains ready supply of Inconel X-750 spring wire, round bars, forgings, and plates in AMS 5670, AMS 5671, and ASTM B637 conditions. Our technical team can advise on the optimal heat treatment selection for your specific application — whether you need maximum spring relaxation resistance, improved notch rupture life, or general structural strength. We provide full heat treatment certification, ultrasonic testing, and spring wire surface quality reports.

For quotations, material certifications, or technical consultation, contact our sales team or call +86-136-1165-6360. We typically respond within 10 minutes.