Inconel 718 vs Inconel X750 – Composition, Heat Treatment, Properties, and Applications

Introduction

Inconel 718 and Inconel X-750 are two of the most commonly specified precipitation‑strengthened nickel‑chromium alloys in aerospace, power generation, and high‑temperature industrial applications. Engineers and procurement teams routinely choose between them when designing components where a balance of elevated‑temperature strength, manufacturability, corrosion resistance, and cost must be achieved. Typical decision contexts include: selecting a material for hot‑section hardware (where sustained high‑temperature strength and creep resistance matter), picking a spring or fastener material (where heat‑treat response and fatigue life are critical), or choosing a weldable alloy for repair and assembly.

The principal technical distinction is how each alloy achieves and retains strength at elevated temperatures. That difference governs selection for parts expected to operate under sustained stress at high temperature, and drives contrasts in alloy chemistry, heat treatment practice, and in‑service behavior. Because both are age‑hardenable nickel alloys and share similar corrosion resistance, the comparison often narrows to high‑temperature mechanical performance, heat‑treatment windows, and fabrication constraints.

1. Standards and Designations

• Inconel 718: UNS N07718 (common designation); widely specified by aerospace and industrial AMs/MS/AMS documents and by product specifications for bars, forgings, plate, and strip. It appears in many ASTM/ASME product specifications for nickel‑base alloys used in pressure and structural parts.
• Inconel X‑750: UNS N07750 (common designation); historically specified by aerospace AMS documents and by industrial specifications for springs, fasteners, and high‑temperature hardware.
• Equivalency and regional standards: These nickel‑base superalloys are most commonly specified by UNS and AMS/ASTM product specs rather than direct EN, JIS, or GB one‑to‑one equivalents. Users frequently call out UNS/AMS numbers on engineering drawings and procurement documentation.
• Classification: Both are nickel‑chromium alloy (age‑hardenable precipitation‑strengthened alloys), not stainless steels, tool steels, carbon steels, or HSLA materials.

2. Chemical Composition and Alloying Strategy

How the chemistry maps to properties: - Inconel 718 uses a combination of Nb (niobium), Mo, Ti and Al to produce a strong $\gamma''$ (Ni3Nb) precipitation response plus $\gamma'$ precipitates. The $\gamma''$ phase gives very high yield and tensile strength, particularly at intermediate elevated temperatures. - Inconel X‑750 relies mainly on $\gamma'$ (Ni3(Al,Ti)) precipitation for age hardening; its Nb and Mo contents are much lower, so its precipitation spectrum, stability, and high‑temperature retention differ from 718. - Chromium provides oxidation and corrosion resistance for both alloys; Ni is the matrix element that preserves toughness and high‑temperature stability.

3. Microstructure and Heat Treatment Response

• Inconel 718 microstructure (typical): a face‑centered cubic (FCC) nickel matrix with fine, coherent $\gamma''$ (Ni3Nb) precipitates as the primary strengthening phase and $\gamma'$ (Ni3(Al,Ti)) as a co‑precipitate. Carbides and minor phases can form depending on thermal history and composition control.
• Inconel X‑750 microstructure (typical): FCC nickel matrix strengthened predominantly by $\gamma'$ precipitates and stable carbides; precipitation kinetics, particle morphology, and volume fraction differ appreciably from 718.

Heat treatment behavior: - Inconel 718: Solution treatment followed by controlled aging produces the $\gamma''$ precipitates. The alloy is relatively tolerant to a range of manufacturing heat treatments and is often supplied in solution‑treated and age‑hardened conditions. Overaging or inappropriate thermal cycles can coarsen $\gamma''$ and reduce strength. - Inconel X‑750: Requires precise solution treatment and aging cycles to obtain the desired $\gamma'$ distribution. It is sensitive to embrittling phases (such as grain boundary precipitation) if cooled or aged improperly; some grades are supplied in cold‑worked plus aged conditions for spring applications.

Processing effects: - Thermo‑mechanical processing (forgings, cold work) refines grain size and affects precipitation kinetics in both alloys; cold work before aging typically increases yield strength after aging but can reduce ductility. - Exposure to prolonged high temperatures can cause phase coarsening (reducing strength) and in some conditions promote grain boundary precipitates that reduce ductility and stress‑rupture life—this is alloy‑ and temperature‑dependent and is central to selection decisions.

4. Mechanical Properties

Explanation: - Inconel 718 achieves higher yield and tensile strengths than X‑750 in many commonly used aged conditions because $\gamma''$ precipitates provide a very effective impediment to dislocation motion. This makes 718 the preferred choice where higher sustained loads at elevated temperature are anticipated. - X‑750 performs reliably as a spring and fastener material and is chosen for applications where good high‑temperature fatigue and relaxation resistance are required, but where the absolute highest static strength at elevated temperature is not necessary.

5. Weldability

Weldability considerations for precipitation‑strengthened nickel alloys depend on base chemistry, hardenability, and the required post‑weld heat treatment.

Useful weldability indices: - Carbon equivalent (IIW) for steels (for reference only): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm formula often used to assess cracking susceptibility in steels: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation for these nickel alloys: - These steel‑based formulas are not directly applicable quantitatively to nickel‑base superalloys, but the qualitative factors still matter: higher levels of segregation‑prone elements (Nb, Ti) and the presence of carbides or brittle phases at grain boundaries increase susceptibility to weld cracking and to loss of properties after welding. - Inconel 718: Generally considered weldable with appropriate filler metals and procedures. Welding typically requires control of heat input, preheat/interpass temperatures in some cases, and a defined post‑weld solution and age treatment to recover precipitation strengthening. Because 718’s primary strengthening phase ($\gamma''$) can be restored by post‑weld processing, welded structures can regain much of their mechanical properties. - Inconel X‑750: More challenging to weld in many applications. X‑750 is more sensitive to heat‑affected‑zone (HAZ) embrittlement and stress‑corrosion cracking in improperly cooled or aged welds. For critical components, welding often requires careful process control and post‑weld heat treatments; for some spring applications, welding is avoided or done only with strict procedures.

Practical notes: - For both alloys, welded assemblies that will see elevated service temperatures should be qualified by testing: tensile, creep, and stress‑rupture tests after the full thermal cycle used for service. - Where repair welding is unavoidable, follow mill or OEM guidelines and use matching filler metals and post‑weld heat treatments recommended for each alloy.

6. Corrosion and Surface Protection

• Neither Inconel 718 nor X‑750 are stainless steels; they are nickel‑base superalloys with good general corrosion and oxidation resistance due to high nickel/chromium levels.
• For localized corrosion indices such as PREN, the metric is designed for stainless steels and is not generally used for nickel‑base superalloys. For reference: $$\text{PREN} = \text{Cr} + 3.3\times \text{Mo} + 16\times \text{N}$$ This index is not applicable as a design tool for Inconel 718/X‑750 because their corrosion behavior is governed by the overall nickel matrix, chromium, alloy stabilizers, and precipitation structure rather than just Cr/Mo/N contributions used in stainless steels.
• Surface protection: Where required, both alloys can be coated or painted. Common industrial protections (ceramic coatings, aluminide diffusion coatings, thermal barrier coatings, or sprayed metal coatings) are applied for high‑temperature oxidation protection in power or aerospace environments.
• Corrosion resistance considerations: 718 generally shows excellent resistance to many corrosive environments and to oxidation at intermediate elevated temperatures. X‑750 also resists oxidation and corrosion but designer must consider crevice, chloride stress‑corrosion cracking, and service‑dependent phenomena; material selection should be validated by service environmental testing.

7. Fabrication, Machinability, and Formability

• Machining: Both alloys are tougher to machine than common steels. Inconel 718 is well known for work hardening and for rapid tool wear if feeds, speeds, and tooling are not optimized. X‑750 is also difficult to machine, especially in aged or cold‑worked conditions. Use of carbide or ceramic tooling, rigid setups, and conservative depth of cut is standard practice.
• Forming: Both alloys are formable in solution‑treated conditions but require higher forces than steels. Cold work prior to aging can increase strength but reduce ductility; thus forming is typically done in the solution‑treated condition followed by a controlled aging cycle.
• Finishing: Grinding and polishing are common for final dimensions and surface finish; chemical milling or electrochemical methods may be used for complex parts.
• Heat treatment sensitivity: Because final mechanical properties depend on precise thermal cycles, fabrication sequences that introduce local heating (welding, bending with high localized temperatures) must be planned so the part can receive the required solution and aging treatments afterward.

8. Typical Applications

Selection rationale: - Choose Inconel 718 for higher static loads at elevated temperatures, for parts where post‑weld heat treatment can be applied to restore strengthening, and when improved creep and tensile performance are necessary. - Choose Inconel X‑750 where spring behavior, stress‑relaxation resistance, and proven fatigue performance in high‑temperature spring applications are the primary concerns, and where maximum static strength is less critical than relaxation and cyclic stability.

9. Cost and Availability

• Cost: Inconel 718 is widely specified and available in many product forms (bars, forgings, plate, wire, powder) and is typically priced at a premium relative to commodity steels. Compared with X‑750, 718 can be similar or somewhat higher in material cost depending on market prices for Nb and Mo, and on processing (forged/aged vs cold‑worked/aged).
• Availability: Both alloys are common in the aerospace and power markets and are available from a broad supply base. Inconel 718 is one of the most commonly stocked nickel‑based superalloys, which often improves lead times and availability. X‑750 is broadly available, especially in forms geared toward springs and fasteners.
• Product forms: 718 tends to be more widely available in large forged shapes and structural forms; X‑750 is readily available in wire, bar, and finished spring forms.

10. Summary and Recommendation

Concluding recommendations: - Choose Inconel 718 if you need the highest combination of tensile and yield strength retained at intermediate to elevated temperatures, require better static load capability, or need an alloy that responds well to restoration after welding by post‑weld aging. - Choose Inconel X‑750 if the primary requirement is high temperature spring performance, stress‑relaxation resistance, or proven fatigue behavior in cyclic applications where the maximum static strength requirement is lower and where cold work plus aging processes are part of the manufacturing route.

Final note: Both alloys demand careful specification of heat treatment, surface condition, and fabrication route to realize expected properties in service. For critical components always reference OEM/mill heat‑treatment charts, validate weld procedures, and qualify parts with representative testing for the intended temperature/stress environment.