Inconel 625 Mechanical Properties: Complete Technical Data

Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum superalloy. Its strength is from the solid-solution strengthening effects of molybdenum and niobium in a nickel-chromium matrix.

This guide provides a comprehensive, data-driven analysis of Inconel 625 mechanical properties across the full spectrum of engineering parameters: tensile strength, yield strength, hardness, impact toughness, creep, stress-rupture, fatigue, and the effects of temperature and heat treatment. All data is referenced to authoritative sources including Special Metals Corporation (the original developer), ASTM specifications (B446, B443, B444), and AMS aerospace material standards (5666, 5599).

Inconel 625 Mechanical Properties at a Glance

The table below summarizes the core mechanical property data for annealed Inconel 625 at room temperature. These values meet or exceed the minimum requirements of ASTM B446 (bar) and ASTM B443 (plate/sheet).

Chemical Composition of Inconel 625: How to Influence Mechanical Strength

Inconel 625 derives its high mechanical strength from solid-solution strengthening: molybdenum and niobium dissolved in a nickel-chromium matrix distort the crystal lattice, increasing the force required to move dislocations. Unlike precipitation-hardened alloys like Inconel 718, this strengthening mechanism does not require complex aging heat treatments and is stable across a wide temperature range.

Understanding the 625 chemical composition is the first step to understanding why Inconel 625 behaves the way it does mechanically. Each alloying element plays a specific role:

Inconel 625 maintains useful strength from cryogenic temperatures (-196 C / -320 F) all the way up to 1093 C (2000 F) without the brittleness that can accompany precipitation-hardened alloys.

What Are the Room-Temperature Mechanical Properties of Inconel 625?

In the annealed condition, Inconel 625 exhibits a minimum tensile strength of 827 MPa (120 ksi), a minimum yield strength of 414 MPa (60 ksi), and a minimum elongation of 30%. These values meet the requirements of ASTM B446 (bar), ASTM B443 (plate and sheet), and AMS 5666 (aerospace bar and forging). Typical measured values often exceed these minimums by 15-30%.

Room-temperature mechanical properties form the baseline for all engineering design calculations. The values below represent the specification minimums and typical measured values for annealed Inconel 625:

Important note on condition: The properties above apply to material in the annealed condition. Cold-worked material can achieve significantly higher strengths: tensile strength up to 1241 MPa (180 ksi) and yield strength up to 827-965 MPa (120-140 ksi), but with reduced elongation (15-25%).

The choice between annealed and cold-worked condition depends on the application: annealed for maximum ductility and weldability; cold-worked for maximum strength.

What Is the Tensile Strength of Inconel 625?

The tensile strength of Inconel 625 is 827 MPa (120 ksi) minimum in the annealed condition per ASTM B446, with typical values of 965-1103 MPa (140-160 ksi). This places Inconel 625 in the upper tier of solid-solution-strengthened nickel alloys, below precipitation-hardened grades like Inconel 718 but well above standard austenitic stainless steels like 316L.

Tensile strength (also called ultimate tensile strength, or UTS) is the maximum stress a material can withstand while being stretched before it breaks. For Inconel 625, the high tensile strength comes from the combined effects of molybdenum and niobium in solid solution:

• Bar (annealed): 827 MPa (120 ksi) minimum for bar per ASTM B446 and AMS 5666.

• Plate/Sheet (annealed): 827 MPa (120 ksi) minimum for plate and sheet per ASTM B443.

• Seamless Pipe (annealed): 827 MPa (120 ksi) minimum for seamless pipe and tube per ASTM B444.

• Typical Measured: 965-1103 MPa (140-160 ksi) typical measured values for fully annealed material.

• Cold-Worked: Up to 1241 MPa (180 ksi) achievable with heavy cold work, though elongation drops to 15-25%.

To put these numbers in context, consider that standard 316L stainless steel has a minimum tensile strength of 515 MPa (75 ksi). Inconel 625 is approximately 60% stronger in tension than 316L, which is why it is selected for high-stress applications in aerospace, oil and gas, and chemical processing where 316L would deform or fail.

What Is the Yield Strength of Inconel 625?

The yield strength of Inconel 625 is 414 MPa minimum in the annealed condition, with typical values of 414-517 MPa. Cold-worked material can reach yield strengths of 827-965 MPa. The relatively large gap between yield and tensile strength (the "strain hardening range") gives Inconel 625 excellent ductility and energy absorption before fracture.

Yield strength is the stress at which a material begins to deform plastically. Below the yield point, the material returns to its original shape when the load is removed. Above the yield point, permanent deformation occurs. For structural and pressure-containing applications, yield strength is often the controlling design parameter.

The yield-to-tensile ratio of annealed Inconel 625 (approximately 0.50) is notably lower than that of precipitation-hardened alloys like Inconel 718 (approximately 0.83). This means Inconel 625 has a wider plastic deformation range before fracture, which translates to better formability, better energy absorption in impact events, and greater tolerance for stress concentrations.

What Is the Hardness of Inconel 625?

Inconel 625 typically has a hardness of 210-250 HB or HRB 90-100 in the annealed condition, equivalent to approximately HRC 25-30. Cold-worked material can reach HRC 35-40. Hardness is not a specification requirement for ASTM B446 but is useful as a quality control indicator and for estimating wear resistance.

Hardness measures a material resistance to localized plastic deformation, typically by indentation. While not a direct design parameter like tensile or yield strength, hardness correlates with wear resistance and machinability and can be used as a rapid non-destructive check to verify heat treatment condition.

For comparison, annealed 316L stainless steel typically measures 160-217 HB (HRB 79-95). Inconel 625 is noticeably harder, which contributes to its superior wear and galling resistance in friction applications. However, higher hardness also means more challenging machining: Inconel 625 work-hardens rapidly during cutting operations, requiring slow speeds, sharp carbide tools, and generous coolant flow.

What Are the Impact Toughness Properties of Inconel 625?

Inconel 625 exhibits excellent impact toughness from cryogenic to elevated temperatures. Charpy V-notch impact values are typically 70-100 J at room temperature and remain above 60 J even at -196 C. This retention of toughness at cryogenic temperatures is a key advantage over many other high-strength alloys.

Impact toughness measures the energy a material absorbs during fracture under rapid loading. High impact toughness means the material bends or deforms rather than shattering, which is critical for applications subject to shock loads, vibrations, or low-temperature service.

The face-centered cubic (FCC) crystal structure of Inconel 625 does not exhibit a ductile-to-brittle transition temperature as body-centered cubic metals like carbon steel do. Carbon steel becomes brittle below approximately -40 C; Inconel 625 remains tough well below -196 C. This makes it ideal for cryogenic applications such as LNG storage and transport, where sudden impact loads must be absorbed without catastrophic brittle fracture.

How Do Inconel 625 Mechanical Properties Change at Elevated Temperatures?

Inconel 625 retains significant strength up to 816°C, with a tensile strength of approximately 655 MPa at 538°C and approximately 345 MPa (50 ksi) at 871°C. The alloy maintains useful mechanical properties up to 1093°C, making it suitable for gas turbine, furnace, and heat exchanger applications.

The relationship between temperature and mechanical properties is one of the most critical considerations in high-temperature alloy selection. As temperature increases, atomic mobility rises, dislocations move more easily, and strength decreases. The rate and extent of this decrease determine how useful an alloy is at elevated temperatures.

What Are the Creep and Stress-Rupture Properties of Inconel 625?

Inconel 625 exhibits strong creep resistance up to approximately 816°C. The 1000-hour stress-rupture strength is approximately 241 MPa (35 ksi) at 649 C (1200 F), 83 MPa (12 ksi) at 816 C (1500 F), and 28 MPa (4 ksi) at 982 C (1800 F).

For applications requiring sustained loads above 816 C, precipitation-hardened alloys like Inconel 718 or solid-solution grades like Inconel 617 may offer better creep performance.

Creep is the slow, progressive deformation of a material under constant stress at elevated temperatures. Unlike room-temperature behavior, where a material either yields or does not, creep means a component slowly stretches over months or years even when the applied stress is well below the yield strength. Creep becomes significant when the service temperature exceeds approximately 40% of the melting point (in Kelvin), which for Inconel 625 means above roughly 450°C.

What Are the Fatigue Properties of Inconel 625?

Inconel 625 exhibits excellent fatigue resistance, with a rotating-beam fatigue endurance limit of approximately 345 MPa at 10^8 cycles at room temperature. At elevated temperatures, fatigue strength decreases but remains useful: approximately 241 MPa at 10^8 cycles at 649 ℃. The alloy also shows good resistance to thermal fatigue cracking.

Fatigue is the failure of a material under cyclic loading at stresses below the ultimate tensile strength. It is the dominant failure mode in rotating equipment, vibrating structures, and components subject to pressure or temperature cycling. Fatigue failures are particularly dangerous because they occur without visible warning, often suddenly and after millions of load cycles.

How Does Heat Treatment Affect Inconel 625 Mechanical Properties?

Inconel 625 is primarily used in the solution-annealed condition (1093-1204°C / 2000-2200°F followed by rapid cooling), which provides the best combination of strength, ductility, and corrosion resistance. An intermediate anneal at 927-1038°C can be used for applications below 649°C where higher strength is desired. The alloy can also be age-hardened, but this is uncommon because it reduces ductility and can impair corrosion resistance.

Inconel 625's primary heat treatment is a simple solution anneal. However, understanding the effects of different thermal treatments is important for optimizing properties for specific applications:

Critical warning: Prolonged exposure of Inconel 625 to temperatures in the 649-816°C range causes precipitation of the delta phase (Ni₃Nb), a needle-like intermetallic that increases hardness but significantly reduces impact toughness and ductility. This is not a heat treatment to deliberately apply, but rather a service condition to avoid. If Inconel 625 components operate in this temperature range, periodic inspection for embrittlement is recommended.

How Do Mechanical Properties Vary by Product Form (Bar, Sheet, Pipe, Forging)?

The specification minimums for Inconel 625 are generally consistent across product forms: tensile strength of 827 MPa (120 ksi) minimum and yield strength of 414 MPa (60 ksi) minimum. However, typical measured values vary by product form due to differences in processing history. Cold-drawn bar tends to have the highest strength, while large-diameter forged rings may have the lowest due to slower cooling rates during annealing.

Inconel 625 is available in a wide range of product forms, each governed by specific ASTM or AMS specifications. While the minimum mechanical property requirements are similar, the typical achieved values differ based on the manufacturing process:

While specification minimums are identical across forms, engineers should be aware that:

• Cold-Drawn Bar: A cold-drawn bar typically tests 10-15% above minimum tensile due to work hardening during drawing.

• Large Forgings: Large-section forgings may test near minimum values because the mass slows cooling during annealing, allowing some grain coarsening.

• Thin Sheet: A thin sheet may show slightly lower elongation values compared to a plate due to the smaller gauge length effect.

• Welded Pipe: Welded pipe must be tested transverse to the weld; the weld metal typically matches or slightly exceeds base metal strength when proper filler metal is used.

Frequently Asked Questions

What is the tensile strength of Inconel 625?

The minimum tensile strength of Inconel 625 is 827 MPa in the annealed condition, per ASTM B446 and AMS 5666. Typical measured values range from 965 to 1103 MPa. Cold-worked material can achieve tensile strengths up to 1241 MPa.

What is the yield strength of Inconel 625?

The minimum yield strength of Inconel 625 is 414 MPa in the annealed condition, per ASTM B446. Typical measured values range from 414 to 517 MPa. Cold-worked material can reach yield strengths of 827 to 965 MPa, though with reduced elongation.

What is the hardness of Inconel 625?

Inconel 625 typically has a hardness of 210-250 HB, equivalent to HRB 90-100 or HRC 25-30, in the annealed condition. Cold-worked material can reach HRC 35-40. Hardness is not a specification requirement for ASTM B446 but is commonly measured for quality control purposes.

How strong is Inconel 625 compared to steel?

Inconel 625 is approximately 60% stronger than 316L stainless steel in tensile strength and about 35% stronger in yield strength. However, it is significantly more expensive and heavier. Compared to high-strength low-alloy steel, Inconel 625 is comparable or stronger, with the added benefit of far superior corrosion and high-temperature resistance.

At what temperature does Inconel 625 lose strength?

Inconel 625 begins to lose significant strength above 649°C, where its tensile strength drops to approximately 690-827 MPa. By 871°C, tensile strength falls to approximately 310-414 MPa. The alloy retains useful mechanical properties up to 1093 C but is generally not recommended for load-bearing applications above 816 C due to creep deformation.

Does Inconel 625 work-harden?

Yes, Inconel 625 work hardens rapidly. Cold working increases both tensile and yield strength significantly: a cold-drawn bar can achieve yield strengths of 690-827 MPa compared to 414 MPa for annealed material. However, work hardening also reduces ductility and can make subsequent machining more difficult. Rapid work hardening is the reason Inconel 625 requires slow machining speeds with sharp carbide tools.

Is Inconel 625 stronger than Inconel 718?

No. Inconel 718 is significantly stronger than Inconel 625: its minimum tensile strength is 1241 MPa versus 827 MPa for Inconel 625, and its minimum yield strength is 1034 MPa versus 414 MPa. However, Inconel 625 offers better corrosion resistance and superior weldability without post-weld heat treatment and does not require the complex solution-plus-aging treatment that Inconel 718 needs to achieve its high strength.

Can Inconel 625 be used at cryogenic temperatures?

Yes. Inconel 625 maintains excellent mechanical properties at cryogenic temperatures, with Charpy V-notch impact values of 60-80 J even at -196 C. Its face-centered cubic crystal structure means there is no ductile-to-brittle transition, so the alloy remains tough and ductile at the lowest temperatures. This makes it suitable for LNG storage, cryogenic piping, and aerospace applications at extreme cold.