Inconel 718 is a nickel-based high-temperature alloy known for its high strength, corrosion resistance, and excellent mechanical properties at elevated temperatures. Its hardness depends on the material’s condition (solution annealed, aged, or cold-worked).
Image source: Rockwell Hardness
Typical Hardness Range of Inconel 718
| Material Condition | Rockwell Hardness (HRC) | Vickers Hardness (HV) | Description |
| Solution Annealed | ~20 HRC | ~220 HV | Softest condition, ideal for machining or forming |
| Aged Condition (Standard Heat Treatment) | 36–44 HRC | 360–450 HV | Solution treated and precipitation hardened; significantly increased strength |
| Cold Worked + Aged | 45–50 HRC | 460–520 HV | Maximum hardness, suitable for extreme service conditions |
Strengthening Mechanism:
Inconel 718 is primarily strengthened by the precipitation of γ″ (Ni₃Nb) and γ′ (Ni₃(Al,Ti)) phases. These fine precipitates hinder dislocation movement, thereby increasing the alloy’s hardness and strength.
From the above, we can see that the hardness of the material is closely related to its heat treatment or processing condition, as different states alter the internal microstructure and strengthening mechanisms, directly affecting hardness and mechanical properties.
Material Condition Determines Microstructure
Different conditions lead to changes in crystal structure, phase composition, and dislocation density, which directly affect hardness:
Annealed: Uniform microstructure, stress-relieved, larger grain size → softer material, lower hardness
Solution Treated: Even distribution of solute elements → moderate strength and hardness
Age Hardened / Precipitation Hardened: Precipitation of strengthening phases (γ′ and γ″) → significantly improved strength and hardness
Cold Worked: High dislocation density and deformed grains → notably higher hardness, but reduced ductility
Different Strengthening Mechanisms Increase Hardness
Inconel 718 is a precipitation-hardening alloy. After aging treatment, the precipitation of fine γ′ and γ″ phases obstructs dislocation movement, significantly improving hardness and strength.
Example:
In the solution-treated condition, Inconel 718 typically has a hardness of HRC 18–22
After standard aging treatment, it can reach HRC 36–44, with tensile strength increasing by more than 50%
Approximate Relationship Between Hardness and Ultimate Tensile Strength (UTS):
| Rockwell Hardness (HRC) | UTS (MPa) | UTS (ksi) |
| 20 | ≈ 750 | ≈ 109 |
| 30 | ≈ 950 | ≈ 138 |
| 35 | ≈ 1100 | ≈ 160 |
| 40 | ≈ 1250 | ≈ 181 |
| 45 | ≈ 1400 | ≈ 203 |
| 50 | ≈ 1550 | ≈ 225 |
Inconel 718 Heat Treatment Reference Values:
Solution Annealed: HRC 18–22, tensile strength ~750–950 MPa
Standard Aged (per ASTM B637): HRC 36–44, tensile strength ~1240–1380 MPa
High Strength Condition (Cold Worked + Aged): HRC up to 50, tensile strength over 1500 MPa
Applications Requiring High Hardness of Inconel 718
Turbine disks and compressor blades
Jet engine fasteners
High-temperature springs
Aerospace structural components
Additional Info: Hardness Conversion Chart (Approximate Values)
| Rockwell C (HRC) | Vickers (HV) | Brinell (HB) | Rockwell B (HRB) |
| 20 | ≈ 220 | ≈ 215 | 96 |
| 25 | ≈ 245 | ≈ 240 | — |
| 30 | ≈ 270 | ≈ 265 | — |
| 35 | ≈ 310 | ≈ 300 | — |
| 40 | ≈ 370 | ≈ 340 | — |
| 45 | ≈ 430 | ≈ 390 | — |
| 50 | ≈ 500 | ≈ 450 | — |
💡 Note: HRC is used for high-hardness materials; HRB is applicable to softer metals (HRB may apply to Inconel 718 in the solution-treated condition).
Conclusion
Inconel 718’s hardness and mechanical strength are not fixed values—they are highly dependent on the alloy’s processing history. Whether used in jet engine components, high-temperature springs, or structural aerospace parts, selecting the appropriate material condition is key to achieving optimal performance. Understanding the relationship between hardness, tensile strength, and heat treatment ensures engineers can confidently apply Inconel 718 in the most demanding environments.
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