What is Precipitation Hardening?
Precipitation hardening (also called age hardening or strengthening by precipitation) is a heat treatment process that significantly increases the strength and hardness of an alloy by forming fine, uniformly distributed strengthening phase particles within the metal matrix.
One-sentence explanation: By heat treatment, many tiny “strengthening particles” precipitate inside the metal, acting like nails that block dislocation movement, thus making the metal stronger.
Resource: What is Precipitation Hardening? A Comprehensive Guide
What is a Precipitation-Hardened Superalloy?
A precipitation-hardened superalloy is a type of high-temperature alloy whose strength and creep resistance at elevated temperatures are significantly improved through precipitation hardening. These alloys are widely used in aerospace, gas turbines, nuclear industries, and other applications requiring excellent mechanical properties and corrosion resistance at high temperatures.
Basic principle: Precipitation hardening improves alloy strength and hardness by precipitating fine and evenly distributed strengthening phases (such as γ′ or γ″ phases) within the metal matrix, which hinder dislocation movement.
Characteristics of Precipitation-Hardened Superalloys
- Excellent high-temperature strength (usable above ~600°C)
- Good creep resistance and long-term strength
- Outstanding oxidation and corrosion resistance
- More complex processing and heat treatment
Typical Alloy Grades and Their Strengthening Features
| Alloy Name | Matrix Type | Strengthening Phase(s) | Features & Applications |
| Inconel 718 | Nickel-based | γ″ + γ′ (Ni₃Nb + Ni₃(Al,Ti)) | Typical dual-phase precipitation strengthening, excellent strength, corrosion resistance and weldability; widely used in aerospace and power industries |
| Waspaloy | Nickel-based | γ′ (Ni₃(Al,Ti)) | Strong γ′ strengthening, higher high-temperature performance than Inconel 718; used in gas turbine hot sections |
| Rene 41 | Nickel-based | γ′ | Excellent high-temperature strength; used in aero engine turbine components |
| A-286 | Iron-based | γ′ (Ni₃(Al,Ti)) | Moderate strength with good corrosion resistance; used for structural parts below 600°C such as fasteners and turbine casings |
| 17-4PH | Iron-based | Cu-rich precipitates | Significant strengthening effect; widely used in chemical, marine, and aerospace industries |
| Co-Al-W alloys | Cobalt-based | Co₃(Al,W) (L1₂ structure) | Good thermal stability and corrosion resistance; advanced research material for extreme high-temperature environments |
How Does Inconel 718 Precipitation Harden?
The precipitation hardening of Inconel 718 is achieved by aging heat treatment to form two main strengthening phases inside the alloy:
- γ″ (Ni₃Nb): the primary strengthening phase providing high-temperature strength
- γ′ (Ni₃(Al,Ti)): the secondary strengthening phase enhancing overall properties
These two phases precipitate uniformly in the matrix, blocking dislocation movement and significantly improving Inconel 718’s strength, hardness, and high-temperature performance.
Typical Inconel 718 Precipitation Hardening Heat Treatment Process:
1. Solution Annealing
- Temperature: ~980°C (1800°F)
- Time: 1 hour or longer
- Purpose: Dissolve carbides and precipitates to form a supersaturated solid solution
- Cooling: Rapid air cooling
2. Double Aging Treatment
This is the key step for Inconel 718 precipitation hardening:
- First aging
- Temperature: ~720°C (1325°F)
- Time: 8 hours
- Cooling: Slow cooling at about 55°C/h to next step
- Second aging
- Temperature: ~620°C (1150°F)
- Time: 8 hours
- Cooling: Air cool to room temperature
This process enables γ″ and γ′ phases to precipitate at optimal size and distribution, significantly improving yield strength and long-term strength.
Inconel 718 Performance Improvements After Precipitation Hardening
| Property | Solution Annealed State | Precipitation Hardened State |
| Yield Strength | Low | High (>1000 MPa) |
| Creep Resistance | Poor | Significantly improved |
| High-Temperature Fatigue Strength | Moderate | Excellent |
Are All Inconel 718 Products Precipitation Hardened?
Not all Inconel 718 products are delivered in the precipitation hardened state; it depends on the intended use, processing method, and customer requirements. Typically, there are two delivery conditions:
| State | Description |
| Solution Annealed State | Solution treated but not aged; softer and suitable for further machining or welding |
| Precipitation Hardened (Aged) State | Fully aged with high strength, ready for final use |
Inconel 718 Application Examples
| Property | Typical Value |
| Tensile Strength (Rm) | ≥ 1240 MPa |
| Yield Strength (Rp0.2%) | ≥ 1035 MPa |
| Elongation (A5) | ≥ 12% |
| Hardness (Rockwell C) | 36 – 44 HRC |
| Impact Toughness (notched) | High |
Heat treatment: Solution annealed + aged (precipitation hardening)
Inconel 718 Available Forms
- Pipes & Tubes (Seamless & Welded)
- Forged Flanges
- Fittings (Elbow, Tee, Reducer, Cap)
- Plates & Sheets
- Rods, Bars, and Fasteners
Standards & Specifications
- ASTM B637, AMS 5662 / 5663
- UNS N07718, DIN 2.4668
- ISO 15156 / NACE MR0175
- ASME Boiler & Pressure Vessel Code approved
Delivery Conditions of Inconel 718 Under Common Standards
Important Notes:
- One major advantage of Inconel 718 is its excellent weldability in both solution annealed and precipitation hardened states.
- Standards such as ASTM B637, AMS 5662, AMS 5663 specify whether materials are aged or solution treated on delivery.
- Manufacturers often supply in solution annealed condition, and users perform final aging heat treatment as needed.
| Standard | Title / Description | Material Condition | Application / Remarks |
| AMS 5662 | Aerospace Material Specification | Solution Annealed | For machining, welding, intermediate processing before aging |
| AMS 5663 | Aerospace Material Specification | Precipitation Hardened (Aged) | Standard double aging, ready for service |
| AMS 5832 | Welding Wire Specification | Solution Annealed wire | For welding Inconel 718 parts |
| ASTM B637 | Standard for Forgings, Bars, etc. | Various states per customer | Includes solution annealed, aged, and hot-worked states |
| API 6A CRA | Corrosion Resistant Alloys for Oil & Gas | Usually solution annealed | For wellhead equipment, aging after welding |
| UNS N07718 | Unified Numbering System | Material designation only | State depends on referenced standard |
Summary
Precipitation hardening is a heat treatment process that significantly increases the strength and hardness of alloys by precipitating fine strengthening phases within the metal matrix. Precipitation-hardened superalloys utilize this mechanism to enhance high-temperature performance and are widely used in aerospace, gas turbines, and other demanding applications.
Inconel 718 is a typical precipitation-hardened superalloy, whose strength improvement mainly depends on the precipitation of two strengthening phases: γ″ (Ni₃Nb) and γ′ (Ni₃(Al,Ti)). The standard heat treatment includes solution annealing followed by double aging to achieve optimal strengthening.
Inconel 718 products are delivered in either solution-annealed or precipitation-hardened (aged) conditions, depending on the application and customer requirements. The solution-annealed state is suitable for further machining and welding, while the aged state is intended for final use. Different standards (such as AMS 5662 and AMS 5663) specify the delivery condition accordingly.
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