In high-performance industries such as aerospace, chemical processing, and power generation, 15-7PH stainless steel is widely recognized for its excellent strength, toughness, and corrosion resistance. But the property that truly sets it apart is its heat treatability. Through controlled thermal processing, this alloy can achieve a wide range of mechanical properties, from high ductility for forming to exceptional hardness for demanding service environments.
This article by SAKYSTEEL provides an in-depth look at the heat treatment conditions available for 15-7PH stainless steel, explaining the procedures, temperature ranges, and results of each condition. Whether you are an engineer, fabricator, or metallurgist, understanding these heat treatment conditions will help you select the right combination of performance and manufacturability for your application.
1. Introduction to 15-7PH Stainless Steel
15-7PH stainless steel (UNS S17700, ASTM A693 Type 631) is a precipitation-hardening martensitic stainless steel that contains approximately 15% chromium, 7% nickel, and 1% aluminum. The “PH” designation refers to its ability to be hardened through precipitation heat treatment.
Developed as an improvement over 17-7PH, this alloy delivers higher strength and better thermal stability. It can be easily formed in its annealed condition and then heat treated to produce high hardness and yield strength suitable for critical structural applications.
2. Chemical Composition
| Element | Typical Content (%) | Purpose |
|---|---|---|
| Chromium (Cr) | 14.0–16.0 | Provides corrosion resistance |
| Nickel (Ni) | 6.5–7.8 | Improves toughness and stability |
| Aluminum (Al) | 0.75–1.5 | Main precipitation hardening agent |
| Carbon (C) | ≤ 0.09 | Strength and hardness control |
| Manganese (Mn) | ≤ 1.00 | Improves strength |
| Silicon (Si) | ≤ 1.00 | Enhances oxidation resistance |
| Phosphorus (P) | ≤ 0.04 | Controlled for ductility |
| Sulfur (S) | ≤ 0.03 | Improves machinability |
| Iron (Fe) | Balance | Base element |
The combination of nickel and aluminum enables precipitation of Ni₃Al compounds during aging, which greatly strengthens the alloy while retaining adequate toughness.
3. Heat Treatment Overview
15-7PH stainless steel can be supplied and processed in several heat treatment conditions to achieve specific combinations of strength, hardness, and ductility. The most common conditions are:
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Condition A: Solution annealed
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Condition RH950: Austenite to martensite transformation and aging
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Condition TH1050: Cold work followed by aging
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Condition RHT 900–1150: Custom high-temperature treatments
Each condition involves different thermal and mechanical steps designed to optimize the material for a particular use.
4. Condition A – Solution Annealed
4.1 Description
In this condition, 15-7PH is solution treated at approximately 1038°C (1900°F) for 1 hour followed by air cooling. The resulting structure is austenitic, which provides maximum ductility and allows the material to be easily formed, deep drawn, or spun into shape before final hardening.
4.2 Typical Applications
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Sheet forming and stamping
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Bellows and diaphragms
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Components requiring complex shaping prior to aging
4.3 Mechanical Properties (Typical)
| Property | Value |
|---|---|
| Tensile Strength | 1050 MPa |
| Yield Strength | 850 MPa |
| Elongation | 18–20% |
| Hardness | 25–30 HRC |
Condition A is not the final state for most industrial applications — it is typically followed by cold working and aging for higher strength.
5. Condition RH950 – Precipitation Aged
5.1 Description
Condition RH950 (RH = “Reversion Heat Treatment”) is achieved by:
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Solution treating the alloy at 1038°C (Condition A).
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Cooling to room temperature.
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Heating to 955°C (1750°F) for 1 hour.
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Rapidly cooling to room temperature.
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Aging at 510°C (950°F) for 4 hours, followed by air cooling.
During this process, the structure changes from austenitic to martensitic, and aluminum-nickel precipitates form, increasing strength and hardness dramatically.
5.2 Mechanical Properties
| Property | Value |
|---|---|
| Tensile Strength | 1400–1450 MPa |
| Yield Strength | 1200–1300 MPa |
| Elongation | 8–10% |
| Hardness | 42–46 HRC |
5.3 Key Features
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Very high strength with good ductility
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Excellent fatigue resistance
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Good dimensional stability
5.4 Typical Applications
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Springs and fasteners
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Aerospace fittings
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Power turbine components
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High-strength pressure diaphragms
Condition RH950 is the most common condition for 15-7PH in aerospace and precision applications.
6. Condition TH1050 – Cold Worked and Aged
6.1 Description
Condition TH1050 (TH = “Transformation Heat Treatment”) is used when even higher strength is required. It involves:
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Solution treatment at 1038°C for 1 hour (Condition A).
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Cold reduction of 60–70% to transform austenite to martensite.
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Aging at 565°C (1050°F) for 1 hour, followed by air cooling.
6.2 Microstructure
The cold working step promotes complete transformation to martensite before aging, resulting in a fine and uniform grain structure with superior strength.
6.3 Mechanical Properties
| Property | Value |
|---|---|
| Tensile Strength | 1500–1600 MPa |
| Yield Strength | 1300–1450 MPa |
| Elongation | 6–8% |
| Hardness | 44–48 HRC |
6.4 Key Features
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Maximum achievable strength for 15-7PH
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Excellent fatigue and wear resistance
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Slightly reduced corrosion resistance compared to RH950
6.5 Typical Applications
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Aerospace structural components
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High-performance springs and valve parts
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Fasteners requiring maximum load-bearing capacity
Condition TH1050 represents the peak strength condition for this alloy.
7. Custom Heat Treatment Variations
In addition to RH950 and TH1050, other customized heat treatment variations are available to tailor the properties of 15-7PH for specialized environments.
| Condition | Temperature (°F) | Description | Result |
|---|---|---|---|
| RHT900 | 900 | Lower aging temp | Higher strength, lower toughness |
| RHT1000 | 1000 | Medium range | Balanced strength and toughness |
| RHT1150 | 1150 | Higher aging temp | Improved ductility and corrosion resistance |
These treatments can fine-tune the alloy’s balance of strength, hardness, and stress-corrosion resistance depending on the intended service environment.
8. Comparison of Heat Treatment Conditions
| Property | Condition A | RH950 | TH1050 |
|---|---|---|---|
| Structure | Austenitic | Martensitic + Precipitates | Martensitic + Precipitates |
| Tensile Strength (MPa) | 1050 | 1450 | 1600 |
| Hardness (HRC) | 30 | 45 | 48 |
| Ductility | High | Moderate | Low |
| Corrosion Resistance | Excellent | Very Good | Good |
| Formability | Excellent | Limited | Limited |
| Typical Use | Forming / Fabrication | Aerospace / Springs | Heavy-duty components |
This comparison highlights how different heat treatments create flexibility in how 15-7PH is used across multiple industries.
9. Metallurgical Mechanism of Precipitation Hardening
The key to 15-7PH’s strength lies in the formation of fine Ni₃Al precipitates during aging. These precipitates act as obstacles to dislocation movement, which increases yield strength.
The transformation sequence can be summarized as:
Austenite → Martensite → Martensite + Ni₃Al Precipitates
By adjusting the aging temperature and duration, engineers can precisely control the size and distribution of these precipitates, tailoring hardness and toughness for each application.
10. Heat Treatment Precautions
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Temperature Control: Over-aging above 600°C may reduce hardness due to coarsening of precipitates.
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Atmosphere Control: Use vacuum or inert gas furnaces to avoid oxidation and scaling.
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Distortion Management: Support complex parts to minimize warping during cooling.
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Post-Weld Aging: Components welded in the annealed condition should be aged afterward to restore strength.
Proper adherence to these guidelines ensures uniform properties and reliable performance.
11. Effect of Heat Treatment on Corrosion Resistance
While 15-7PH provides good corrosion resistance in all heat treatment conditions, the annealed condition (A) exhibits the best resistance due to its austenitic structure. As hardness increases through aging, corrosion resistance slightly decreases, particularly in chloride-containing environments.
To enhance surface protection, passivation and polishing are recommended after aging treatments.
12. Example Heat Treatment Procedure (RH950)
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Solution Anneal: Heat to 1038°C (1900°F) for 1 hour, air cool.
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Condition RH Step: Heat to 955°C (1750°F) for 1 hour, air cool.
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Age Hardening: Heat to 510°C (950°F) for 4 hours, air cool.
Result: Tensile strength ~1450 MPa, hardness 45 HRC, elongation 10%.
This sequence produces a balanced combination of high strength and ductility ideal for aerospace applications.
13. Applications by Condition
| Condition | Industry | Typical Components |
|---|---|---|
| Condition A | Aerospace / Forming | Bellows, diaphragms, springs |
| RH950 | Energy / Chemical | High-pressure valves, turbine seals |
| TH1050 | Aerospace / Defense | Structural fasteners, actuators |
| Custom RHT | Marine / Power | Corrosion-resistant bolts and rings |
The ability to tailor mechanical properties through heat treatment makes 15-7PH a truly versatile alloy.
14. Temperature Stability and Service Limits
15-7PH stainless steel maintains excellent mechanical strength up to 550°C (1020°F). Beyond this point, over-aging can reduce hardness and fatigue resistance. In cryogenic applications, it retains toughness down to -200°C, making it suitable for both high and low-temperature service.
15. Quality Control and Testing at SAKYSTEEL
SAKYSTEEL applies stringent quality assurance standards throughout every heat treatment stage to guarantee performance consistency.
All materials are:
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Heat treated in controlled-atmosphere furnaces
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Verified through hardness and tensile testing
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Checked for grain size and microstructure
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Supplied with EN 10204 3.1 / 3.2 certificates
(Logo SAKYSTEEL)
Every bar, plate, and wire is fully traceable, ensuring reliability for critical industrial applications.
16. SAKYSTEEL Supply Range for 15-7PH
We provide 15-7PH stainless steel in the following forms and conditions:
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Round Bars & Rods – Annealed, RH950, or TH1050
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Plates & Sheets – Solution-treated or aged
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Wires & Strips – Cold-drawn and precipitation-hardened
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Forgings & Rings – Heat treated to custom specifications
Customized heat treatment services are also available based on client requirements.
17. Advantages of Heat Treated 15-7PH
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High Strength-to-Weight Ratio
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Dimensional Stability During Aging
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Excellent Fatigue Life
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Tailorable Mechanical Properties
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Consistent Performance at Elevated Temperatures
These characteristics make 15-7PH an ideal solution for demanding engineering challenges across multiple industries.
18. Comparison with Other PH Stainless Steels
| Alloy | Hardening Element | Max Strength (MPa) | Max Temp (°C) | Weldability |
|---|---|---|---|---|
| 15-7PH | Aluminum | 1500+ | 550 | Good |
| 17-4PH | Copper | 1300 | 315 | Excellent |
| 17-7PH | Aluminum | 1300 | 425 | Good |
15-7PH outperforms 17-4PH and 17-7PH in high-temperature strength and fatigue resistance, though 17-4PH remains easier to weld and more corrosion-resistant in marine conditions.
19. Summary of Heat Treatment Conditions
| Condition | Temp Range (°C) | Key Result |
|---|---|---|
| A | 1038 | Annealed, soft, formable |
| RH950 | 510 | High strength, good ductility |
| TH1050 | 565 | Maximum hardness, high fatigue resistance |
| RHT900–1150 | 480–620 | Custom strength/toughness balance |
By selecting the appropriate condition, designers can match the alloy’s performance to exact operational requirements.
20. Conclusion
15-7PH stainless steel offers exceptional flexibility through its range of heat treatment conditions. From the soft, formable Condition A to the ultra-high-strength TH1050 condition, this alloy can be engineered to meet diverse operational demands — from aerospace springs and diaphragms to turbine components and valve parts.
The available heat treatment options — RH950, TH1050, and custom RHT variants — provide unmatched control over mechanical performance, ensuring reliability even under severe stress, temperature, or fatigue conditions.
At SAKYSTEEL, we supply certified 15-7PH stainless steel materials in all standard and custom heat treatment conditions, produced under strict quality standards and supported by full technical documentation.
Post time: Oct-29-2025