The aerospace industry is one of the most demanding environments for material performance. Aircraft, spacecraft, and engine components are constantly exposed to extreme conditions — high stress, rapid temperature changes, vibration, and corrosion. To ensure long-term safety and reliability, materials must combine high strength, low weight, corrosion resistance, and stability at elevated temperatures.
Among the many advanced alloys used in aerospace, 15-7PH stainless steel has earned a distinguished place. Known for its outstanding mechanical properties and versatility, it remains a key material for aircraft structures, springs, bellows, fasteners, and turbine components.
In this article, SAKYSTEEL explains why 15-7PH stainless steel is widely used in aerospace applications, examining its composition, properties, processing advantages, and proven track record in the industry.
1. What Is 15-7PH Stainless Steel?
15-7PH stainless steel (UNS S17700, also known as Type 631) is a precipitation-hardening (PH) martensitic stainless steel. The “15-7” refers to its composition: approximately 15% chromium and 7% nickel, while “PH” indicates that it gains its strength through precipitation hardening — a heat treatment that forms fine intermetallic compounds (mainly Ni₃Al) to increase hardness and yield strength.
This alloy was developed as an improvement over 17-7PH, offering higher strength and better temperature stability without sacrificing corrosion resistance. It provides engineers with the flexibility to form complex shapes in the soft condition (annealed) and later harden the parts by heat treatment to achieve extremely high strength.
2. Chemical Composition
| Element | Content (%) | Purpose |
|---|---|---|
| Chromium (Cr) | 14.0 – 16.0 | Corrosion resistance and hardness |
| Nickel (Ni) | 6.5 – 7.8 | Ductility and toughness |
| Aluminum (Al) | 0.75 – 1.5 | Precipitation hardening element |
| Carbon (C) | ≤ 0.09 | Controls strength and toughness |
| Manganese (Mn) | ≤ 1.00 | Improves strength |
| Silicon (Si) | ≤ 1.00 | Increases oxidation resistance |
| Phosphorus (P) | ≤ 0.04 | Controlled for ductility |
| Sulfur (S) | ≤ 0.03 | Enhances machinability |
| Iron (Fe) | Balance | Base metal |
This unique chemistry enables 15-7PH to achieve very high strength (up to 1500 MPa) while retaining good corrosion resistance and fatigue performance.
3. Why Aerospace Engineers Choose 15-7PH
Aerospace engineers need materials that can withstand extreme conditions while minimizing weight and maintenance. 15-7PH satisfies these requirements because it offers a rare combination of properties that few alloys can match:
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High strength-to-weight ratio
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Excellent fatigue resistance
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Good corrosion resistance in aerospace environments
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Outstanding dimensional stability after heat treatment
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Capability to operate up to 550°C (1020°F)
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Ease of forming, machining, and heat treating
These features make 15-7PH one of the most versatile stainless steels used in flight-critical components.
4. High Strength and Hardness
A major advantage of 15-7PH is its ability to reach exceptional tensile and yield strength through heat treatment. After precipitation hardening (typically RH950 or TH1050 condition), the alloy can achieve:
| Property | Typical Value |
|---|---|
| Tensile Strength | 1400–1600 MPa |
| Yield Strength | 1200–1450 MPa |
| Hardness | 42–48 HRC |
| Elongation | 6–10% |
Such strength allows engineers to design lighter components without sacrificing load-bearing capability, a key advantage in aerospace structures where weight reduction directly translates into higher fuel efficiency and performance.
5. Excellent Strength-to-Weight Ratio
In aerospace design, weight is everything. The density of 15-7PH is around 7.80 g/cm³, similar to other stainless steels but offering far higher strength. This means components can be made smaller and thinner, reducing overall mass.
For example, a spring or diaphragm made from 15-7PH can handle the same load as one made from conventional stainless steel but at 30–40% less weight. This makes it invaluable in aircraft systems, where every kilogram saved increases payload capacity and reduces fuel burn.
6. Superior Fatigue and Creep Resistance
Aircraft components such as landing gear springs, actuators, and fasteners experience repeated cyclic loading. 15-7PH stainless steel offers outstanding fatigue resistance, maintaining mechanical integrity over millions of stress cycles.
Furthermore, its creep strength — the ability to resist deformation under continuous stress at elevated temperatures — makes it suitable for turbine and exhaust components that operate at up to 550°C (1020°F).
7. Dimensional Stability During Heat Treatment
Precision is critical in aerospace manufacturing. Any distortion during heat treatment can lead to costly rework or part rejection. 15-7PH stainless steel exhibits minimal dimensional change during aging, typically less than 0.05%.
This stability ensures that complex parts such as bellows, diaphragms, and precision springs maintain their design tolerances even after final hardening. Engineers can rely on consistent performance across large production batches.
8. Corrosion Resistance for Harsh Aerospace Environments
Aircraft and spacecraft operate in conditions where materials face exposure to humidity, salt, jet fuel, and hydraulic fluids. The high chromium content (14–16%) of 15-7PH forms a protective oxide layer that prevents rust and pitting.
While not as corrosion-resistant as fully austenitic grades like 316, 15-7PH performs exceptionally well in:
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Aircraft hydraulic systems
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Fuel systems
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Landing gear mechanisms
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Airframe structural joints
Its corrosion resistance can be further improved through passivation and surface polishing, which are standard aerospace finishing processes.
9. Excellent Performance at High and Low Temperatures
15-7PH retains its mechanical strength and stability over a wide temperature range:
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Maintains yield strength above 1000 MPa up to 550°C (1020°F)
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Retains good ductility and toughness at cryogenic temperatures (down to -200°C)
This makes it suitable for both hot-section engine components and cryogenic control systems, showing versatility unmatched by many other stainless steels.
10. Heat Treatment Flexibility
One of the greatest advantages of 15-7PH is its multiple heat treatment conditions, allowing engineers to tailor properties precisely to application needs.
| Condition | Temperature (°C) | Characteristics |
|---|---|---|
| A (Annealed) | 1038 | Soft and formable |
| RH950 | 510 | High strength with good ductility |
| TH1050 | 565 | Maximum strength and hardness |
| Custom RHT | 480–620 | Tuned balance of properties |
This adaptability means a single alloy can be optimized for different components — from flexible bellows to rigid structural fasteners — simplifying supply chains and maintenance.
11. Fabrication and Machinability
In its annealed (Condition A) form, 15-7PH is easily formed, machined, and welded, similar to 17-4PH. After aging, it becomes very hard but maintains excellent dimensional control. This dual capability is ideal for aerospace manufacturing, where complex geometries are required.
For example:
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Forming: Aircraft diaphragms and bellows are formed in Condition A, then aged to final strength.
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Machining: Fasteners and shafts are precision-machined in the annealed condition and aged to hardness afterward.
This sequence reduces tooling wear and ensures precise tolerances on finished parts.
12. Resistance to Stress Corrosion Cracking
Aerospace structures often operate under high tensile loads in humid or salt-laden environments, which can lead to stress corrosion cracking (SCC) in some materials.
15-7PH exhibits excellent SCC resistance in mild environments due to its aluminum precipitation structure and stable martensitic matrix. Aging at moderate temperatures (RH950 condition) provides an optimal balance between strength and SCC resistance, ensuring long-term reliability in flight-critical components.
13. Common Aerospace Applications of 15-7PH
13.1 Aircraft Springs and Diaphragms
The high fatigue strength and dimensional stability make 15-7PH perfect for springs, washers, and diaphragms in aircraft control systems, actuators, and hydraulic assemblies.
13.2 Bellows and Flexible Joints
Its combination of elasticity, corrosion resistance, and high strength after aging allows it to serve in expansion bellows and flexible couplings, which must withstand both vibration and pressure.
13.3 Fasteners and Structural Hardware
15-7PH is commonly used for bolts, nuts, pins, and shafts that require high strength and precision under load. Its non-galling nature also prevents seizing during assembly and disassembly.
13.4 Turbine and Engine Components
Components in turbine engines, including retainers, springs, and compressor blades, benefit from the alloy’s high-temperature strength and oxidation resistance.
13.5 Spacecraft and Satellite Systems
Because of its low magnetic permeability and high strength, 15-7PH is ideal for non-magnetic satellite parts and connectors that require both lightness and stability in vacuum environments.
14. Comparison with Other Aerospace Steels
| Property | 15-7PH | 17-4PH | 17-7PH |
|---|---|---|---|
| Hardening Element | Aluminum | Copper | Aluminum |
| Max Service Temp (°C) | 550 | 315 | 425 |
| Strength (MPa) | 1500+ | 1300 | 1250 |
| Corrosion Resistance | Good | Very Good | Good |
| Formability | Excellent (annealed) | Moderate | Excellent |
| Fatigue Resistance | Excellent | Good | Good |
Compared to 17-4PH and 17-7PH, 15-7PH offers superior strength and temperature capability, making it better suited for hot and dynamic aerospace environments.
15. Real-World Case Example
A leading aircraft manufacturer replaced 17-4PH with 15-7PH for a high-temperature actuator spring.
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The new spring maintained load retention up to 540°C,
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Reduced relaxation under cyclic loading by 25%,
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And extended service life by more than 50%.
This demonstrates the real operational advantage of using 15-7PH in aerospace systems where performance and safety cannot be compromised.
16. Certification and Standards
15-7PH stainless steel is produced and certified under strict international standards to meet aerospace requirements:
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ASTM A693 – Precipitation-hardening stainless plate, sheet, and strip
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AMS 5520 / AMS 5659 – Aerospace material specifications
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UNS S17700 – Unified numbering system
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EN 1.4532 – European equivalent designation
SAKYSTEEL supplies all 15-7PH products with full traceability and EN 10204 3.1/3.2 certificates to ensure compliance with aerospace procurement standards.
(Logo SAKYSTEEL)
17. SAKYSTEEL Product Range for Aerospace
SAKYSTEEL offers a complete selection of 15-7PH stainless steel in:
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Bars and Rods – Annealed, RH950, or TH1050
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Plates and Sheets – For aircraft structures and tooling
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Wires and Strips – Precision spring wire for aerospace use
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Forgings and Rings – Custom shapes for turbine and actuator components
Our products are used by clients worldwide in aviation, space, and defense industries, backed by consistent quality and technical expertise.
18. Advantages of Choosing 15-7PH from SAKYSTEEL
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Guaranteed Alloy Authenticity with full PMI and heat number traceability
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Multiple Heat Treatment Options tailored to project requirements
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High Dimensional Accuracy through precision processing
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Compliance with AMS, ASTM, and EN standards
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Fast Global Delivery supported by professional export experience
Every batch is heat treated, inspected, and certified according to aerospace quality standards.
19. Future Role of 15-7PH in Aerospace Innovation
As aerospace technology advances toward lighter, more fuel-efficient aircraft and reusable space systems, the need for high-strength, temperature-stable materials continues to grow.
15-7PH remains a trusted alloy for additive manufacturing, advanced actuators, and high-efficiency turbine systems, and ongoing research focuses on optimizing its performance through vacuum melting and powder metallurgy.
20. Conclusion
The success of 15-7PH stainless steel in aerospace applications is no coincidence. Its high strength, fatigue resistance, thermal stability, and corrosion protection make it one of the most reliable materials for flight-critical components.
By offering engineers flexibility in heat treatment and excellent formability before aging, 15-7PH allows for efficient production of complex, high-performance parts.
At SAKYSTEEL, we proudly supply certified 15-7PH stainless steel products for aerospace and defense clients worldwide — ensuring the highest standards of quality, traceability, and performance.
Post time: Oct-29-2025