In the world of advanced engineering materials, 15-7PH stainless steel stands out as one of the most versatile and high-strength precipitation-hardening stainless steels available today. Its ability to combine excellent mechanical strength, corrosion resistance, and thermal stability makes it an ideal material for industries that demand reliability under extreme conditions.
In this article, SAKYSTEEL provides a detailed overview of what 15-7PH stainless steel is, its chemical composition, key properties, and main industrial applications, helping engineers and procurement specialists make informed decisions when selecting materials for high-performance components.
1. What Is 15-7PH Stainless Steel?
15-7PH stainless steel (UNS S17700, also known as Type 15-7 PH or 631 alloy) is a precipitation-hardening martensitic stainless steel containing approximately 15% chromium and 7% nickel, with additions of aluminum to facilitate hardening by heat treatment.
The “PH” in its name stands for Precipitation Hardening, a heat-treatment process that enhances the alloy’s mechanical strength through the controlled precipitation of fine intermetallic compounds within the steel’s structure.
Developed as an improvement over 17-7PH and 17-4PH, 15-7PH offers a balance of high strength, corrosion resistance, and excellent formability, making it suitable for both aerospace and industrial applications.
2. Chemical Composition
The carefully controlled chemical composition of 15-7PH stainless steel is key to its outstanding performance.
| Element | Typical Content (%) | Function |
|---|---|---|
| Chromium (Cr) | 14.0 – 16.0 | Corrosion resistance and hardness |
| Nickel (Ni) | 6.5 – 7.8 | Toughness and stability of the martensitic matrix |
| Aluminum (Al) | 0.75 – 1.5 | Precipitation hardening element |
| Carbon (C) | ≤ 0.09 | Strength without reducing ductility |
| Manganese (Mn) | ≤ 1.0 | Deoxidizer and strength enhancer |
| Silicon (Si) | ≤ 1.0 | Improves oxidation resistance |
| Phosphorus (P) | ≤ 0.04 | Kept low to avoid brittleness |
| Sulfur (S) | ≤ 0.03 | Controlled for machinability |
| Iron (Fe) | Balance | Base element |
This composition allows the alloy to transform from austenitic to martensitic phase through heat treatment, which can then be aged to obtain high strength and hardness.
3. Microstructure and Metallurgy
In its solution-annealed condition, 15-7PH is austenitic, meaning it can be easily formed and fabricated. After cold working and heat treatment, it converts to a martensitic structure, which provides its signature high strength.
When aged (typically between 510–565°C), aluminum-containing precipitates such as Ni₃Al form within the matrix. These finely distributed particles significantly increase hardness and yield strength, while maintaining good toughness and corrosion resistance.
4. Key Mechanical Properties
The following table summarizes typical mechanical properties of 15-7PH stainless steel in different heat treatment conditions:
| Condition | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) | Hardness (HRC) |
|---|---|---|---|---|
| Solution Annealed (Condition A) | 850 | 1050 | 15–20 | 25–30 |
| Condition CH900 | 1300 | 1450 | 5–8 | 42–46 |
| Condition RH950 | 1170 | 1300 | 8–12 | 40–44 |
These results show the alloy’s ability to achieve very high strength after aging, often comparable to low-alloy tool steels, but with the added benefit of corrosion resistance typical of stainless grades.
5. Key Features and Advantages
5.1 High Strength and Hardness
After aging, 15-7PH develops yield strengths above 1300 MPa, which allows designers to reduce section thickness and weight in critical components.
5.2 Excellent Corrosion Resistance
The high chromium content gives 15-7PH excellent resistance to atmospheric corrosion and mild chemical environments. It performs well in marine and industrial atmospheres, although not as corrosion-resistant as fully austenitic grades like 316.
5.3 Good Formability
In its annealed (A) condition, the material can be easily formed, drawn, or bent, making it suitable for complex parts that are later heat treated for high strength.
5.4 Outstanding Fatigue and Stress Resistance
The fine, uniform microstructure after aging gives 15-7PH outstanding fatigue strength and resistance to stress corrosion cracking, even under cyclic loading conditions.
5.5 Dimensional Stability
It exhibits minimal distortion during heat treatment and excellent dimensional control, making it ideal for precision mechanical components.
6. Heat Treatment and Processing
15-7PH can be heat treated to various tempers to achieve specific mechanical properties.
Step 1: Solution Annealing (Condition A)
-
Heat to 1038°C (1900°F) for 1 hour and air cool.
-
Material remains austenitic and easily formable.
Step 2: Transformation to Martensitic Condition (Condition TH1050)
-
Cold work (typically 60–70% reduction) to induce martensitic structure.
-
Then heat to 565°C (1050°F) for 1 hour and air cool.
Step 3: Aging Treatment (Condition RH950)
-
Age at 510°C (950°F) for 1–4 hours.
-
Results in maximum hardness and strength.
Proper control of these steps ensures consistent properties across large or complex components.
7. Physical Properties
| Property | Value |
|---|---|
| Density | 7.8 g/cm³ |
| Elastic Modulus | 200 GPa |
| Thermal Conductivity | 16.7 W/m·K |
| Coefficient of Expansion | 10.9 × 10⁻⁶ /°C |
| Electrical Resistivity | 0.00074 Ω·cm |
These physical parameters allow 15-7PH to perform well in both low and moderately elevated temperatures up to 550°C (1020°F).
8. Comparison with Other PH Stainless Steels
| Property | 15-7PH | 17-4PH | 17-7PH |
|---|---|---|---|
| Main Hardening Element | Aluminum | Copper | Aluminum |
| Strength (Aged) | Very High | High | Medium |
| Corrosion Resistance | Good | Very Good | Good |
| Fabricability | Excellent | Good | Excellent |
| Max Working Temp | 550°C | 315°C | 425°C |
| Magnetic Behavior | Magnetic after aging | Magnetic | Magnetic |
15-7PH stainless steel offers higher strength and temperature capability compared to 17-4PH and 17-7PH, while maintaining good corrosion performance and fabrication flexibility.
9. Applications of 15-7PH Stainless Steel
Due to its unique combination of strength, toughness, and corrosion resistance, 15-7PH is widely used in critical applications where reliability is essential.
9.1 Aerospace Industry
-
Springs, diaphragms, and bellows
-
Aircraft fittings and fasteners
-
High-performance actuators
-
Structural components exposed to heat and stress
9.2 Chemical and Petrochemical Equipment
-
Pressure vessels and valve components
-
Chemical process springs and seals
-
Corrosion-resistant shafts
9.3 Power Generation
-
Turbine blade retainers
-
Heat-resistant bolts and pins
9.4 Automotive and Industrial
-
Precision springs and washers
-
High-strength tools and instrumentation parts
Because of its excellent fatigue resistance, 15-7PH is particularly valued in components subject to repeated cyclic loading.
10. Machinability and Fabrication
10.1 Machinability
15-7PH machines similarly to 17-4PH stainless steel. It should be machined in the annealed condition to minimize tool wear. After aging, the material becomes much harder and requires carbide tooling.
10.2 Welding
It can be welded using conventional fusion methods, such as GTAW (TIG) and GMAW (MIG). However, post-weld heat treatment is recommended to restore corrosion resistance and strength.
10.3 Forming
In its annealed condition, 15-7PH offers good formability. After cold work and aging, it hardens significantly, limiting further forming operations.
11. Corrosion Resistance in Detail
15-7PH performs well in many environments including:
-
Marine atmospheres
-
Mildly acidic solutions
-
Industrial gases and steam
However, it is not recommended for strongly acidic or chloride-rich environments like seawater immersion, where pitting may occur. For maximum corrosion resistance, the surface should be properly cleaned and passivated after fabrication.
12. Temperature Performance
The alloy maintains its mechanical properties up to about 550°C (1020°F), above which over-aging may reduce strength. It also retains good toughness and fatigue strength at sub-zero temperatures, making it versatile for both hot and cold service applications.
13. Quality Control and Certification
At SAKYSTEEL, all 15-7PH stainless steel materials are produced under strict quality control systems, ensuring:
-
Full traceability and heat number identification
-
Chemical composition analysis by spectrometer
-
Mechanical testing for each batch
-
Ultrasonic and surface inspection
-
Certificates according to EN 10204 3.1 / 3.2 standards
(Logo SAKYSTEEL)
14. Typical Product Forms Supplied by SAKYSTEEL
-
Round Bars & Rods: Cold drawn, peeled, ground, or centerless polished
-
Plates & Sheets: Hot rolled or cold rolled
-
Wire & Strip: Precision spring wire for aerospace and industrial use
-
Forgings: Custom shapes according to client specifications
SAKYSTEEL provides flexible size ranges, cutting services, and fast global shipping to meet project timelines.
15. Advantages of Choosing 15-7PH from SAKYSTEEL
-
Guaranteed Alloy Authenticity – Verified through PMI and heat-number traceability
-
Stable Mechanical Properties – Uniform strength and hardness in every batch
-
Professional Heat Treatment – Condition A, TH1050, and RH950 available
-
Excellent Surface Finish – Polished or bright-turned options for precision components
-
Global Export Experience – Compliant with ASTM, AMS, and EN standards
SAKYSTEEL has supplied 15-7PH materials to aerospace, defense, marine, and chemical industries across Asia, Europe, and North America.
16. Comparison: 15-7PH vs. 17-4PH in Real Applications
| Property | 15-7PH | 17-4PH |
|---|---|---|
| Hardness (Max HRC) | 46 | 42 |
| Operating Temperature | Up to 550°C | Up to 315°C |
| Fatigue Strength | Higher | Moderate |
| Weldability | Good | Excellent |
| Machinability | Moderate | Better |
| Application Example | Springs, Diaphragms, Valves | Shafts, Bolts, Pump parts |
For high-temperature strength and fatigue performance, 15-7PH clearly outperforms 17-4PH. When weldability and cost are priorities, 17-4PH may be preferred.
17. Storage and Handling Recommendations
-
Store in dry, clean areas to avoid contamination and corrosion.
-
Avoid contact with carbon steel surfaces during handling.
-
Use non-metallic separators when stacking sheets or bars.
-
Apply protective coating or wrapping for export shipments.
Proper handling preserves the alloy’s surface quality and corrosion performance.
18. Conclusion
15-7PH stainless steel is a high-strength, precipitation-hardening alloy that offers a unique blend of mechanical performance, corrosion resistance, and thermal stability.
With its ability to achieve yield strengths exceeding 1300 MPa and maintain stability up to 550°C, it is an excellent choice for aerospace, petrochemical, and high-precision industrial applications.
At SAKYSTEEL, we provide certified 15-7PH stainless steel materials with full traceability, precision processing, and customized dimensions to support our clients’ most demanding projects.
When performance, reliability, and quality matter — trust SAKYSTEEL for your stainless steel solutions.
Post time: Oct-29-2025