Among the many types of stainless steels used in modern industry, precipitation-hardening (PH) stainless steels are highly valued for their combination of high strength, toughness, and corrosion resistance. Two of the most popular grades in this category are 15-7PH and 17-4PH.
Although they belong to the same family and share some similarities, there are important differences in their chemical composition, mechanical properties, and applications.
In this detailed article, SAKYSTEEL explains the key distinctions between 15-7PH and 17-4PH stainless steel, helping engineers, fabricators, and purchasing managers select the most appropriate material for their specific needs.
1. Introduction to Precipitation-Hardening Stainless Steels
Precipitation-hardening (PH) stainless steels achieve their strength through a two-step process:
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Solution treatment (annealing) to create a uniform structure.
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Aging (precipitation hardening) to form fine particles that block dislocation movement, increasing strength and hardness.
This process allows PH stainless steels to achieve much higher mechanical strength than standard austenitic grades (like 304 or 316), while maintaining good corrosion resistance and fabrication flexibility.
Both 15-7PH and 17-4PH belong to this family — but their alloying elements and strengthening mechanisms are different.
2. Chemical Composition Comparison
| Element | 15-7PH (UNS S17700) | 17-4PH (UNS S17400) |
|---|---|---|
| Chromium (Cr) | 14.0 – 16.0% | 15.0 – 17.5% |
| Nickel (Ni) | 6.5 – 7.8% | 3.0 – 5.0% |
| Aluminum (Al) | 0.75 – 1.5% | — |
| Copper (Cu) | — | 3.0 – 5.0% |
| Carbon (C) | ≤ 0.09% | ≤ 0.07% |
| Manganese (Mn) | ≤ 1.0% | ≤ 1.0% |
| Silicon (Si) | ≤ 1.0% | ≤ 1.0% |
| Phosphorus (P) | ≤ 0.04% | ≤ 0.04% |
| Sulfur (S) | ≤ 0.03% | ≤ 0.03% |
| Iron (Fe) | Balance | Balance |
Key Difference:
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15-7PH uses aluminum as its main precipitation-hardening element.
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17-4PH uses copper as its strengthening element.
This difference results in distinct microstructures, aging behaviors, and performance characteristics between the two alloys.
3. Metallurgical Differences
3.1 15-7PH: Aluminum-Hardened Martensitic Alloy
15-7PH develops high strength through the precipitation of Ni₃Al (nickel-aluminum intermetallic compounds) during aging. It transitions from an austenitic to a martensitic structure after cold working and heat treatment.
This aluminum-based precipitation mechanism provides exceptionally high strength and hardness with good corrosion resistance.
3.2 17-4PH: Copper-Hardened Martensitic Alloy
17-4PH is strengthened by fine copper-rich precipitates formed during aging. The alloy remains martensitic after solution treatment and aging, and it is known for its excellent weldability and ease of processing.
While it offers slightly lower maximum strength than 15-7PH, it provides better general corrosion resistance and superior toughness.
4. Mechanical Properties Comparison
| Property | 15-7PH | 17-4PH |
|---|---|---|
| Yield Strength (MPa) | 1100–1300 | 1000–1200 |
| Tensile Strength (MPa) | 1250–1450 | 1100–1300 |
| Elongation (%) | 6–10 | 10–18 |
| Hardness (HRC) | 42–46 | 38–44 |
| Density (g/cm³) | 7.8 | 7.75 |
Summary:
15-7PH offers higher tensile strength and hardness, while 17-4PH provides better ductility and impact toughness. Both can be tailored through heat treatment to suit specific application requirements.
5. Corrosion Resistance
Both alloys resist atmospheric corrosion, mild acids, and industrial environments better than carbon or low-alloy steels. However:
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17-4PH has slightly better corrosion resistance, thanks to its higher chromium and copper content. It performs well in marine and chloride environments if properly passivated.
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15-7PH is also corrosion-resistant but may show pitting in severe chloride conditions. It is best suited for moderately corrosive environments where strength is the priority.
6. Heat Treatment Differences
6.1 15-7PH
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Solution Anneal (Condition A): 1038°C for 1 hour, air cool.
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Transformation to Martensite (Condition TH1050): Cold work followed by heating at 565°C for 1 hour.
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Aging (Condition RH950): 510°C for 4 hours, air cool.
This treatment sequence produces a very high-strength martensitic structure.
6.2 17-4PH
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Solution Treatment: 1040°C for 30–60 minutes, air cool.
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Aging Conditions: 480°C (H900), 550°C (H1025), 580°C (H1075), or 620°C (H1150).
Different aging temperatures allow designers to tune strength and toughness depending on service needs.
7. Fabrication and Machinability
| Property | 15-7PH | 17-4PH |
|---|---|---|
| Machinability | Good in annealed condition | Excellent in solution-treated condition |
| Formability | Excellent before aging | Moderate |
| Weldability | Good with precautions | Excellent |
| Surface Finish | Excellent | Excellent |
15-7PH is easier to form in the annealed condition before aging but becomes hard after heat treatment.
17-4PH, on the other hand, is more forgiving during machining and welding, making it a popular choice for complex or thick components.
8. Temperature Resistance
| Property | 15-7PH | 17-4PH |
|---|---|---|
| Max Service Temperature | 550°C (1020°F) | 315°C (600°F) |
| Strength Retention at High Temp | Excellent | Moderate |
| Thermal Expansion (µm/m°C) | 10.9 | 10.8 |
15-7PH retains its strength better at elevated temperatures, while 17-4PH is typically limited to applications below 315°C due to over-aging and loss of strength.
9. Applications Comparison
15-7PH Stainless Steel Applications
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Aerospace components (springs, bellows, diaphragms)
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High-pressure vessels and valves
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Chemical process equipment
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Precision mechanical parts
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Springs and fasteners requiring high fatigue strength
17-4PH Stainless Steel Applications
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Pump shafts and impellers
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Turbine components
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Nuclear and power generation parts
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Marine fittings
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Valve bodies, bolts, and gears
Summary:
If the application requires maximum strength and high-temperature performance, choose 15-7PH.
If corrosion resistance, toughness, and weldability are more critical, 17-4PH is the better choice.
10. Microstructure and Hardening Mechanisms
| Feature | 15-7PH | 17-4PH |
|---|---|---|
| Strengthening Mechanism | Ni₃Al (Aluminum precipitates) | Cu-rich precipitates |
| Phase after Aging | Martensite + Precipitates | Martensite + Precipitates |
| Microstructure Stability | Very stable up to 550°C | Stable up to 315°C |
| Aging Sensitivity | Narrow range | Wide range, easier to control |
The Ni₃Al phase in 15-7PH provides superior high-temperature stability, while the Cu-rich precipitates in 17-4PH provide better weldability and toughness.
11. Performance Summary
| Feature | 15-7PH | 17-4PH |
|---|---|---|
| Strength | Higher | High |
| Hardness | Higher | Moderate |
| Ductility | Moderate | Better |
| Corrosion Resistance | Good | Very Good |
| High-Temp Performance | Excellent | Limited |
| Machinability | Moderate | Better |
| Weldability | Fair | Excellent |
| Fatigue Resistance | Excellent | Good |
| Cost | Slightly higher | Slightly lower |
Conclusion:
15-7PH is optimized for strength and heat resistance, while 17-4PH excels in weldability, toughness, and corrosion resistance.
12. Choosing Between 15-7PH and 17-4PH
To select the right grade, consider the following:
| Requirement | Recommended Alloy |
|---|---|
| Maximum strength and hardness | 15-7PH |
| Excellent corrosion resistance | 17-4PH |
| High-temperature stability (up to 550°C) | 15-7PH |
| Better weldability | 17-4PH |
| Aerospace and precision parts | 15-7PH |
| Marine and industrial equipment | 17-4PH |
Engineers often choose 15-7PH when the design demands high strength and minimal deformation, and 17-4PH when fabrication ease and corrosion resistance are top priorities.
13. Cost and Availability
Both materials are commercially available in a wide range of forms — round bar, plate, wire, and strip.
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17-4PH is generally more economical and widely stocked due to its popularity in industrial applications.
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15-7PH, while slightly more expensive, is preferred in aerospace and defense sectors for its superior strength-to-weight ratio.
14. Real-World Example: Aerospace Spring Application
An aerospace manufacturer compared both alloys for a high-temperature spring application:
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17-4PH springs began to relax after exposure above 400°C.
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15-7PH springs retained full load capacity up to 520°C with minimal creep deformation.
As a result, 15-7PH was chosen for the final design, ensuring long-term reliability in service.
15. Quality Assurance and Supply from SAKYSTEEL
At SAKYSTEEL, we manufacture and supply both 15-7PH and 17-4PH stainless steels under strict quality control.
All materials come with:
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ASTM A693 / AMS 5520 / AMS 5659 compliance
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EN 10204 3.1 / 3.2 certificates
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Positive Material Identification (PMI)
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Ultrasonic and surface inspection
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Custom heat treatment options
(Logo SAKYSTEEL)
Our facilities are equipped for precision cutting, polishing, and packaging to meet global export requirements.
16. SAKYSTEEL Product Range
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Round Bars & Rods: From 6 mm to 350 mm
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Plates & Sheets: Thickness 1 mm to 100 mm
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Wire & Strip: For springs and precision components
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Forgings & Rings: Custom dimensions for aerospace and oilfield applications
SAKYSTEEL ensures fast delivery, consistent quality, and expert technical support.
17. Industry Certifications
SAKYSTEEL’s stainless steel materials meet or exceed:
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AMS, ASTM, and ASME standards
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NACE MR0175 / ISO 15156 compliance
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Aerospace and defense specifications
Every product is tested and certified for chemical, mechanical, and metallurgical conformity.
18. Summary: 15-7PH vs. 17-4PH
| Comparison Factor | 15-7PH | 17-4PH |
|---|---|---|
| Hardening Element | Aluminum | Copper |
| Strength | Higher | Slightly Lower |
| Corrosion Resistance | Good | Better |
| Temperature Limit | 550°C | 315°C |
| Weldability | Fair | Excellent |
| Cost | Higher | Lower |
| Common Use | Aerospace, Springs, Valves | Pumps, Shafts, Marine Parts |
In short:
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Choose 15-7PH when strength and temperature resistance are critical.
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Choose 17-4PH when ease of fabrication and corrosion protection are more important.
19. Conclusion
Both 15-7PH and 17-4PH stainless steels are outstanding materials with proven track records across multiple industries.
15-7PH offers higher strength, better fatigue resistance, and superior performance at elevated temperatures, while 17-4PH provides excellent weldability, corrosion resistance, and cost efficiency.
When sourced from a trusted supplier like SAKYSTEEL, both grades deliver exceptional reliability, traceability, and consistent quality — ensuring that your components perform as designed under the toughest conditions.
Post time: Oct-29-2025