Precipitation-Hardening Stainless Steels (PH Stainless Steels) are a specialized family of stainless steels designed to deliver exceptionally high strength, hardness, and fatigue resistance while maintaining good to excellent corrosion resistance.
Their unique advantage lies in the precipitation-hardening heat treatment, where finely dispersed intermetallic compounds form within the steel matrix, dramatically increasing strength without significant distortion.
Metallurgical Classification
Martensitic PH Stainless Steels
• Example: 17-4 PH (UNS S17400)
• Highest strength and hardness
• Most widely used PH grade
Semi-Austenitic PH Stainless Steels
• Examples: 17-7 PH, PH 15-7 Mo
• Excellent formability before aging
• Very high strength after heat treatment
Austenitic PH Stainless Steels
• Example: A-286 (UNS S66286)
• Best high-temperature strength and corrosion resistance
Typical Chemical Composition
| Element | Typical Range (%) | Purpose |
|---|---|---|
| Chromium (Cr) | 15 – 18 | Corrosion resistance |
| Nickel (Ni) | 4 – 9 | Austenite stabilization |
| Copper (Cu) | 3 – 5 | Precipitation hardening |
| Al / Nb | Small additions | Precipitate formation |
| Carbon (C) | ≤ 0.07 | Strength control |
| Iron (Fe) | Balance | Base metal |
Precipitation-Hardening Mechanism
Solution Annealing (Condition A)
• Dissolves alloying elements
• Produces a soft, machinable structure
Aging / Precipitation Hardening
• Cu-rich or Ni-Al precipitates form
• Strength and hardness increase dramatically
• Minimal dimensional distortion
Typical Mechanical Properties
| Property | Typical Range |
|---|---|
| Tensile Strength | 930 – 1400 MPa |
| Yield Strength (0.2%) | 690 – 1310 MPa |
| Hardness | 30 – 44 HRC |
| Elongation | 10 – 25% |
| Density | ~7.75 g/cm³ |
| Modulus of Elasticity | ~200 GPa |
Corrosion Resistance
• Comparable to 304 / 316 in many environments
• Resistant to atmospheric, marine, and industrial corrosion
• Improved SCC resistance at higher aging temperatures
Key Advantages
✔ Extremely high strength-to-weight ratio
✔ Excellent fatigue and creep resistance
✔ Good weldability (weld before aging)
✔ Excellent dimensional stability
✔ Precision-machining friendly
Common PH Stainless Steel Grades
| Grade | Key Feature |
|---|---|
| 17-4 PH | Most widely used, high strength |
| 17-7 PH | Excellent formability before aging |
| 15-5 PH | Improved toughness |
| PH 13-8 Mo | High fracture toughness |
| A-286 | High-temperature strength |
Applications
Aerospace: Structural parts, turbine components, landing gear
Oil & Gas: High-pressure valves, pump shafts, downhole tools
Industrial: Gears, springs, fasteners, precision shafts
Medical & Instrumentation: Surgical tools, high-load housings
Why Choose PH Stainless Steel
Precipitation-hardening stainless steels offer a unique combination of ultra-high strength, corrosion resistance, and dimensional precision. They are the preferred choice for aerospace, defence, oil & gas, and high-performance engineering applications where reliability and long service life are critical.