17-7PH (UNS S17700, also known as grade 631) is a semi-austenitic precipitation-hardening stainless steel that combines high strength and excellent corrosion resistance. Its unique metallurgical structure allows it to be formed in a soft, ductile condition and then heat-treated to achieve tensile strengths exceeding 1650 MPa. This makes it indispensable in aerospace applications where weight reduction, fatigue resistance, and environmental stability are critical.
1. Basic Information of 631 (17-7PH)
17-7PH is classified as a semi-austenitic stainless steel because, in the annealed condition, it retains an austenitic structure that is easily formable. Upon precipitation hardening (aging), it transforms to martensite and precipitates intermetallic compounds, primarily Ni₃Al, which dramatically increase strength. The alloy is available in various conditions: Condition A (annealed), TH 1050 (tempered and hardened at 1050°F), and RH 950 (refrigerated and hardened at 950°F). The RH 950 condition typically yields the highest strength, with ultimate tensile strengths up to 1825 MPa.
2. Chemical Composition
The precise chemical composition of 17-7PH is tightly controlled to ensure consistent precipitation hardening response. The key alloying elements are chromium for corrosion resistance, nickel for austenite stabilization, and aluminum for the formation of the hardening precipitate.
| Element | Composition Range (wt%) |
|---|---|
| Carbon (C) | ≤ 0.09 |
| Manganese (Mn) | ≤ 1.00 |
| Silicon (Si) | ≤ 1.00 |
| Chromium (Cr) | 16.00 – 18.00 |
| Nickel (Ni) | 6.50 – 7.75 |
| Aluminum (Al) | 0.75 – 1.50 |
| Phosphorus (P) | ≤ 0.040 |
| Sulfur (S) | ≤ 0.030 |
| Iron (Fe) | Balance |
3. Mechanical Properties
The mechanical properties of 17-7PH vary significantly depending on the heat treatment condition. Below are the typical values for the most common aerospace specifications (AMS 5528 for sheet, AMS 5643 for bar).
| Condition | Tensile Strength (MPa) | Yield Strength 0.2% (MPa) | Elongation (%) | Hardness (HRC) |
|---|---|---|---|---|
| Condition A (Annealed) | ≤ 1030 | ≤ 450 | ≥ 25 | ≤ 30 |
| TH 1050 | 1379 – 1550 | 1275 – 1448 | ≥ 6 | 43 – 47 |
| RH 950 | 1655 – 1825 | 1517 – 1724 | ≥ 6 | 47 – 50 |
4. Typical Aerospace Applications
17-7PH is used extensively in aerospace for components that require a high strength-to-weight ratio and resistance to stress corrosion cracking. Common applications include:
- Aircraft structural components: Wing skins, bulkheads, and fuselage frames where high strength and fatigue resistance are required.
- Springs and fasteners: Belleville washers, coil springs, and bolts that must maintain load at elevated temperatures (up to 315°C).
- Actuator and valve components: Housings, pistons, and shafts exposed to hydraulic fluids and high cyclic loads.
- Honeycomb panels: Core material for sandwich structures due to its formability and post-heat-treatment strength.
- CNC machined parts: Precision-machined brackets, fittings, and sensor housings requiring tight tolerances.
5. CNC Machining Performance
Machining 17-7PH requires careful parameter selection due to its work-hardening tendency and high strength in the hardened condition. For optimal results, machining is often performed in the annealed (Condition A) state, followed by heat treatment and a final light finishing pass.
| Operation | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Tool Material |
|---|---|---|---|---|
| Turning (Annealed) | 60 – 90 | 0.15 – 0.30 | 2.0 – 4.0 | Carbide (C2/C3) |
| Turning (Hardened) | 30 – 50 | 0.08 – 0.15 | 0.5 – 1.5 | Ceramic or CBN |
| Milling (Annealed) | 50 – 80 | 0.10 – 0.20 (per tooth) | 1.0 – 3.0 | Carbide (TiAlN coated) |
| Drilling | 20 – 35 | 0.05 – 0.12 | N/A | HSS-Co or Carbide |
Key Machining Tips:
- Use high-pressure coolant (minimum 40 bar) to control heat and prevent work hardening.
- Maintain a constant chip load; avoid dwell or rubbing which causes surface hardening.
- For tapping, use roll-form taps in the annealed condition to avoid breakage.
- Surface finish in hardened condition can achieve Ra ≤ 0.4 µm with proper finishing passes.
6. Our Services
Dongguan Stirling Metal Products Co., Ltd. specializes in providing 631 material + CNC machining One-Stop Service:
- ✅ Material procurement: genuine product guarantee, complete material certificates (MTC per AMS 5528/5643)
- ✅ CNC machining: precision ±0.01mm, surface finish down to Ra 0.2 µm
- ✅ Heat treatment: in-house solution annealing and precipitation hardening (TH 1050 / RH 950)
- ✅ Sample production: fast delivery in 3-5 days for prototype validation
- ✅ Batch production: delivery time of 7-15 days with full PPAP documentation
For a Free Quote, please feel free to contact us!
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