This article references the authoritative materials of the Metal Materials Encyclopedia to provide a detailed analysis of the performance characteristics and machining considerations of 17-4PH (UNS S17400 / SUS630) precipitation hardening stainless steel. 17-4PH is a martensitic precipitation hardening stainless steel that achieves a combination of high strength and good corrosion resistance through solution treatment + aging treatment. In CNC machining and metal component manufacturing, it is commonly used in high-strength application scenarios such as aerospace, energy, and precision machinery.
1. Basic Information on 17-4PH
17-4PH (UNS S17400, Japanese grade SUS630) is a precipitation hardening martensitic stainless steel containing copper and niobium. Hardening is achieved by precipitating a copper-rich phase through aging treatment in the 480-620°C range. Typical hardness ranges are approximately 40-47 HRC for H900 (480°C) and 28-38 HRC for H1150 (620°C). It combines high strength, good corrosion resistance, and excellent resistance to stress corrosion cracking, and is widely used in aerospace fasteners, turbine blades, valve stems, shaft components, and nuclear industry structural parts.
2. Chemical Composition (ASTM A564 / AMS 5643)
| Element | Content (%) | Function |
|---|---|---|
| Carbon (C) | ≤0.07 | Controls martensitic hardness |
| Silicon (Si) | ≤1.00 | Deoxidizer |
| Manganese (Mn) | ≤1.00 | Deoxidation and strength |
| Phosphorus (P) | ≤0.040 | Impurity element |
| Sulfur (S) | ≤0.030 | Impurity element |
| Chromium (Cr) | 15.00-17.50 | Corrosion resistance and hardening |
| Nickel (Ni) | 3.00-5.00 | Toughness and corrosion resistance |
| Copper (Cu) | 3.00-5.00 | Core element for precipitation hardening |
| Niobium + Tantalum (Nb+Ta) | 0.15-0.45 | Grain refinement, prevents intergranular corrosion |
3. Mechanical Properties (Comparison of Different Aging Conditions)
| Heat Treatment Condition | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|
| Solution Annealed (Condition A) | ≤1310 (reference) | — | — | ≤363 HB / ≤38 HRC |
| H900 (480°C×1h) | ≥1310 | ≥1170 | ≥10 | 40-47 HRC |
| H925 (495°C×4h) | ≥1170 | ≥1070 | ≥10 | 38-45 HRC |
| H1025 (550°C×4h) | ≥1070 | ≥1000 | ≥12 | 33-39 HRC |
| H1075 (580°C×4h) | ≥1000 | ≥860 | ≥13 | 31-38 HRC |
| H1150 (620°C×4h) | ≥930 | ≥725 | ≥16 | 28-38 HRC |
Data source: ASTM A564 bars and shapes (longitudinal, diameter ≤75mm)
4. Corrosion Resistance
17-4PH exhibits the following performance in various environments after solution treatment + aging:
- Atmospheric environment: Comparable to 304 stainless steel, with excellent atmospheric corrosion resistance
- Fresh water / seawater: Good fresh water resistance; certain corrosion resistance in seawater, but inferior to 316
- Weak acid environments: Good resistance to dilute nitric acid and organic acids
- Chlorides: Sensitive to pitting and crevice corrosion; long-term use in high-chloride environments is not recommended
- Stress corrosion: In H1150 and above aging conditions, resistance to stress corrosion cracking is superior to conventional martensitic stainless steels
- Hydrogen embrittlement susceptibility: Sensitive to hydrogen embrittlement when hardness exceeds 33 HRC; the H1150 condition is more resistant to hydrogen embrittlement than H900
5. Typical Applications
- Aerospace: Fasteners, landing gear components, turbine blades, structural brackets
- Energy industry: Valve stems, pump shafts, valve bodies, nuclear reactor components
- Precision machinery: High-precision shaft components, mold inserts, gears
- Medical devices: Surgical instruments, orthopedic implants
- Chemical equipment: Corrosion- and wear-resistant valves, sealing rings
- Marine engineering: Ship propeller shafts, fasteners (H1150 condition)
- CNC machined parts: High-strength precision connectors, specialty fasteners
6. CNC Machinability
The machining characteristics of 17-4PH differ significantly across various conditions:
- Solution annealed (Condition A): Hardness of approximately 28-32 HRC, good machinability, controllable chip formation
- H1150 aged condition: Hardness of 28-38 HRC, good machinability
- H900 aged condition: Machining in the H900 condition is not recommended due to excessive hardness (40-47 HRC), which causes severe tool wear
- Recommendation: Perform rough machining in the solution annealed (Condition A) state, then finish machining to final dimensions after aging
- Cutting speed: 40-80 m/min (carbide tools)
- Feed rate: 0.05-0.15 mm/rev
- Tool selection: Coated carbide (TiAlN / AlTiN), CBN for finishing
- Cooling: Adequate cooling is essential to prevent work hardening and thermal deformation
- Precautions: 17-4PH has a moderate work hardening rate; avoid prolonged idle friction of the tool against the workpiece
7. Heat Treatment Process
| Step | Temperature | Time | Cooling Method |
|---|---|---|---|
| Solution treatment | 1038±14°C | Calculated based on cross-section thickness (≥30min/25mm) | Air cool or oil cool to <32°C |
| Aging hardening | 480-620°C | 1-4 hours (depending on aging temperature) | Air cool |
Note: After solution treatment, the material must be cooled to below 32°C within 1 hour to ensure complete martensitic transformation. Higher aging temperatures result in lower hardness and strength, but better toughness and resistance to stress corrosion cracking.
8. Procurement Guide
Market price reference:
- Plate: 30-55 RMB/kg (depending on thickness and aging condition)
- Bar: 35-65 RMB/kg
- Tube: 40-80 RMB/kg
Quality verification:
- Verify the material test certificate (MTC) to confirm compliance with ASTM A564 or AMS 5643
- Spectroscopic analysis to confirm Cr, Ni, and Cu contents
- Check the heat treatment condition designation (Condition A / H900 / H1025, etc.)
- Surface must be free of cracks, laps, and inclusions
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