17-7 PH is a precipitation-hardening stainless steel as famous and important as 17-4 PH, but it follows a different technical path with more unique performance characteristics.
- 17-4 PH is a martensitic precipitation-hardening steel, known for its high strength Dan excellent machinability.
- 17-7 PH is a semi-austenitic precipitation-hardening steel, renowned for its exceptional fatigue strength, high elasticity, outstanding relaxation resistance (resistance to permanent set), Dan better formability.
Property Comparison: 17-7 PH vs. 17-4 PH
| Milik | 17-7 PH | 17-4 PH |
| Strengthening Type | Semi-Austenitic Precipitation Hardening | Martensitic Precipitation Hardening |
| Core Advantage | Exceptional elasticity, relaxation resistance, fatigue strength | Higher ultimate tensile strength, excellent machinability |
| Perlakuan Panas | More complex, typically three steps (Solution → Conditioning → Aging) | Simpler, typically two steps (Solution → Aging) |
| Kemampuan dibentuk | Excellent in Solution-Treated condition (Condition A), suitable for complex cold forming | Good, but generally not as good as 17-7 PH |
| Typical Tensile Strength | ~1240 – 1450 MPa (Condition RH950) | ~1000 – 1380 MPa (Condition H1150-H900) |
| Primary Applications | Springs, diaphragms, bellows, aircraft skin, fasteners | Structural parts, shafts, gears, valve stems, high-strength bolts |
| Common Conditions | Condition A (Sol. Ann.), Condition C (Cold Rolled), RH950, TH1050 | Condition A, H900, H1025, H1150 |
17-7 PH Stainless Steel Equivalent Designations
17-7 PH is recognized worldwide under various national and international standards. Below is a comprehensive reference table:
| Negara/Wilayah | Standar | Grade/Designation | Equivalent Status |
| Amerika Serikat | ASTM A564/A564M | Type 631 | Primary U.S. standard |
| PBB | S17700 | Sistem Penomoran Terpadu | |
| Bahasa Indonesia: AMS | AMS 5528, 5544, 5673 | Aerospace Material Specifications | |
| International (ISO) | ISO 683-13 | X7CrNiAl17-7 | International standard designation |
| European Union | EN 10088-3 | 1.4568 | Main European grade number |
| X7CrNiAl17-7 | European material designation | ||
| Jerman | DIN/Werkstoff Nr. | 1.4568 | German material number |
| DIN EN 10088-3 | X7CrNiAl17-7 | German/European designation | |
| Jepang | JIS G4303 | SUS 631 | Standar Industri Jepang |
| Cina | GB/T 20878 | 07Cr17Ni7Al | Current Chinese standard |
| (Old Designation) | 0Cr17Ni7Al | Previously used designation | |
| France | AFNOR NF | Z8CNA17-7 | French standard |
| Inggris Raya | Sarjana | 301S81, 631S17 | British standards (obsolete but referenced) |
| Russia/GOST | GOST 5632 | 09Х17Н7Ю / 09Ch17N7Yu | Russian standard |
17-7 PH Stainless Steel Chemical Composition
17-7 PH (UNS S17700) is a semi-austenitic precipitation-hardening stainless steel. Its unique composition enables the complex heat treatment that gives it exceptional elastic properties.
| Elemen | Minimum | Maksimum | Key Role |
| Kromium (Cr) | 16.00% | 18.00% | Provides corrosion resistance; forms passive oxide layer |
| Nikel (Ni) | 6.50% | 7.75% | Critical for austenite stability; enables phase transformation during heat treatment |
| Aluminium (Al) | 0.75% | 1.50% | Primary precipitation-hardening element; forms NiAl intermetallic strengthening phases |
| Karbon (C) | — | 0.09% | Kept low to maintain good weldability and corrosion resistance |
| Mangan (Mn) | — | 1.00% | Austenite stabilizer; improves hot workability |
| Silikon (Si) | — | 1.00% | Deoxidizer; improves oxidation resistance |
| Fosfor (P) | — | 0.04% | Impurity (kept as low as possible) |
| Belerang (S) | — | 0.03% | Impurity (kept as low as possible) |
| Besi (Fe) | Keseimbangan | — | Base metal |
17-7 PH Stainless Steel Heat Treatment Conditions and Uses
Properties are entirely determined by the heat treatment condition. The condition code is part of the material designation.
| Condition Code | Heat Treatment & Properties | Primary Uses |
| Condition A | Solution Annealed. Very soft, fully austenitic microstructure, best ductility and formability. | Used for complex cold forming, deep drawing, bending. Must be followed by subsequent heat treatment after forming. |
| Condition C | Solution annealed followed by cold rolling. Provides medium to high strength and improves spring properties. | Used for flat springs or strip materials. |
| TH1050 | Solution → Condition at 955°C → Sub-zero cool at -73°C → Age at 565°C. High strength, good toughness, good corrosion resistance. | General-purpose high-strength condition for most parts requiring high strength and some ductility. |
| RH950 | Solution → Condition at 955°C → Condition at room temperature → Age at 510°C. Achieves the highest strength and hardness, but with lower toughness. | Used for springs and elastic elements requiring maximum strength. |
Primary Application Areas
17-7 PH is used in the most demanding “elastic” applications due to its超凡 (extraordinary) elastic properties:
Antariksa:
- Aircraft springs, bellows, diaphragms.
- Engine seal rings, lock washers.
Precision Instruments:
- Pressure sensor diaphragms.
- Precision spring plates, collets.
Chemical & General Engineering:
- Corrosion-resistant springs, valve discs.
- High-strength fasteners requiring high relaxation resistance.
Common FAQs of 17-7 PH Stainless Steel
In terms of ultimate tensile strength and hardness, their top conditions (17-7 PH RH950 vs. 17-4 PH H900) are comparable, both exceeding 1300 MPa. However, 17-7 PH typically has superior "elastic strength" (e.g., yield ratio, fatigue limit, relaxation resistance).
The key properties for spring materials are high elastic limit, low relaxation rate, and high fatigue life. The microstructure of 17-7 PH, achieved through its complex heat treatment, has inherent advantages in these elastic-related properties. While 17-4 PH is strong, its resistance to permanent set is generally inferior to 17-7 PH for spring applications.
Similar to 17-4 PH, its corrosion resistance is better than common martensitic steels and approaches that of 304 stainless steel, suitable for most mild to moderate corrosive environments. It exhibits the best corrosion resistance in the TH1050 condition.
Ringkasan
17-7 PH is the “King of Elastic Components”. If you need to manufacture a spring that maintains precise force over millions of cycles, or a diaphragm that must not permanently deform under pressure, 17-7 PH is often the material of choice. Its value lies not in simply being “stronger,” but in being “more elastic, more fatigue-resistant, and more relaxation-resistant.” When selecting it, you must precisely specify the condition based on forming needs (use Condition A) and final performance requirements (choose TH1050 or RH950, etc.).
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