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Product Overview: Custom 631 Semi-Austenitic Stainless Steel
Custom Steel 631 (17Cr-7Ni, UNS S17700) is a semi-austenitic precipitation-hardening stainless steel that uniquely integrates ultra-high strength and superior formability, with corrosion resistance comparable to conventional 304 stainless steel. In the fully annealed state, the steel retains a stable austenitic microstructure and delivers excellent cold drawing performance, making it an optimal material for manufacturing high-strength steel wires.
Its superior mechanical strength is achieved through precise cold drawing followed by professional aging treatment. Critically, the cold drawing process lays a solid foundation for structural stability and reliable performance in the subsequent aging hardening procedure. Key product advantages include:
- Superior Formability in Soft State: In the solution-annealed condition, it offers outstanding plasticity and moderate hardness, making it ideal for precision cold forming (bending, stamping, cold drawing) of complex components while minimizing scrap rates.
- Exceptional Strength & Fatigue Resistance: Through low-temperature aging (e.g., CH900/RH950), it achieves a maximum tensile strength of up to 1720 MPa and hardness exceeding 48 HRC, delivering outstanding deformation resistance and fatigue performance.
- Reliable Corrosion Resistance: Comparable to 304 stainless steel, it effectively prevents oxidation and corrosion in standard operating environments, including dry atmospheres, freshwater, and weak acid solutions.
- High-Temperature Dimensional Stability: Maintains long-term performance stability and prevents deformation at elevated temperatures up to 315°C, ensuring durability and reliability for precision equipment and elastic components.
Application
- Aerospace: Aircraft engine components, landing gear, thin-walled structures, fuel systems, and high-strength fasteners.
- Medical Devices: Surgical instruments, dental tools, precision components, and orthopedic implants requiring high strength and fatigue resistance.
- Automotive & Transportation: Critical load-bearing parts including engine valve springs, exhaust systems, chassis, and suspension components.
- Chemical & Marine Engineering: Equipment for moderately corrosive environments, such as chemical containers, pipelines, high-pressure valves, pump shafts, and offshore platforms.Chemical composition
Chemical composition
| Element | Symbol | Content Range / Max Value |
| Nickel | Nil | 6.50% – 7.75% |
| Chromium | Cr | 16.00% – 18.00% |
| Aluminum | Al | 0.75% – 1.50% |
| Carbon | C | ≤ 0.09% |
| Silicon | Si | ≤ 1.00% |
| Manganese | Mn | ≤ 1.00% |
| Phosphorus | P | ≤ 0.04% |
| Sulfur | S | ≤ 0.03% |
| Iron | Fe | Balance |
Mechanical Properties
| Heat Treatment Condition | Tensile Strength | Yield Strength | Elongation | Hardness | Characteristics |
| Solution Treated (Condition A) | ≤ 1030 MPa | ≤ 380 MPa | ≥ 20% | ≤ 229 HB (≤ 28 HRC) | Excellent ductility; ideal for cold forming, stamping, and bending. |
| CH900 (Aged at 480℃) | ≥ 1720 MPa | ≥ 1650 MPa | ≥ 6% | ≥ 48 HRC | Achieves ultra-high strength. |
| RH950 (Aged at 510℃) | ≥ 1550 MPa | ≥ 1450 MPa | ≥ 8% | ≥ 45 HRC | Optimal balance of strength and toughness. |
| TH1050 (Aged at 565℃) | ≥ 1380 MPa | ≥ 1170 MPa | ≥ 10% | ≥ 42 HRC | Better toughness and stress-corrosion resistance. |
Physical Properties
| Property | Value | Unit | Notes |
| Density | 7.8 – 7.93 | g/cm³ | Varies slightly by source/standard |
| Melting Range | 1390 – 1430 | °C | |
| Specific Heat | 0.50 | J/(g·K) | At 0–100°C |
| Thermal Conductivity | 16.3 – 17.9 | W/(m·K) | At 20–100°C |
| Electrical Resistivity | 0.80 – 1.05 | μΩ·m | At 20°C |
| Modulus of Elasticity | 200 | GPa | At 20°C |
| Coefficient of Thermal Expansion | 15.3 – 17.1 | μm/(m·K) | 0–100°C / 0–630°C |
| Magnetic Permeability | Weak / Non-magnetic | – | Condition A (Solution Treated) |
| Magnetic Permeability | Magnetic | – | Aged Condition (After Heat Treatment) |
Heat Treatment
Custom 631 is a semi-austenitic precipitation-hardening stainless steel that undergoes a transformation from “soft and easily formable” to “ultra-high strength” through four steps of heat treatment:
1.Solution Treatment (Condition A)
Heat to 1040～1080℃ followed by rapid cooling. The material is delivered in soft condition with hardness ≤28 HRC and excellent ductility, suitable for cold forming such as stamping and bending.
2.Conditioning Treatment (Condition T)
Either heat and hold at 760℃ then rapid cool, or perform heavy cold working directly. This step reduces austenite stability in preparation for subsequent phase transformation.
3.Cryogenic Treatment
Soak at -73℃ for 8 hours to fully transform retained austenite into 2low-carbon martensite.
4.Age Hardening (Key Strengthening Process)
Strengthening precipitates form within the matrix. Three aging options available per application requirement:
- CH900 (480℃ / 1h): Ultra-high strength, tensile ≥1720 MPa, hardness ≥48 HRC.
- RH950 (510℃ / 1h): Balanced strength & toughness (most commonly used), tensile ≥1550 MPa, hardness ≥45 HRC.
- TH1050 (565℃ / 1.5h): High toughness & good corrosion resistance, tensile ≥1380 MPa, hardness ≥42 HRC.
Custom 631 (17-7PH) features favorable corrosion resistance, which is significantly affected by service environments and heat treatment conditions:Favorable service environments: It delivers outstanding performance in dry atmospheric conditions, fresh water as well as weak acid and weak alkaline environments, outperforming 304 stainless steel in such scenarios.High-risk service environments: Pitting corrosion tends to occur readily in chlorine-rich environments such as seawater and salt spray; the material is susceptible to stress corrosion cracking (SCC) under high-strength aged conditions.Impact of material condition: The solution-annealed condition (Condition A) boasts the optimal corrosion resistance; corrosion resistance declines as tensile strength rises after aging treatment.Material comparison: Its corrosion resistance surpasses that of conventional martensitic stainless steels (410/420), yet falls short of molybdenum-containing stainless steels (316L, 15-7PH, 17-4PH).
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C&N Industrial Group Limited is a leading pipe, flange and fitting manufacturer. These products can be with epoxy and 3HDPE coating and lining. All raw materials are provided by highly reputable suppliers, and our products all go through strict quality inspection under ISO 17025 regulation.
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