X7CrNiAl17-7 (EN 10088-3 standard, AISI 631) is a typical precipitation-hardening stainless steel featuring a dual-phase austenitic-martensitic microstructure. By adjusting heat treatment parameters, it attains a combination of high strength, excellent corrosion resistance, and good machinability, widely used in aerospace, chemical processing, and medical device industries where performance demands are stringent.

2. Chemical Composition

Main elements (wt%):

• Carbon (C): ≤0.09% (with “X7” indicating approximately 0.07%)

• Chromium (Cr): 16–18% (provides corrosion resistance)

• Nickel (Ni): 6.5–7.5% (stabilizes austenitic structure)

• Aluminum (Al): 0.75–1.5% (key precipitation strengthening element)

• Others: trace amounts of Manganese (Mn), Silicon (Si), Phosphorus (P), Sulfur (S), etc.

3. Material Characteristics

• High Strength: Aging treatment precipitates intermetallic compounds such as Ni₃Al, significantly enhancing strength.

• Corrosion Resistance: Superior to common martensitic stainless steels, comparable to austenitic 304 stainless steel, though caution is advised in chloride environments.

• Heat Treatment Flexibility: Solution treatment, conditioning, and aging can be tailored to optimize strength and toughness.

• Magnetism: Exhibits weak magnetism after aging due to increased martensitic phase.

4. Mechanical Properties (Typical Values)

Performance varies with heat treatment:

（1）Solution annealed (Condition A):

• Tensile Strength: ~850 MPa

• Yield Strength: ~380 MPa

• Elongation: ~25%

（2）After aging (e.g., TH1050 condition):

• Tensile Strength: ≥1450 MPa

• Yield Strength: ≥1310 MPa

• Elongation: ≥6%

5. Applications

• Aerospace: Aircraft fasteners, springs, structural components.

• Chemical Equipment: Valves, pump shafts (moderate corrosion resistance environments).

• Medical Devices: Surgical instruments, orthopedic implants (requiring biocompatibility).

• Precision Instruments: High-stress springs, sensor components.

6. Heat Treatment Process

（1）Solution Treatment: Heat to 1000–1100°C, hold, then rapidly cool (water or air quench) to form austenite.

（2）Conditioning (Optional):

• Cryogenic treatment at –73°C to promote austenite-to-martensite transformation.

• Heat conditioning at 750°C for short duration to adjust microstructure.

（3）Aging Treatment: Hold at 480–620°C for 1–4 hours to precipitate strengthening phases (e.g., Ni₃Al), significantly increasing strength.

7. Physical Properties

• Density: 7.8 g/cm³

• Melting Point: ~1400–1450°C

• Thermal Expansion Coefficient: 11.5×10⁻⁶/°C (20–100°C)

• Thermal Conductivity: 16 W/(m·K) (aged state)

• Electrical Resistivity: 0.85 μΩ·m

8. Corrosion Resistance

• Resists atmospheric corrosion, freshwater, and mild acids such as acetic acid.

• Limitations: Susceptible to pitting and stress corrosion cracking in chloride-containing environments (e.g., seawater) and strong acids (e.g., hydrochloric acid).

• Surface treatments like passivation or plating can enhance corrosion resistance.

9. Processing Considerations

• Cold Working: Recommended to be done in solution-treated condition before aging.

• Welding: Requires preheating (150–200°C) and matching filler materials (e.g., AWS ER630), with post-weld aging treatment.

• Machining: Hard carbide tools needed due to high hardness after aging; use low speed and high feed to reduce tool wear.

10. Comparison with Other Materials

• 17-7 PH:

Extremely high strength (1450 MPa)；

Moderate corrosion resistance;

Complex processing with heat treatment;

High cost

• 17-4 PH:

High strength (1310 MPa);

Moderate corrosion resistance;

Somewhat complex processing;

Moderate-high cost

• 304 Stainless Steel:

Low strength (520 MPa);

Excellent corrosion resistance;

Easy processing;

Low cost

11. Conclusion

X7CrNiAl17-7 achieves a balanced combination of strength and corrosion resistance through flexible heat treatment, making it particularly suitable for applications requiring stringent mechanical performance with moderate corrosion protection. While its complex heat treatment and cost limit broader industrial use, it remains indispensable in aerospace and high-end manufacturing sectors.

12.About Suzhou Bolaibao Metal

Suzhou Bolaibao Metal specializes in high-performance alloy material solutions, including precipitation-hardening stainless steels like X7CrNiAl17-7. We provide professional material supply, technical support, and customized services for aerospace, automotive, medical device, and precision engineering clients. Committed to quality and innovation, Bolaibao Metal is your trusted partner for premium alloy solutions.

Choose Suzhou Bolaibao Metal for top-tier precipitation-hardening stainless steel solutions tailored to meet the most demanding engineering challenges.
Contact us for more technical information and cooperation opportunities:

• Phone: +86 15366282159

• Email: sale@bolaibao1.com

• Website: http://blbmetal.com