631 / 17-7PH

631 (UNS S17700) is a precipitation-hardening martensitic stainless steel, mainly composed of chromium, nickel, and aluminum. It is renowned for ultra-high strength (achieved via precipitation hardening heat treatment), excellent low-temperature toughness, and good corrosion resistance. It maintains stable mechanical properties in a wide temperature range (from -200 °C to 315 °C) and offers diverse performance options via different heat treatment states (e.g., RH950, TH1050). Its balance of high tensile strength (1100 – 1400 MPa after aging) and good formability makes it ideal for applications requiring high strength, corrosion resistance, and fatigue resistance.​.

Behaviour that matters in service

• Ultra-High Mechanical Strength — After precipitation hardening (aging treatment), it exhibits ultra-high tensile strength (1100 – 1400 MPa) and yield strength (950 – 1200 MPa), exceeding most austenitic stainless steels and even some 630 stainless steel grades.​.
• Excellent Low-Temperature Toughness — Maintains good toughness at temperatures as low as -200 °C, avoiding brittle fracture in low-temperature environments (a key advantage over conventional martensitic stainless steels).​.
• Good Corrosion Resistance — Resists atmospheric corrosion, fresh water, and weak corrosive media (e.g., dilute acids, alkalis); performance is comparable to 304 stainless steel in mild environments, suitable for non-severe corrosion scenarios.​.
• Diversified Heat Treatment States — Different aging temperatures produce distinct performance states (e.g., RH950 for high strength, TH1050 for better ductility), adapting to diverse application requirements.​.
• Controllable Formability — In the solution-annealed (soft) state, it has good cold working performance (e.g., bending, stamping, coiling); after aging, hardness increases significantly, limiting secondary processing—ideal for pre-forming then strengthening.​.

Typical applications

• Aerospace & Aviation — Widely used in aircraft structural components (e.g., fasteners, brackets), engine springs, and missile guidance system parts; high strength and low-temperature toughness meet extreme aerospace environment requirements.​.
• Spring & Elastic Components — Applied to high-performance springs (e.g., aircraft landing gear springs, chemical equipment pressure springs) and elastic fasteners; excellent fatigue resistance and strength ensure long service life.​.
• Chemical & Petrochemical Industry — Used in high-pressure valves, pump shafts, and pipeline connectors; resists corrosion from oilfield/media and maintains strength under high pressure and cyclic loads.​.
• Medical Equipment — Employed in high-strength medical springs (e.g., surgical instrument springs, implantable elastic components) and low-temperature storage equipment parts; biocompatibility and corrosion resistance meet medical standards.​.
• Precision Machinery — Used in precision mold cores, high-strength gears, and instrument structural parts; high dimensional stability and strength ensure precision operation of mechanical systems.