631 Stainless Steel Tube – 17-7PH Semi-Austenitic Precision Pipe for Ultra-High-Strength Medical & Aerospace Lines with 304-Class Corrosion

631 Stainless Steel Tube—globally known as 17-7PH or UNS S17700—is the only semi-austenitic precipitation-hardening (PH) tubular grade that combines 304-level cold formability in Condition A with 1380 MPa tensile after a simple refrigeration + age cycle. Containing 16–18 % Cr, 6.5–7.75 % Ni and 0.75–1.5 % Al, 631 Stainless Steel Tube is supplied as ultra-soft cold-rolled strip (Condition A), deep-drawn or bent into complex shapes, then refrigerated to –75 °C and aged at 480–560 °C to reach 46 HRC—double the strength of annealed 630 while retaining 304-equivalent corrosion resistance. The grade stays non-magnetic before hardening, experiences near-zero distortion during aging, and can be welded without pre-heat. From 0.2 mm medical catheter walls to 25 mm aerospace hydraulic lines, 631 Stainless Steel Tube gives designers the only PH stainless that truly combines ultra-high strength, excellent corrosion resistance, limitless formability and vacuum-grade cleanliness in one cost-effective tubular form.

Key Features & Benefits – Why 631 Stainless Steel Tube (17-7PH) Outperforms 630 and Austenitic Tubes on Formability & Strip Strength

Semi-Austenitic Structure = Unlimited Cold Formability Before Aging

• Condition A (annealed) offers 35 % elongation and 90 HRB—allows 70 % deep-draw ratio in bellows and 3 mm bend radius in 0.5 mm wall—impossible with as-supplied 630 tube.

Transformation to Martensite + Precipitation Hardening with Aluminum

• Refrigeration at –75 °C or 15 % cold work converts austenite to martensite; aging at 480 °C (CH900) precipitates nano-aluminum particles, reaching 46 HRC without quench distortion.

Various Aging Conditions for Strength-Toughness Balance

• CH900 (480 °C) = 1380 MPa tensile / 1310 MPa yield / 46 HRC—ideal for high-rate springs and thin-wall pressure tubing.
• RH950 (510 °C) = 1240 MPa tensile / 1170 MPa yield / 42 HRC—balances strength with 16 % elongation for fatigue-critical tubing.

Excellent Corrosion Resistance in Atmospheric & Mild Chemical Environments

• PREN ≈ 18 meets ASTM G48 pitting test at 25 °C; outperforms 410/420 and equals 304 in neutral chlorides; passes 1000 h ASTM B117 with <2 % red rust.

High Fatigue and Stress Corrosion Cracking Resistance

• Rotating-beam fatigue limit 620 MPa (CH900)—double that of annealed 316; perfect for aircraft landing-gear hydraulic circuits and high-cycle medical drive cables.

Good Weldability in Annealed Condition

• Full-penetration TIG welds with 17-7PH or 304L filler pass ASTM A262 Practice E without pre-heat on walls < 0.5 mm—critical for capillary medical tubing.

Low Magnetic Permeability Options

• µr < 1.02 in Condition A—suitable for sensor housings and MRI-compatible guide wires before final hardening.

Cost-Effective & Price-Stable Strip-to-Tube Solution

• Nickel content 30 % lower than 304; precipitation hardening eliminates costly quench/temper furnaces, cutting total tube cost 10 % versus 630 for complex shapes.

Technical Specifications

Typical Tube Dimensions

• Seamless: OD 0.3–25 mm, wall 0.05–3 mm, length up to 6 m
• Welded: OD 6–219 mm, wall 0.1–3 mm, length up to 12 m

Example Pressure Rating (CH900, 3 mm OD × 0.3 mm wall)

• Allowable stress 900 MPa → max internal pressure 600 bar at 20 °C

Standards: ASTM A269/A632 (seamless/welded stainless tubes), AMS 5528/5529 (strip/tube), ISO 13485 (medical), AS9100 (aerospace).

Applications and Use Cases – Proven 17-7PH Tube Performance from Aerospace Hydraulic Lines to Medical Cannula

Aerospace Hydraulic and Pneumatic Tubing

Cold-drawn OD 6 mm × 1 mm 631 Stainless Steel Tube aged to CH900 withstands 1 000 bar cyclic pressure; 46 HRC resists erosion while non-magnetic Condition A allows eddy-current inspection.

Oil & Gas Instrumentation Tubes

H1150 631 tube (OD 12 mm, wall 1 mm) resists seawater pitting while carrying 1 070 MPa yield—40 % higher than 316L, permitting smaller OD and lower weight on subsea trees.

Medical and Surgical Device Tubing (Endoscopes, Catheters)

Hypodermic stylets use 0.3 mm OD × 0.05 mm wall 631 tube aged to CH900; 46 HRC provides needle sharpness while 304-level passivation meets FDA biocompatibility.

Marine Control Lines and Propeller Shaft Liners

H1025 631 tube (OD 25 mm, wall 2 mm) resists seawater pitting while carrying 1 170 MPa yield—non-magnetic before hardening avoids compass interference.

High-Pressure Gas Cylinders

Cold-drawn OD 50 mm × 2 mm 631 tube aged to RH950 withstands 700 MPa hoop stress—double that of annealed 304, eliminating thick-wall carbon steel.

Precision Bellows and Diaphragms Tubing

Deep-drawn 0.2 mm wall 631 bellows convert 40 MPa pressure pulses; RH950 condition gives 16 % elongation for 10 million fatigue cycles.

Defense Weapon Components

Cold-bent OD 10 mm × 1 mm 631 tube forms recoil-spring housings; CH900 provides 1 380 MPa tensile while 304-level corrosion resists battlefield humidity.

Spring-Like Flexible Tubing

Coiled OD 3 mm × 0.2 mm 631 tube acts as a torsion spring; Condition A allows 5 mm bend radius, final CH900 delivers 1 310 MPa spring force.

Comparison with Other Grades – 631 vs 630, 304, 316, 640, 321 Tube

Pros: 631 Stainless Steel Tube offers the best combination of ultra-high strip strength and excellent cold formability among standard PH tube grades.
Cons: Higher alloy cost and more complex heat treatment (refrigeration + age); not suitable for non-magnetic after hardening or strong reducing-acid piping.

Manufacturing Process and Quality Assurance

631 Stainless Steel Tube is produced from VIM+VAR-cleaned strip (Condition A), slit to width, then roll-formed and TIG-welded with matching filler, or cold-pilgered to seamless dimensions. Solution-annealing at 1060 °C produces soft austenitic Condition A. For maximum strength, tube is refrigerated at –75 °C or cold-worked 15 % to transform to martensite, then aged at 480 °C (CH900) or 510 °C (RH950). Finishes include pickled matte, bright annealed and electropolished Ra ≤ 0.1 µm for medical cannula. All tubes are 100 % eddy-current inspected, dimensionally checked to ASTM A269/A632, and certified to AMS 5528/5529, ISO 13485 (medical) and AS9100 (aerospace). Tests include ICP chemistry, tensile per condition, bend fatigue, ASTM G48 pitting corrosion, and cleanliness per ASTM F138.

Maintenance and Care Tips for 631 Stainless Steel Tube

Clean tube interiors with warm water and neutral detergent; avoid chloride bleach. For medical service, periodic passivation in 2 % citric acid maintains passive layer. Do not expose aged (CH900) sections to >400 °C for prolonged periods to avoid over-aging. Store strip or tube in controlled humidity (RH < 60 %) to prevent discoloration. Inspect cyclic-bend samples for fatigue after 10 000 cycles in aerospace service.

Frequently Asked Questions on 631 Stainless Steel Tube

Q: What are the main aging conditions for 631 Stainless Steel Tube?
A: Condition A → refrigeration or 15 % CW → CH900 (480 °C) for max strength 1380 MPa, or RH950 (510 °C) for balanced strength/toughness.

Q: How does formability compare to 630 in tube manufacturing?
A: 631 offers 70 % deep-draw ratio vs. 30 % for 630, enabling thinner walls and tighter bend radii before aging.

Q: Is 631 Stainless Steel Tube suitable for medical applications?
A: Yes—electropolished CH900 meets ASTM F138 and ISO 5832-5 for surgical and endoscopic devices.

Q: What post-weld heat treatment is required?
A: None—weld in Condition A, then refrigerate + age the entire assembly; no post-weld anneal needed.

Ready to form complex ultra-thin tubing and still reach 1380 MPa without quench distortion? Contact us today for custom 631 Stainless Steel Tube quotes, mill test certificates and global delivery—full metallurgical support included.