ASTM A269

Covers — austenitic stainless steel tubing in three manufacturing types:

Grades covered (austenitic stainless steel):

Size range — OD from 1/16″ to 2″ (1.59 mm to 50.8 mm); wall thickness from ~0.020″ upward. Most instrument tubing is 1/4″, 3/8″, or 1/2″ OD × 0.035″ or 0.049″ wall.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Boiler, superheater, and heat-exchanger pressure tubes → ASTM A213 (SA-213)
• Austenitic stainless welded boiler tubes → ASTM A249
• Stainless steel pipe (NPS/schedule) → ASTM A312
• Welded stainless mechanical tubing (structural/general) → ASTM A554
• Duplex/super-duplex tubing → ASTM A789
• Compression tube fittings → fitting manufacturer's tube qualification standard (e.g., Swagelok MS, Parker A-Lok)

A269 tubing is always specified by OD × wall (e.g., "¼″ OD × 0.035″ wall, TP316L, Type S, ASTM A269"). Never use NPS/schedule for ordering tubing to this standard.

A269 establishes the following minimum requirements:

Chemical Composition

Each grade has defined limits for C, Mn, P, S, Si, Cr, Ni, Mo, and (where applicable) Ti, Nb/Cb. Key distinctions:

• "L" grades (304L, 316L) — carbon ≤ 0.035% to minimise sensitisation in the weld heat-affected zone
• Stabilised grades (321, 347) — Ti or Nb additions tie up carbon to prevent sensitisation at high temperatures

Mechanical Properties

Hardness

• For compression fitting compatibility: ≤ 90 HRB (≤ 200 HV) in the annealed condition. This is critical — harder tube does not cold-work under ferrule load.

Dimensional Tolerances

OD tolerance controls compression-fitting make-up. Tube at the high end of OD tolerance requires more make-up turns; low-end OD may produce insufficient ferrule grip.

Manufacturing Condition

• Annealed (solution annealed) — heated to ≥ 1040°C and rapidly quenched to dissolve carbides and relieve cold-work. Required for all grades; hardness is the verification test.

Required Tests per Lot

A269 Tubing vs. Pipe — The Fundamental Difference

ASTM A269 governs tubing — a product sized by actual OD and wall, machined to tight OD tolerances so a compression fitting's ferrule can reliably bite and seal. This is fundamentally different from pipe (governed by A312), which is sized by nominal bore (NPS) and schedule and relies on threaded or welded connections.

The distinction matters in procurement and design:

• Specifying "1/4-inch stainless pipe" and "1/4-inch stainless tube" are different orders with different dimensions, tolerances, and fittings.
• NPS ¼ pipe has an OD of 13.72 mm (0.540″); ¼″ A269 tube has an OD of exactly 6.35 mm (0.250″).

How Compression Fittings Depend on the Tube

A twin-ferrule compression fitting (Swagelok, Parker, Gyrolok) works by driving two ferrules against the tube OD as the nut is tightened:

1. The front ferrule swages into the tube OD and creates a primary metal-to-metal seal.
2. The back ferrule collets the tube and prevents pull-out under vibration.

Both actions require the tube to plastically deform locally under the ferrule. If the tube is too hard (cold-drawn, work-hardened), no deformation occurs and the connection leaks. If the OD is out of tolerance, ferrule contact geometry changes and sealing load is non-uniform.

Common Instrument Tubing Sizes

Burst pressures are illustrative (using Barlow's formula with 515 MPa UTS); actual allowable working pressure depends on temperature, applicable code, and design factor.

Grade Selection Guide

A269 vs. Related Stainless Tube Specifications