ASTM A312: Seamless and Welded Stainless Steel Pipe Standard

Quick Answer

ASTM A312/A312M is the governing specification for austenitic stainless steel pipe in the United States and in international projects referencing ASTM standards. "TP" in the grade designation stands for "tubular product," distinguishing pipe grades from flat product grades covered by A240. The standard covers both seamless pipe (no longitudinal weld) and welded pipe (with or without filler metal addition).

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Scope

A312 applies to:

• Seamless pipe: hot-finished or cold-finished, full wall thickness
• Welded pipe: longitudinally welded (straight seam), with or without filler metal
• Cold-worked pipe: cold-worked to improve mechanical properties (specifically noted for duplex grades)

The standard covers nominal pipe sizes (NPS) from 1/8 inch to 12 inches (DN 6 to DN 300) in standard wall thicknesses defined by ASME B36.19M. Larger sizes may be available as "special" items, typically requiring supplementary agreement.

Not covered by A312:

• Welded fittings (A403)
• Flanges and forged fittings (A182)
• Pipe with heavy wall (> nominal Schedule 80S for large diameters) — may fall under A358 or A376

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Grade Coverage

A312 grades are prefixed with "TP" and cover primarily austenitic grades. Common grades:

Duplex grades (e.g., S31803, S32205) were added in later revisions of A312.

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Chemical Composition Requirements

Composition limits for the most frequently specified grades:

Ti ≥ 5×C for TP321. Nb ≥ 10×C for TP347. Refer to current A312 edition for full limits including P, S, Si, Mn.

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Mechanical Property Requirements

Elongation is measured on 2-inch gauge length for wall thickness ≥ 5 mm; proportional gauge lengths apply for thin-wall pipe.

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Mandatory Testing Requirements

A312 requires the following tests on each lot of material:

Chemical Analysis

• One analysis per heat (ladle analysis)
• Product analysis may be required by the buyer (one piece per lot)

Tensile Test

• One test per lot (lot = same grade, heat, nominal size, and heat treatment condition)
• Test coupons machined from pipe body in the longitudinal direction

Hydrostatic Test (or NDE)

• Every length of pipe must be hydrostatically tested or subjected to a nondestructive examination (eddy current or ultrasonic), at the manufacturer's option
• For welded pipe: the weld seam must be verified — either by NDE or by hydrostatic test pressure sufficient to stress the weld

Flattening Test (Welded Pipe)

• Three flattening tests per lot for welded pipe to verify weld integrity
• The pipe is compressed until the distance between platens equals a specified fraction of the outside diameter (typically 67% of OD)

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Seamless vs. Welded: When Does It Matter?

In most corrosive process service, seamless (S) and welded (W) pipe have equivalent performance, and A312 allows interchangeability unless the purchase order specifically requires one form. However, there are important distinctions:

For pulsating pressure service (reciprocating compressors, pumps), seamless is often specified. For static process service, welded is generally acceptable.

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Supplementary Requirements

A312 includes optional supplementary requirements that must be invoked on the purchase order:

For piping serving under ASME B31.3 Normal Fluid Service, the above are optional. For Category M Fluid Service (highly toxic or high-pressure steam), additional requirements apply and S2 (radiography) is often mandatory.

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Reading the A312 MTC

Key data points to verify on an A312 mill test certificate:

• Grade designation (e.g., TP316L) and form (seamless or welded)
• Heat number and lot number
• Nominal pipe size (NPS) and wall thickness (Schedule)
• Chemical analysis: all elements within A312 limits
• Tensile strength and yield strength ≥ minimums
• Elongation ≥ minimum
• Test method: hydrostatic test pressure or NDE method stated
• For welded pipe: flattening test results stated
• Annealing temperature and condition noted
• Any supplementary requirements addressed

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Frequently Asked Questions