ASTM A312 A778 A358 Stainless Steel Industrial Pipe Specifications

ASTM A312, A778 and A358 are three complementary specifications for austenitic stainless steel pipe. Choosing between them is a matter of service pressure, diameter and manufacturing route. A312 is the workhorse specification for seamless and welded pipe in general and pressure service; A358 covers large-diameter electric-fusion-welded (EFW) pipe for demanding pressure and corrosion duty; A778 covers welded pipe intended mainly for non-pressure or low-stress service.

The Three Standards at a Glance

Item	ASTM A312	ASTM A778	ASTM A358
Type	Seamless and welded austenitic pipe	Welded austenitic pipe for non-pressure service	Electric-fusion-welded (EFW) austenitic pipe
Manufacturing route	Seamless, ERW or EFW	Welded, typically without filler metal	Double-welded EFW with filler metal
Common grades	TP304, TP304L, TP316, TP316L, TP321, TP347	304, 304L, 316L, 321	304, 304L, 316, 316L, 321, plus H grades
Typical supply OD	about 6-530 mm	about 12-609 mm	about 16-1219 mm (large diameters)

The size ranges above are typical mill supply data rather than standard limits; the scope clause of each specification governs.

ASTM A312 in Detail

A312 covers seamless and straight-seam welded austenitic pipe for high-temperature and general corrosive service. It is a full pressure-piping specification: material must meet chemical and tensile requirements, and pipe is subjected to flattening, hydrostatic or non-destructive electric testing and dimensional checks. Representative requirements for TP304 and TP316: tensile strength ≥515 MPa, yield strength ≥205 MPa, elongation ≥35%. A312 covers small and medium diameters, typically up to about DN 750.

ASTM A358 in Detail

A358 covers electric-fusion-welded austenitic pipe made with filler metal, in classes distinguished by welding procedure, heat treatment and inspection level. It is the standard route for large-diameter heavy-wall pipe in power generation, petrochemical and corrosive service where a seamless product is not commercially available. Mechanical requirements are comparable to A312, with tensile strength ≥515 MPa, yield strength ≥205 MPa and elongation ≥35% for the 304/316 classes, and hydrostatic testing is required.

ASTM A778 in Detail

A778 covers welded austenitic stainless steel pipe intended for non-pressure and low-stress corrosive service. Because it is not a pressure-piping specification, it is commonly applied to decorative, architectural and light structural systems where the corrosion resistance of stainless steel is wanted but internal pressure is not a design factor. Grades include 304, 304L, 316L and 321.

How to Select

For pressurized systems in corrosive or high-temperature service, use A312 for small and medium diameters, and A358 for large diameters or high-integrity EFW requirements. For decorative, architectural or low-stress applications where cost matters, A778 is appropriate. When specifying, state the standard, grade, size and schedule, and confirm whether seamless or welded construction is required.

FAQ

1. What is the difference between A312 and A358?

A312 covers seamless and welded pipe for general and pressure service in small-to-medium diameters. A358 covers electric-fusion-welded pipe with filler metal, mainly for large diameters and heavy-duty pressure and corrosion service.

2. Can A778 pipe be used for pressure?

No. A778 pipe is intended for non-pressure or low-stress service such as decorative and light structural use.

3. Which grades are most commonly ordered?

TP304, TP304L, TP316 and TP316L dominate A312; TP316L is common in A358 for corrosive service; 304 and 316L are typical for A778.

4. Is seamless pipe always better than welded?

Not automatically. Both A312 welded and seamless pipe must meet the same chemical, mechanical and testing requirements. Seamless is often preferred for severe service, while welded pipe is economical and fully acceptable for most applications.

5. Which standard should I use for high-temperature service?

Use A312 or A358 with the appropriate grade, including H grades such as TP316H where elevated-temperature strength is required; A778 is not intended for pressure or high-temperature duty.