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Stainless Steel Pipe (A312, 304/316, L Grades)

ASTM A312 is the main specification for seamless and welded austenitic stainless pipe. This sheet explains 304 and 316, the L grades, the 5S to 80S schedules and the difference between welded and seamless.

Ref PV-0006ASTM A312ASME B36.19M

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

ASTM A312 covers seamless, straight-seam welded and heavily cold worked welded austenitic stainless steel pipe for high-temperature and general corrosive service. Grades are written with a TP prefix, such as TP304L or TP316L. Dimensions and the S schedules (5S, 10S, 40S, 80S) are in ASME B36.19M. Heavier walls without an S schedule are taken from ASME B36.10M.

How to read it

The stencil gives the maker, "A312," the grade, welded (WLD) or seamless (SMLS), the size and schedule, and the heat number. Most 304 and 316 pipe is stocked dual certified, for example "TP304/304L." Dual-certified pipe meets the low carbon limit of the L grade and the strength of the straight grade, so one stock item covers both.

Grades and schedules

Item	Choice	What it means
304	General purpose	Chromium-nickel stainless; good general corrosion resistance
316	Added molybdenum	Better resistance to pitting from chlorides, such as salt water and many chemicals
L suffix (304L, 316L)	Low carbon	Lower risk of carbide precipitation and corrosion next to welds
H suffix (304H, 316H)	Controlled higher carbon	Higher strength at elevated temperatures
Schedule 5S and 10S	Thin wall	Low-pressure process, food, sanitary-adjacent and roll-grooved systems
Schedule 40S	Standard wall	Common general-purpose wall; matches Sch 40 through NPS 10
Schedule 80S	Extra strong	Threaded and higher-duty service; matches Sch 80 through NPS 8
Welded (WLD)	Formed and seam welded	Most common in thin walls and larger sizes; lower cost
Seamless (SMLS)	Pierced and drawn, no seam	Often specified for high pressure or critical service

Key points

Specify grade, schedule, welded or seamless, and any finish or testing requirements. All four affect price and lead time.
Thin-wall 5S and 10S pipe cannot be threaded. Use butt welding, roll grooving or press fittings made for it.
Match fittings and filler metal to the pipe grade. 316L pipe usually takes 316L fittings and 316L filler. See the stainless filler sheet in the welding library.
Keep stainless away from carbon steel tools, brushes and racks. Iron contamination causes rust spots.
Sanitary tubing for food and pharmaceutical work is a different product, sold by OD under other standards, not by NPS.

Common mistakes

Treating 40S and Sch 40 as the same at every size.
Shipping 304 where the customer has chloride exposure and needs 316.
Threading 10S pipe.
Assuming sanitary tube and A312 pipe of similar size are interchangeable.

Governing standards: ASTM A312; ASME B36.19M (stainless steel pipe); ASME B36.10M; the governing piping code.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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