ASTM A312 TP304 Stainless Steel Welded Pipes: Specifications and Applications

ASTM A312 covers seamless, welded and heavily cold-worked austenitic stainless steel pipe for general corrosive and high- or low-temperature service. Grade TP304, UNS S30400, is the workhorse of the specification: an 18-8 austenitic chemistry with good corrosion resistance, excellent formability and weldability, and a wide range of available sizes and wall thicknesses.

Welded TP304 pipe is produced by forming strip into a tube and joining the longitudinal seam by electric resistance welding or fusion welding. For agricultural machinery, food processing, irrigation and general plant piping, welded pipe offers the same corrosion performance as seamless pipe at a lower cost and with tighter wall-thickness control.

Chemical Composition per ASTM A312

Element	TP304, UNS S30400 (wt%)
Carbon, max	0.08
Manganese, max	2.00
Phosphorus, max	0.045
Sulfur, max	0.030
Silicon, max	0.75
Chromium	18.0–20.0
Nickel	8.0–11.0

For heavy welded sections where sensitization is a concern, the low-carbon version TP304L, UNS S30403, is specified with carbon 0.03 percent maximum; TP304H, UNS S30409, with carbon 0.04-0.10 percent, is used where elevated-temperature strength is required.

Mechanical Properties and Inspection per ASTM A312

Property	Value
Tensile strength, min (MPa)	515
0.2% yield strength, min (MPa)	205
Elongation, min (%)	35

The heat treatment for welded pipe is solution annealing: heating above 1040 C followed by rapid cooling, which restores full corrosion resistance to the seam and the parent metal. A312 pipe is supplied with hydrostatic testing or an equivalent non-destructive examination, and the heat treatment condition is declared. Buyers should request a mill test certificate with heat numbers, chemical analysis and mechanical test results, and specify supplementary requirements such as radiography of the seam when the service demands it.

Manufacturing Routes: ERW, EFW and Seamless

Electric resistance welded pipe is formed from strip with the longitudinal seam joined by resistance heating and pressure; it is economical in small and medium diameters, offers a narrow seam, and is the standard route for schedules such as 40S and 80S. Electric fusion welded pipe is joined by arc welding, typically for large diameters and thicker walls, often with filler metal and full radiography. Seamless pipe, produced from a solid billet, has no seam at all and is specified where the weld is unacceptable for pressure or fatigue reasons. The choice of route is part of the A312 order; a pipe marked ERW has a longitudinal seam and must not be confused with spiral-welded line pipe, which is made to other specifications such as API 5L.

Dimensions, Schedules and Design Pressure

Stainless steel pipe dimensions follow ASME B36.19, which defines the lettered schedules 5S, 10S, 40S and 80S. Representative wall thicknesses:

NPS	Outside diameter (mm)	Sch 40S wall (mm)	Sch 80S wall (mm)
1/2	21.3	2.77	3.73
3/4	26.7	2.87	3.91
1	33.4	3.38	4.55
1-1/2	48.3	3.68	5.08
2	60.3	3.91	5.54
3	88.9	5.49	7.62
4	114.3	6.02	8.56
6	168.3	7.11	10.97

Allowable working pressures are calculated from the wall thickness, outside diameter and the allowable stress for TP304 at the design temperature, per ASME B31.3. For elevated-temperature piping, the practical upper limit for TP304 in non-impact service is about 816 C under the ASME B31.3 rules; above that range, high-carbon or stabilized grades should be evaluated.

Applications in Agricultural Machinery

Welded TP304 pipe is specified in agricultural and food equipment for fertilizer and pesticide sprayer booms and lines, irrigation and fertigation systems, dairy and beverage processing lines, air and hydraulic circuits on harvesters and sprayers, and structural frames that are washed down with detergents and sanitizers. The 18-8 chemistry resists the mild organic acids and cleaning chemicals found on farms, while the welded route keeps cost low and delivery fast.

FAQ

What is the difference between TP304 and TP304L pipe?

TP304L, UNS S30403, limits carbon to 0.03 percent maximum, which prevents sensitization and intergranular corrosion in heavy welded sections. TP304 is the general-purpose grade; TP304L is specified for welded service above about 425 C or for very thick welds.

Is welded TP304 pipe as corrosion resistant as seamless?

After proper solution annealing, the weld seam is fully recrystallized and the pipe meets the same corrosion requirements as seamless pipe. The seam itself is inspected and tested to the specification.

What does schedule 40S mean?

The S suffix indicates a stainless-steel schedule defined by ASME B36.19 rather than the carbon-steel schedule 40 of ASME B36.10. For most sizes up to NPS 12 the wall thicknesses are identical, but the S designation is mandatory when ordering stainless pipe.

Can TP304 welded pipe be used for food contact?

Yes, in most jurisdictions 304-type stainless is an accepted food-contact material for equipment and piping; verify the relevant national food-contact regulations and keep surfaces clean and passivated.

What is the maximum service temperature of TP304?

For pressure design, ASME B31.3 allows TP304 up to about 816 C in non-impact service, subject to allowable stress limits. For continuous oxidation resistance, 304 is commonly rated to about 870 C in dry air, but strength falls rapidly above 600 C.

How is the seam of ERW pipe tested?

Under A312, welded pipe is hydrostatically tested; supplementary eddy-current or ultrasonic examination of the seam can be specified by the buyer for critical service.