ASTM A358 Class 1 TP304H Stainless Steel Welded Pipe

What Is ASTM A358 TP304H?

ASTM A358/A358M covers electric-fusion-welded austenitic chromium-nickel stainless steel pipe for high-temperature and general corrosive service. The specification is commonly used for large-diameter pipe, typically NPS 8 (DN 200) and larger, produced by forming plate or strip and joining the longitudinal seam with an electric fusion welding process such as TIG or MIG. The grade designation TP304H refers to the high-carbon version of TP304 with the same chromium-nickel base but a controlled carbon content of 0.04-0.10%, which preserves strength at elevated temperature where low-carbon variants would soften. The corresponding UNS number is S30409. Pipe is normally supplied in the solution-annealed condition and may be annealed and pickled, bright annealed, or polished to the required surface finish.

Chemical Composition (ASTM A358 / A312 TP304H)

Mechanical Properties (ASTM A358)

The tensile requirements below apply to the pipe in the heat-treated condition and are the minimum values specified for TP304H. Higher strength is not a function of the pipe class; the class defines the welding and inspection regime.

Class 1 vs Class 3 in ASTM A358

A358 defines several classes that differ in welding method and nondestructive examination. Class 1 and Class 3 are the most frequently specified.

Class 4 (double-side welded with 100% radiography) is also available for very thick walls. In every case the weld seam must be finished so that it meets the requirements of the specification, and for sour or cyclic service the weld cap is normally ground flush with the pipe surface.

ASTM A358 vs ASTM A312

ASTM A358 applies specifically to electric-fusion-welded (EFW) pipe and is the usual route for large-diameter stainless steel pipe. ASTM A312 covers seamless and welded (ERW) pipe of the same austenitic grades and is used for the full range of sizes, including small bores. For a given grade such as TP304H, the composition and tensile requirements are consistent between the two documents; the choice depends mainly on diameter, wall thickness, and the availability of the forming process. Very large diameters and heavy walls that cannot be produced as seamless pipe are welded to A358 or A358M.

Typical Applications of TP304H Welded Pipe

Steam piping, superheater and reheater lines in power generation

High-temperature process lines in petrochemical and refinery service

Heat exchanger and furnace piping where creep resistance above 500 C is required

LNG and cryogenic systems where fully radiographed Class 3 welds are required

Corrosive media handling where the molybdenum-free 304 base is acceptable but elevated temperature rules out 304L

Frequently Asked Questions

What does Class 1 mean in ASTM A358?

Class 1 pipe is welded from one side with filler metal permitted. Radiographic examination of the weld is performed only when the purchase order requires it, which keeps cost lower for general corrosive service where the risk of weld defects is acceptable.

What is the difference between Class 1 and Class 3?

The essential difference is inspection: Class 3 requires 100% radiographic examination of the longitudinal weld, while Class 1 does not unless specified. Class 3 is therefore used where a weld failure would be dangerous or expensive, such as high-pressure steam or toxic media lines.

What is the difference between ASTM A358 and ASTM A312?

A358 covers electric-fusion-welded pipe only and is preferred for large diameters and heavy walls. A312 covers seamless and welded pipe of austenitic grades across a wider size range. Composition and tensile minima for the same grade are comparable in both specifications.

Why is TP304H used instead of TP304 or TP304L at high temperature?

TP304H keeps carbon at 0.04-0.10%, which maintains creep and stress-rupture strength above roughly 500 C. TP304L with maximum 0.030% carbon is easier to weld without sensitization but loses strength at elevated temperature, so design codes favor the H grade for hot service.

Is TP304H pipe supplied in the annealed condition?

Yes. The pipe is solution annealed at a minimum temperature of approximately 1040 C (1900 F) and then cooled, which restores the austenitic structure and corrosion resistance after welding and forming. It is then typically pickled or bright annealed and may be polished.

Can TP304H welded pipe be used for seawater service?

No. Without molybdenum, TP304H has limited resistance to chloride pitting and is unsuitable for seawater or high-chloride media. For such service, molybdenum-bearing grades such as TP316L or higher-alloy duplex and super-austenitic grades should be specified.