ASTM A312 TP304H Pickled Seamless Pipe Guide

What Is TP304H?

ASTM A312/A312M covers seamless, straight-seam welded and heavily cold-worked austenitic stainless steel pipe for corrosive and elevated- or low-temperature general service. The H suffix in TP304H stands for high carbon: the grade is controlled to 0.04-0.10% carbon (UNS S30409, EN 1.4948, JIS SUS304H), compared with 0.08% maximum for TP304. The deliberate carbon addition improves creep rupture strength and resistance to graphitization at sustained elevated temperatures, which is why TP304H is the standard 18-8 choice for superheater, reheater and high-temperature process piping. It is available as seamless pipe, produced by hot extrusion or piercing of billets followed by solution annealing, and as ERW (electric resistance welded) pipe where the specification and service permit.

Chemical Composition

Element	TP304H (weight %)
Carbon	0.04-0.10
Chromium	18.0-20.0
Nickel	8.0-10.5
Manganese	2.00 max
Silicon	0.75 max
Phosphorus	0.045 max
Sulfur	0.030 max

Mechanical and Physical Properties

Property	Value
Tensile strength (min)	515 MPa
Yield strength 0.2% offset (min)	205 MPa
Elongation (min, in 50 mm)	40%
Hardness	92 HRB / 201 HB max
Density	8000 kg/m³
Modulus of elasticity	193 GPa
Mean CTE (0-538°C)	18.4 x 10⁻⁶ m/m·°C
Thermal conductivity (500°C)	21.5 W/m·K

Why the Pickled Surface Condition Matters

Hot finished and annealed pipe carries a layer of chromium-depleted mill scale and heat tint that must be removed before service. Pickling with a nitric-hydrofluoric acid solution dissolves the scale and the chromium-depleted layer beneath it, exposing clean metal that can form a uniform passive film. For TP304H pipe in high-temperature and boiler service, the pickled condition also provides a smooth surface that sheds deposits, resists localized attack under scale, and gives the clean appearance expected for refinery and power-plant piping. The same principle applies after welding: any heat tint left on the pipe surface should be removed by pickling or mechanical cleaning and re-passivation.

Applications

Superheater and reheater piping in thermal power plants, where metal temperatures exceed the design envelope of TP304.

Boiler components, pressure vessels and heat exchangers operating in the 500-800°C range.

Refinery heater tubes and transfer lines in clean hydrocarbon service.

Gas processing, petrochemical and specialty chemical plants requiring oxidation resistance at elevated temperature.

Steam lines and instrument tubing where high-temperature strength and cleanliness are specified together.

TP304 vs TP304H vs TP304L

Grade	Carbon	Best suited for
TP304 (S30400)	0.08 max	General corrosive and moderate-temperature service; best all-round 18-8 pipe
TP304H (S30409)	0.04-0.10	Sustained elevated-temperature service where creep strength controls design
TP304L (S30403)	0.035 max	Welded fabrication and chloride-bearing service where sensitization must be avoided

Buying Considerations

State the standard edition, grade, size (NPS or OD x wall), length and surface condition (pickled, or solution annealed and pickled) in the purchase order.

For seamless pipe, confirm the manufacturing route (hot finished or cold finished) and the annealing temperature; TP304H is solution annealed before delivery.

Specify the required tests: hydrostatic or NDE, flattening and flaring for pipe, plus PMI where grade verification is critical.

Request the mill certificate per EN 10204 3.1 with heat numbers and heat analysis, and verify carbon content is within the 0.04-0.10% band.

For ERW pipe, confirm that the weld seam meets the A312 requirements and that the pipe is used within the temperature and pressure limits allowed by the standard and the design code.

Common Misconceptions

"304H is just 304 with more carbon for no reason." The higher carbon is deliberate: it raises creep rupture strength for sustained high-temperature service, which is the entire point of the grade.

"304H welds like standard 304." The higher carbon increases sensitization risk in the heat-affected zone; low-carbon filler and controlled heat input are required for welded fabrication.

"Pickled pipe is rougher than polished pipe." Pickling is a chemical cleaning step for corrosion performance, not a cosmetic finish; the two are different surface programs with different purposes.

"UNS S30409 is a 304L designation." S30409 is TP304H; 304L is S30403. Mixing the two in procurement causes wrong material on site.

"A312 pipe is suitable for boiler tubing." Boiler and superheater tubes follow A213 (seamless) or A249 (welded) with tube-specific requirements; A312 is general pipe. Select the standard for the actual product form and code requirement.

FAQ

What does the H in 304H stand for?

High carbon. The grade is controlled to 0.04-0.10% carbon, versus 0.08% maximum for TP304, to increase creep rupture strength at elevated temperatures.

What is the difference between TP304H and TP304L?

TP304H raises carbon for high-temperature strength; TP304L drops carbon to 0.035% maximum to prevent sensitization during welding. They serve opposite design objectives.

Can TP304H pipe be welded?

Yes, with austenitic stainless procedures, but the higher carbon demands low-carbon or matching filler, limited heat input, and where necessary post-weld solution annealing to restore corrosion resistance.

What temperature range suits TP304H?

It is selected for sustained metal temperatures roughly above 500°C, where creep strength governs, and remains oxidation-resistant in air up to about 800-870°C depending on stress and atmosphere.

Is TP304H available as welded pipe?

Yes. A312 covers welded (including ERW) austenitic pipe. Welded TP304H is used where the design allows, typically at lower pressure ratings; seamless remains the norm for critical high-temperature lines.

Why is pickled surface specified for boiler and refinery pipe?

Pickling removes mill scale and the chromium-depleted surface layer after annealing, restoring the passive film and giving a clean deposit-shedding surface that extends service life in high-temperature and process service.