ASTM A249 TP304H Stainless Steel Welded Tubes for High-Temperature Service

What ASTM A249 TP304H Tubing Is

ASTM A249 TP304H is a welded austenitic stainless steel tube made to the A249 specification and intended for elevated-temperature service in boilers, superheaters, heat exchangers and condensers. It corresponds to UNS S30409 and is the high-carbon version of the 18-8 austenitic grade: chromium is held at 18.0-20.0% and nickel at 8.0-11.0%, while carbon is deliberately raised into the 0.04-0.10% band.

That higher carbon level is the whole point of the grade. At temperature, fine carbide precipitation strengthens the austenitic matrix and raises creep and stress-rupture resistance, so TP304H retains useful strength where TP304 would slowly deform under sustained load. Because the tube is welded rather than seamless, it is produced from strip by forming and welding, the weld is cold worked, and the finished tube is solution heat treated so that the joint and the parent metal behave as one material.

Chemical Composition of TP304H (UNS S30409)

Element	Content (mass %)
Carbon (C)	0.04 - 0.10
Manganese (Mn)	2.00 max
Phosphorus (P)	0.045 max
Sulfur (S)	0.030 max
Silicon (Si)	0.75 max
Chromium (Cr)	18.0 - 20.0
Nickel (Ni)	8.0 - 11.0
Iron (Fe)	Balance

The carbon range is a specification requirement rather than a target: a heat below 0.04% is not TP304H, and one above 0.10% would compromise weldability and corrosion resistance. Producers therefore control carbon closely in the melt and verify it by product analysis on the finished tube.

Mechanical Properties and Heat Treatment

Property	Requirement
Tensile strength, min	515 MPa (75 ksi)
Yield strength, 0.2% offset, min	205 MPa (30 ksi)
Elongation, min, in 50 mm (2 in.)	35%
Hardness, max	90 HRB or 200 HV
Solution heat treatment, min	1040 C (1900 F), rapid cooling

Note that the room-temperature minimum properties of TP304H are the same as those of TP304; the H designation buys high-temperature performance, not higher ambient strength, and it is accompanied by a slightly lower minimum elongation of 35%. Hardness is limited to about 90 HRB or 200 HV, which keeps the tube ductile enough for bending, coiling and U-bend manufacture while confirming that the solution anneal was properly performed.

Sizes, Schedules, Forms and Ends

Nominal bore from 1/8 in. up to 30 in., with larger sizes produced as special orders.

Schedules covering SCH 20, SCH 30, SCH 40, STD, SCH 60, SCH 80, XS, SCH 120, SCH 140, SCH 160 and XXS.

Type: welded and ERW, with the weld cold worked and the tube annealed after welding.

Form: round, square, rectangular, hydraulic and coiled tube sections.

Length: single random, double random and cut length.

Ends: plain, bevelled or threaded.

Welded TP304H tube is normally supplied bright annealed or annealed and pickled, and for pressure duty it is tested with a hydrostatic pressure test and an eddy current examination of the full length. Larger diameters are a common requirement in utility and process work, and wall thickness is ordered as average or minimum wall depending on whether the design code takes credit for the nominal or the thinnest acceptable wall.

High-Temperature Service, Welding and Inspection

The strength of TP304H at temperature comes from its carbon content and from the stabilising effect of the austenitic structure, which resists the spheroidisation and graphitisation that limit carbon and low-alloy steels. In practice the grade is used for superheater and reheater tube, header connections, process heaters, steam piping and furnace fittings, with service temperatures typically in the range of about 550 C to 800 C.

Because oxidation and scaling, rather than mechanical strength, usually set the practical upper limit for an 18-8 austenitic grade, designers treat roughly 800-815 C as the ceiling for long-term pressure service and refer to the applicable code stress tables before fixing the wall thickness. The grade is readily welded by TIG, MIG and other common arc processes; matching filler metal is used, and the finished joint is normally solution annealed or at least stress relieved where the service is severe, to restore corrosion resistance around the heat-affected zone. Inspection for this product follows the A249 framework and covers product analysis, tensile and hardness testing, flattening and bend tests, hydrostatic testing, eddy current or ultrasonic examination of the weld and full length, and dimensional checks before the tube is packed in moisture-proof export crates.

FAQ

Q: What does the H in TP304H mean?
The H indicates a high-carbon austenitic grade. Carbon is specified at 0.04-0.10%, compared with 0.08% maximum for standard TP304, which improves creep and stress-rupture strength for long-term service at elevated temperature.

Q: What is the maximum service temperature for ASTM A249 TP304H tube?
The grade is used from roughly 550 C up to about 800 C, and code stress tables normally extend 304H pressure service to around 815 C. Above that range oxidation and scaling, not strength, govern, so the actual limit depends on the atmosphere and the applicable design code.

Q: Is TP304H stronger than TP304 at room temperature?
No. The minimum tensile strength of 515 MPa, yield strength of 205 MPa and hardness limit of 90 HRB are the same as for TP304. The advantage of TP304H appears only at elevated temperature, where the higher carbon content resists creep.

Q: Why must the weld be cold worked before final heat treatment?
Cold working the weld refines the cast structure produced by welding and removes the bead geometry that would otherwise act as a stress raiser. Combined with the solution anneal, it lets the welded joint match the parent metal in bending and pressure service, which the A249 specification requires.

Q: Can TP304H tube be bent and coiled for heat exchanger bundles?
Yes. The hardness limit of about 90 HRB and the minimum 35% elongation keep the annealed tube ductile enough for U-bend, coil and fabrication work, provided bending follows normal practice for austenitic tube and the bent sections are handled without contamination.

Q: What tests are supplied with A249 TP304H tube?
Typical documentation includes product analysis, tensile and hardness results, flattening and bend tests, a hydrostatic pressure test, and eddy current or ultrasonic examination of the weld and full length, all traceable to the heat number on the mill certificate.