ASME SA213 TP321 Bright Annealed Tube: Stabilised Grade for High-Temperature Service

What ASME SA213 TP321 Covers

ASME SA213 is the ASME Boiler and Pressure Vessel Code edition of ASTM A213, the specification for seamless ferritic and austenitic alloy-steel boiler, superheater and heat exchanger tubes. TP321 is the titanium-stabilised 18-8 austenitic grade, UNS S32100, which was developed for service in the temperature band where plain 304 would suffer chromium carbide precipitation. Because titanium has a higher affinity for carbon than chromium does, the carbon is tied up as titanium carbides and the chromium stays in solution, so the tube keeps its corrosion resistance after welding and after long exposure at temperature. The grade is widely dual-certified to ASTM A213 and ASME SA213, and the two documents are technically identical.

Chemistry of TP321 and TP321H

The titanium-to-carbon relationship is the defining feature of the grade. TP321H carries a higher and controlled carbon range together with a lower titanium multiplier, which raises creep rupture strength for the hottest service. Minimum room-temperature properties for both grades are 515 MPa tensile strength, 205 MPa 0.2 % proof stress and 35 % elongation, and the stabilised grades are also specified for grain size control when the tube is ordered for elevated-temperature creep duty.

Why Bright Annealing Matters

Bright annealing is carried out in a hydrogen or cracked-ammonia atmosphere, or in vacuum, at a temperature high enough to solution-anneal the tube while the reducing atmosphere prevents oxide from forming. The result is a metallically bright surface with no scale and no pickling residue, typically finished to about 0.4 micrometre Ra or better. Three practical benefits follow. First, the inner bore stays free of oxide that would otherwise trap chlorides and start pitting in a heat exchanger. Second, surface roughness that would disturb the boundary layer in condenser tubes is eliminated, which helps thermal performance. Third, an oxide-free surface allows reliable eddy-current testing, the standard non-destructive method used to screen heat exchanger tubes for wall defects.

Service Range and Typical Applications

TP321 is the conventional answer for metal temperatures between 427 and 816 degrees C, where chromium carbide precipitation would otherwise sensitise an unstabilised grade. Typical duties include power station superheater and reheater tubes, steam piping, refinery heater tubes, high-temperature chemical reactor coils, and aircraft exhaust components. The tubes are normally supplied from 12.7 mm to 114.3 mm outside diameter with 0.89 mm to 4.78 mm wall thickness, with tolerances that are tightened for specific exchanger designs. Below about 400 degrees C, plain 304L or 316L tube is more economical and equally adequate.

Inspection and Documentation

A complete mill package for SA213 TP321 bright annealed tube includes a certified material test report with heat analysis and mechanical results, hydrostatic or eddy-current test records, and dimensional inspection data. Flaring and flattening tests are specified for small-diameter tube to demonstrate ductility of the finished product. Where the tube is destined for pressure service, the allowable stresses are taken from ASME BPVC Section II, Part D, and the tube is then qualified under the applicable construction code such as ASME B31.1 or Section I.

Frequently Asked Questions

Q: What is the difference between TP321 and TP321H?
A: TP321 has carbon 0.08 % max with titanium at 5 x (C + N) min; TP321H has carbon 0.04 to 0.10 % with titanium at 4 x (C + N) min, which increases creep strength for the hottest service.

Q: Is ASME SA213 the same as ASTM A213?
A: Yes. SA213 is the ASME Code version of A213 and the requirements for TP321 and TP321H are identical in both documents.

Q: Why is bright annealed tube preferred for heat exchangers?
A: The reducing-atmosphere anneal leaves an oxide-free bore that resists chloride pitting, supports eddy-current testing and avoids scale that would foul the tube.

Q: Can TP321 tube be welded in the field?
A: Yes. It is welded with ER347 or ER321 filler wire (AWS A5.9) and the stabilised chemistry means post-weld heat treatment is not required for normal service.

Q: What is the maximum service temperature of TP321?
A: The grade is normally selected for metal temperatures from 427 to 816 degrees C. For continuous duty at the upper end, the higher-carbon TP321H grade is preferred.

Q: What minimum strength does SA213 TP321 guarantee?
A: 515 MPa tensile strength, 205 MPa 0.2 % proof stress and 35 % elongation at room temperature.