ASTM A213 TP321 Stainless Steel Bright Annealed Tube: Properties and Uses

What Is ASTM A213 TP321 Stainless Steel Tube?

ASTM A213 covers seamless ferritic and austenitic steel boiler, superheater and heat-exchanger tubes. Grade TP321 is a titanium-stabilized austenitic stainless steel. Titanium combines preferentially with carbon, so that chromium carbides do not precipitate at grain boundaries when the steel is held in the sensitization range of 427-816 degrees Celsius (800-1500 degrees Fahrenheit). TP321 therefore resists intergranular corrosion after welding and in high-temperature service where 304-type steels would sensitize.

Compared with 304 and 304H, TP321 offers superior creep and stress-rupture properties in the elevated-temperature range, combined with good low-temperature toughness. It cannot be hardened by heat treatment; strengthening is achieved only by cold working. The steel is readily welded and fabricated with standard shop practice.

Equivalent Grades

Standard	Grade	UNS	EN Designation	JIS	BS
ASTM A213 / A312	TP321	S32100	1.4541 / X6CrNiTi18-10	SUS321TP	321S31
ASTM A213 / A312	TP321H	S32109	1.4878 / X8CrNiTi18-10	SUS321HTP	321S51

Chemical Composition (ASTM A213)

The titanium limits are expressed relative to the carbon content, since the purpose of titanium is to stabilize the carbon in the steel.

Element, %	TP321	TP321H
Carbon C	0.08 max	0.04-0.10
Manganese Mn	2.00 max	2.00 max
Phosphorus P	0.045 max	0.045 max
Sulfur S	0.030 max	0.030 max
Silicon Si	0.75 max	0.75 max
Chromium Cr	17.0-19.0	17.0-19.0
Nickel Ni	9.0-12.0	9.0-12.0
Titanium Ti	5 x C min, 0.70 max	4 x C min, 0.70 max

Mechanical Properties (ASTM A213)

Property	TP321	TP321H
Tensile strength Rm	515 MPa (75 ksi) min	515 MPa (75 ksi) min
0.2% yield strength Rp0.2	205 MPa (30 ksi) min	205 MPa (30 ksi) min
Elongation in 50 mm	35% min	35% min
Hardness	192 HBW / 90 HRB max	192 HBW / 90 HRB max
Solution annealing temperature	1040 degrees C min	1095 degrees C min

Bright Annealed Finish

Bright annealing is a final heat treatment performed in a protective atmosphere, typically hydrogen or a hydrogen-nitrogen mixture, instead of air. Because no oxygen reaches the surface, no oxide scale forms, and the tube exits the furnace with a smooth, bright, reflective surface that requires no pickling or mechanical cleaning.

For TP321 tube, the bright annealed finish provides several practical benefits: a clean surface that is easier to sanitize in food and pharmaceutical service, improved corrosion resistance from a continuous passive film, and a low surface roughness commonly specified at Ra of 0.4 micrometers or less. BA tube is therefore favored for hygienic lines, instrument tubing, heat exchanger tubes and any application where surface cleanliness matters.

Typical Applications

Superheater, reheater and boiler tubes in power generation.

Shell-and-tube heat exchangers and condensers in chemical and petrochemical plants.

Food preparation equipment, particularly in chloride-containing environments.

Coastal architectural panelling, railings and trim.

Aircraft exhaust systems and marine fittings.

Chemical containers, including transport vessels.

Frequently Asked Questions

Q1. Why is titanium added to TP321?

Titanium is a strong carbide former. It reacts with carbon in preference to chromium, so chromium carbides do not precipitate at grain boundaries during exposure to 427-816 degrees Celsius. This prevents sensitization and intergranular corrosion.

Q2. What is the difference between TP321 and TP321H?

TP321 has a carbon limit of 0.08% maximum. TP321H has a controlled carbon range of 0.04-0.10%, which provides higher creep and rupture strength at elevated temperature. Both grades require titanium stabilization, and TP321H is solution annealed at a higher minimum temperature.

Q3. What is the difference between bright annealing and conventional annealing?

Conventional annealing is done in air and produces an oxide scale that must be removed by pickling. Bright annealing is performed in a protective hydrogen or nitrogen atmosphere, so the surface remains oxide-free, smooth and reflective without further treatment.

Q4. What is the maximum service temperature of TP321?

TP321 is suitable for continuous service up to about 816 degrees Celsius and is widely used in the 427-816 degrees Celsius range where 304 would sensitize. Above this range, creep strength and oxidation resistance become limiting factors.

Q5. How does TP321 compare with TP304?

TP304 has no stabilizing element and can sensitize in the 427-816 degrees Celsius range. TP321 resists sensitization and has superior creep and stress-rupture properties at high temperature, which is why it is chosen for superheaters and high-temperature chemical service.

Q6. Which specification covers seamless stainless boiler and heat-exchanger tubes?

ASTM A213 / ASME SA213 covers seamless ferritic and austenitic boiler, superheater and heat-exchanger tubes. Related specifications are ASTM A312 for pipe and ASTM A269 for general-service seamless and welded tube.