ASTM A213 TP321 Stainless Steel Pickled Annealed Tube

ASTM A213 is the standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater and heat-exchanger tubes. Grade TP321, with the corresponding UNS designation S32100, is a titanium-stabilized austenitic stainless steel whose main advantage is resistance to intergranular corrosion after exposure in the chromium carbide precipitation range, combined with good high-temperature strength. Tube supplied to this specification in the pickled and annealed condition has a clean oxide-free surface and a fully solution-annealed microstructure, which makes it a reliable choice for petrochemical, power-generation and heat-transfer service.

Standard and Product Forms

Parameter	Description
Standard	ASTM A213 / ASME SA213
Grade	TP321 (UNS S32100), TP321H (UNS S32109)
Tube type	Seamless
Surface conditions	Annealed and pickled, or bright annealed
Typical size range	Outer diameter about 1/8 in to 5 in (3.2-127 mm); wall about 0.015 in to 0.500 in (0.4-12.7 mm)
Typical service	Boilers, heat exchangers, superheaters, high-temperature process lines

Chemical Composition of TP321 and TP321H

The composition limits below follow ASTM A213:

Element	TP321 (wt%)	TP321H (wt%)
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)	17.00-19.00	17.00-19.00
Nickel (Ni)	9.00-12.00	9.00-12.00
Titanium (Ti)	5 x C min to 0.60	4 x C min to 0.60

Titanium is the key element: it combines preferentially with carbon to form stable titanium carbides, preventing chromium from being depleted at grain boundaries during welding or high-temperature service. The specification requires the titanium content to be at least five times the carbon content for TP321 and four times for TP321H, which is the mechanism behind the grade's intergranular corrosion resistance.

Mechanical and Physical Properties

Property	Value
Tensile strength	515 MPa (75 ksi) min
Yield strength (0.2% offset)	205 MPa (30 ksi) min
Elongation	35% min
Density	8.0 g/cm3
Melting range	Approximately 1400-1454°C

The minimum values follow ASTM A213. TP321H, with its higher carbon range of 0.04-0.10%, offers improved creep strength for service above about 540°C, which is why it is specified for superheater and reheater tubing.

Heat Treatment, Pickling and Testing

Per A213, TP321 tubes must be solution annealed at a temperature of at least 1040°C (1900°F) followed by rapid cooling, which dissolves precipitated carbides and restores a fully austenitic structure. For TP321H the same minimum anneal applies, and a subsequent stabilization treatment in the 850-900°C range is commonly applied to precipitate titanium carbides and maximize resistance to sensitization during service. Pickling removes the oxide scale formed during annealing, using a nitric-hydrofluoric acid bath, leaving a clean surface; bright annealing in a protective atmosphere achieves a bright finish without pickling. Each tube is then subjected to non-destructive testing, typically eddy current or ultrasonic, and a hydrostatic test to verify soundness and pressure capability, in accordance with the standard's requirements.

Typical Applications

TP321 tube is used for boiler shell and pressure parts, heat exchanger and condenser tubes, superheater and reheater elements, oil and gas exhaust combustion piping, engine exhaust manifolds and tubes, diesel engine muffler components, furnace tubes and dryers, chemical transport lines, and expansion joints. Its combination of stabilization, moderate cost and good creep resistance makes it a frequent choice where service temperatures exceed the practical limit of 304 but do not justify the higher cost of fully heat-resistant grades.

FAQ

Q1: What is the difference between TP321 and TP321H?
A1: The main difference is carbon content: TP321 has 0.08% carbon maximum, while TP321H is controlled to 0.04-0.10%. The higher carbon of TP321H increases creep strength at elevated temperature, making it preferred for superheater and reheater service.

Q2: Why is titanium added to TP321?
A2: Titanium stabilizes carbon by forming titanium carbides, preventing chromium carbide precipitation at grain boundaries during welding and high-temperature service. This preserves the corrosion resistance of the heat-affected zone.

Q3: What does pickled and annealed mean?
A3: It means the tube was solution annealed above 1040°C, then pickled in acid to remove oxide scale. The result is a clean, corrosion-resistant surface without the need for further finishing.

Q4: Can TP321 replace 304 in high-temperature service?
A4: Yes, and it is often chosen precisely for this reason. TP321 offers better high-temperature creep and stress-rupture strength than 304 while retaining good weldability, which is why it appears in boilers, exhaust systems and heat exchangers.

Q5: How is the tube quality verified before shipment?
A5: A213 requires non-destructive testing, usually eddy current or ultrasonic, plus a hydrostatic test. Buyers may also request flattening, flange, bend and intergranular corrosion tests, and certified mill test reports for chemistry and mechanicals.