Why is it that in shell-and-tube heat exchangers, some stainless steel tubes still suffer from intergranular corrosion, performance degradation, or even premature failure following welding or prolonged high-temperature operation? The critical factor lies in whether the material's heat treatment state-as well as its dimensional accuracy and surface quality control-truly meets the rigorous requirements of engineering applications.
Our ASME SA213 TP321 tubes for shell-and-tube heat exchangers are specifically engineered for high-temperature and welded service conditions. TP321 is a titanium-stabilized austenitic stainless steel; by strictly controlling its titanium content (typically Ti ≥ 5×C), it effectively inhibits the formation of chromium carbides, thereby eliminating the risk of intergranular corrosion. This makes it particularly well-suited for high-temperature heat exchange applications and environments involving frequent thermal cycling.
Regarding heat treatment, all our TP321 tubes undergo a standard solution annealing process. Typically performed within the temperature range of 1040–1100°C followed by rapid cooling, this process ensures the complete dissolution of carbides and results in a uniform, stable microstructure. Simultaneously, it relieves residual stresses induced by manufacturing and welding, thereby enhancing the material's long-term reliability and oxidation resistance in high-temperature environments.
In terms of dimensional tolerances and surface quality, we strictly adhere to the ASME SA213 standard. We maintain outer diameter tolerances within ±0.3% and wall thickness tolerances within ±10%, ensuring a precise fit during tube-to-tubesheet assembly and effectively minimizing the risk of leaks. Furthermore, both the inner and outer surfaces undergo meticulous finishing and cleaning processes to ensure they are free from scale, oil residues, and impurities. The inner surface roughness (Ra) is typically controlled to ≤0.8 μm, which effectively reduces fluid flow resistance and lowers the propensity for fouling by approximately 15–20%, thereby significantly enhancing overall heat exchange efficiency.
Chemical Composition:
| Grade | UNS | C | Mn | P | S | Si | Cr | Ni | Ti |
| 321 | S32100 | 0.08 max | 2.00 max | 0.045 max | 0.03 max | 1.00 max | 17.0-19.0 | 9.0-12.0 | 5(C+N)-0.07 |
| 321H | S32109 | 0.04-0.10 | 2.00 max | 0.045 max | 0.03 max | 1.00 max | 17.0-19.0 | 9.0-12.0 | 4(C+N)-0.07 |
Mechanical Properties:
| Grade | Tensile Strength, min, ksi (MPa) |
Yield Strength, min, ksi (MPa) |
Elongation in 2 in. or 50mm, min (%) |
Hardness |
Solutioning Temperature, min, °F(°C) |
|
| Rockwell | Brinell/Vickers | |||||
| 321 | 75(515) | 30(205) | 35 | 90HRB | 192HBW/200HV | 1900(1040) |
| 321H | 75(515) | 30(205) | 35 | 90HRB | 192HBW/200HV | 2000(1090) |
SS 321 Tube ASTM A213 Tolerances
| Specification | Allowable Outside Diameter Variation in mm | Allowable Wall Thickness Variation | Exact Length Tolerance in mm | Testing | ||||
| Nominal Diameter | Over | Under | %Over | %Under | Over | Under | ||
| ASTM-A213 TP321 Seamless Boiler Superheater and Heat Exchanger Tubes |
Under 25.4 | .1016 | .1016 | +20 | -0 | 3.175 | 0 | Flattening Test |
| 25.4-38.1 incl | .1524 | .1524 | +22 | -0 | 3.175 | 0 | Tension Test | |
| 38.1-50.8 excl | .2032 | .2032 | +22 | -0 | 3.176 | 0 | Flare Test | |
| 50.8-63.5 excl | .254 | .254 | +2 | -0 | 4.46 | 0 | Hardness Test | |
| 63.5-76.2 excl | .3218 | .3218 | +22 | -0 | 4.76 | 0 | 100% Hydrostatic test | |
| 76.2-101.6 incl | .381 | .381 | +22 | -0 | 4.76 | 0 | Refer to ASTM A-450 | |
ASTM A213 / ASME SA213 TP321 Seamless Tube Applications
Coastal architectural panelling
Boat fittings
Chemical containers
including for transport
Heat exchanger
Testing Requirements
In addition to standard tensile and hardness tests, the following are mandatory requirements:
Flattening/Flaring Tests: To ensure the ductility of the tubing and prevent cracking during tube expansion.
Non-Destructive Testing (NDT): 100% Eddy Current Testing (ET) or Ultrasonic Testing (UT), as well as Hydrostatic Testing.
Intergranular Corrosion Testing: Focuses specifically on compliance with the requirements of ASTM A262 Practice E.
MTC (Material Test Certificate): Must comply with EN 10204 3.1 or 3.2 (in cases involving third-party inspection).
Packaging and Marking:
Packaging shall consist of bundles or plywood crates, wrapped in plastic, and shall incorporate appropriate protective measures to ensure seaworthy transport, or shall be carried out in accordance with specific requirements.
Marking shall comply with the provisions of Specification A1016/A1016M and shall indicate whether the tubing is hot-finished or cold-finished; markings shall include, but not be limited to: standard, grade, dimensions, heat number, and lot number.
FAQ
Q: Is a stabilizing anneal mandatory for TP321?
A: The default requirement under ASME SA213 is solution annealing (heating to a minimum of 1040°C followed by rapid cooling). This process is typically performed within the range of 845°C to 900°C. Although the SA213 standard does not mandate it, for extremely aggressive corrosive environments or operating conditions where the design temperature exceeds 400°C, many users will specifically request a stabilizing anneal in their orders to ensure that the titanium effectively captures the carbon.
Q: Is TP321 resistant to chloride stress corrosion cracking (SCC) in heat exchangers?
A: No. Like all 300-series austenitic stainless steels, TP321 is highly susceptible to chloride stress corrosion cracking. If the circulating water contains high levels of chloride ions, the use of duplex stainless steels (such as S32205) or high-nickel alloys should be considered.
Q7: Are there any special requirements for welding TP321?
A: Filler Metal Selection: Typically, ER321 or ER347 (niobium-stabilized) filler metals are selected.
Shielding Gas: High-purity argon must be used for back-purging; otherwise, oxidation of the titanium will result in slag formation, thereby reducing the corrosion resistance of the weld seam.