321 stainless steel seamless pipe is titanium-stabilized TP321, UNS S32100, supplied under ASME SA-312 with the corresponding ASTM A312 designation. Its signature is not a fixed titanium percentage: Table 1 note F ties the titanium minimum to the measured carbon-plus-nitrogen content and sets 0.70 % as the maximum.
That distinction matters when reviewing a mill certificate. A buyer should see C, N and Ti together, then check the ratio and cap. It also matters mechanically: SA-312 Table 4 gives a separate lower strength row for seamless TP321 above 0.375 in. wall, so a single ‘321 tensile strength’ number is incomplete without route and thickness.
Grade, UNS and Specification
| Grade | UNS | Specification | Stabilizing requirement | Product form |
|---|---|---|---|---|
| TP321 | S32100 | ASME SA-312 / ASTM A312 | Ti ≥ 5 × (C + N), 0.70 % max | Seamless pipe |
Chemical Composition (ASME SA-312 Table 1 and note F)
| Element, weight % | TP321 (S32100) |
|---|---|
| Carbon, max | 0.08 |
| Manganese, max | 2.00 |
| Phosphorus, max | 0.045 |
| Sulfur, max | 0.030 |
| Silicon, max | 1.00 |
| Chromium | 17.0–19.0 |
| Nickel | 9.0–12.0 |
| Nitrogen, max | 0.10 |
| Titanium | Not less than 5 × (C + N); 0.70 max |
| Molybdenum | No requirement |
Tensile Requirements (ASME SA-312 Table 4)
| Route / wall | Tensile min, ksi (MPa) | 0.2 % yield min, ksi (MPa) | Elongation longitudinal / transverse, min % |
|---|---|---|---|
| Welded | 75 (515) | 30 (205) | 35 / 25 |
| Seamless, t ≤ 0.375 in. (9.50 mm) | 75 (515) | 30 (205) | 35 / 25 |
| Seamless, t > 0.375 in. (9.50 mm) | 70 (485) | 25 (170) | 35 / 25 |
Size Range and OD × Wall
| NPS | OD, in. (mm) | Sch 5S wall, in. (mm) | Sch 10S wall, in. (mm) | Sch 40S wall, in. (mm) | Sch 80S wall, in. (mm) |
|---|---|---|---|---|---|
| 1/2 | 0.84 (21.34) | 0.065 (1.65) | 0.083 (2.11) | 0.109 (2.77) | 0.147 (3.73) |
| 3/4 | 1.05 (26.67) | 0.065 (1.65) | 0.083 (2.11) | 0.113 (2.87) | 0.154 (3.91) |
| 1 | 1.315 (33.4) | 0.065 (1.65) | 0.109 (2.77) | 0.133 (3.38) | 0.179 (4.55) |
| 1-1/2 | 1.9 (48.26) | 0.065 (1.65) | 0.109 (2.77) | 0.145 (3.68) | 0.2 (5.08) |
| 2 | 2.375 (60.33) | 0.065 (1.65) | 0.109 (2.77) | 0.154 (3.91) | 0.218 (5.54) |
| 3 | 3.5 (88.9) | 0.083 (2.11) | 0.12 (3.05) | 0.216 (5.49) | 0.3 (7.62) |
| 4 | 4.5 (114.3) | 0.083 (2.11) | 0.12 (3.05) | 0.237 (6.02) | 0.337 (8.56) |
| 6 | 6.625 (168.28) | 0.109 (2.77) | 0.134 (3.4) | 0.28 (7.11) | 0.432 (10.97) |
| 8 | 8.625 (219.08) | 0.109 (2.77) | 0.148 (3.76) | 0.322 (8.18) | 0.5 (12.7) |
| 10 | 10.75 (273.05) | 0.134 (3.4) | 0.165 (4.19) | 0.365 (9.27) | 0.5 (12.7) |
| 12 | 12.75 (323.85) | 0.156 (3.96) | 0.18 (4.57) | 0.375 (9.52) | 0.5 (12.7) |
Selected rows from ANSI B36.19 Table 1 as reproduced in the ASME library. NPS is a designation; the OD shown is the controlling outside diameter. Other schedules and sizes are subject to the specification and enquiry. Wall tolerance is governed separately by SA-312 Table 3.
Manufacturing Route and Heat Treatment
Seamless TP321 is hot or cold finished and solution annealed under the general SA-312 Table 2 condition of 1900 °F [1040 °C] minimum, followed by water quench or rapid cooling. Stabilization during alloy manufacture and stabilization heat treatment are not the same operation. Any supplementary thermal treatment, corrosion test or surface requirement must be identified separately on the purchase order.
Inspection, Testing and Certification
- Heat and product verification: review heat number, UNS/grade, chemical analysis and the specified mechanical result against the named standard, not against a generic grade datasheet.
- Pipe tests: SA-312 Sections 11 and 13 cover tension/flattening and hydrostatic or nondestructive electric examination as applicable. Additional UT, eddy current, impact, PMI or hardness requirements must be defined on the order.
- Dimensions and surface: confirm actual OD, wall, length, end preparation and surface condition. Hydrostatic P = 2St/D is not a design-pressure calculation.
- Documentation: request the MTC route at enquiry stage. EN 10204 3.1 or 3.2, purchaser witnessing and third-party inspection are order-specific options to be confirmed in the quotation, not an automatic certificate bundle.
Packing & Delivery
Plastic end caps; steel frames or wooden crates for larger diameters; VCI protective wrapping where required. Beveled or plain ends are confirmed with the order. Marking should maintain the link between each length, standard, grade/UNS, size and heat number. Delivery schedule, quantity, MOQ and price are confirmed only against a specific enquiry.
Applications
Heat-transfer and furnace-adjacent piping within project temperature limits, exhaust and thermal-process lines, refinery and petrochemical fabricated assemblies, expansion and manifold pipework, and welded equipment where a titanium-stabilized austenitic grade is specified. Creep allowable values and oxidation limits must come from the design code, not Section II Part A.
Technical Notes
1. Titanium stabilization is a calculated acceptance check. The rule Ti ≥ 5(C+N), with Ti ≤ 0.70 %, links three analyses. Nitrogen itself has a 0.10 % maximum. Reviewing titanium alone can miss a nonconforming ratio; reporting carbon without nitrogen is also insufficient for the check.
2. Stabilized does not mean insensitive to every thermal cycle. The purpose of tying carbon with titanium carbides is to reduce chromium-carbide sensitization during fabrication exposure. Weld procedure, heat input and subsequent thermal history still require control. SA-312 Table 6 lists ER321 or ER347 for TP321, providing two specification-recognized filler routes rather than permission to reuse any 308-series procedure.
3. Thick-wall TP321 needs its own line on the technical datasheet. For seamless pipe over 0.375 in. [9.50 mm] wall, Table 4 reduces the minima to 70 ksi tensile and 25 ksi yield; welded and thinner seamless rows remain 75/30 ksi. This is exactly why Site A pairs the mechanical table with OD and wall rather than publishing a grade-only property card.
For non-stabilized alternatives see 304/304L and 316/316L seamless pipe.
Frequently Asked Questions
What is UNS S32100?
It is the UNS identifier for TP321, the titanium-stabilized austenitic stainless pipe grade in ASME SA-312 / ASTM A312.
How much titanium does SA-312 TP321 require?
Titanium must be at least five times the sum of measured carbon and nitrogen and not more than 0.70 %. Nitrogen is limited to 0.10 % max.
Does thick-wall 321 seamless pipe have different strength requirements?
Yes. Above 0.375 in. [9.50 mm] wall, SA-312 Table 4 gives 70 ksi tensile and 25 ksi yield minima; thinner seamless pipe has 75/30 ksi.
Which filler metals are listed for TP321?
SA-312 Table 6 lists ER321 or ER347, with their corresponding AWS/UNS entries. The project WPS remains controlling.
Is a stabilization heat treatment included automatically?
Do not assume so. Solution annealing is required by Table 2; any additional stabilization or corrosion-test requirement should be explicitly stated and confirmed on the order.
