Stainless Steel 321H
Stainless Steel 321H (UNS S32109) is the high-carbon variant of titanium-stabilised 321, with carbon controlled between 0.04% and 0.10%. This promotes a coarser austenitic grain structure that maximises creep rupture strength in long-duration, high-temperature pressure service above 550°C.
1. Chemical Composition
The table shows 321H elemental ranges. The elevated carbon minimum and titanium requirement are what distinguish it from standard 321 in high-temperature design.
| Grade | C | Mn | P | S | Si | Cr | Ni | Mo | Other |
| TP321H | 0.04–0.10 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | ≤ 1.00 | 17.00–19.00 | 9.00–12.00 | — | Ti: 4x(C+N)–0.70 |
2. Mechanical Properties
The following minimum mechanical property requirements apply to 321H in the annealed condition. They reflect its intended use in elevated-temperature pressure vessel and boiler service.
| Grade | Condition | Tensile Strength Min, MPa (ksi) | Yield Strength Min, MPa (ksi) | Elongation Min, % | Heat Treatment Temp |
| TP321 | Annealed | 515 (75) | 205 (30) | 35 | 1040°C (1900°F) min |
Note: Allowable stress values at elevated temperatures for 321H exceed those of standard 321 in ASME Section II Part D tables due to its coarser grain structure and improved creep resistance.
3. Equivalent Grade
This table provides international cross-references for 321H, useful for material substitution verification and specification alignment across ASTM, European, Japanese, Chinese, and Russian standards.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| 321H | S32109 | S32169 | 07Cr19Ni11Ti | SUS321HTP | X8CrNiTi18-10 | 1.4878 | 12X18H10T |
*DIN 1.4878 covers this grade; the H variant is specifically defined under ASTM/ASME designations with a mandatory grain size requirement.
3. Key Technical Advantages
- Superior Creep and Stress Rupture Strength: The minimum carbon content of 0.04% produces coarse-grained austenite (ASTM grain size No. 7 or coarser) after solution annealing. This microstructure significantly outperforms fine-grained 321 in creep-limited design scenarios at temperatures above 550°C.
- ASME Code Compliance for High-Temperature Design: 321H is accepted under ASME Section I and Section VIII Division 1 with higher allowable stress values than 321 at elevated temperatures. It is the preferred choice for boiler superheater and reheater tube specifications.
- Titanium Stabilisation Retained: Despite the higher carbon content, the titanium addition (minimum 4x(C+N)) preserves resistance to intergranular sensitisation during welding, while the coarser grain supports long-term creep performance.
- Technical Note: When ordering 321H, specify ASTM grain size requirement (No. 7 or coarser) and request mill certification confirming solution anneal temperature (typically 1038°C minimum). Materials not meeting grain size criteria will not qualify for elevated-temperature allowable stresses in ASME design codes.
4. Common Manufacturing Standards
- ASTM A213: Standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes — TP321H designation with grain size requirement.
- ASTM A312: Standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipes — TP321H designation covered.
- ASTM A269: Standard specification for seamless and welded austenitic stainless steel tubing for general service — applicable to 321H grades. Please note: ASTM A269 covers general-service tubing and may apply where 321H composition is specified, but elevated-temperature allowable stresses per ASME are not required.
- ASTM A249: Standard specification for welded austenitic steel boiler, superheater, heat-exchanger, and condenser tubes — TP321H applicable for high-temperature service.
- DIN EN 10216-5: Seamless steel tubes for pressure purposes — stainless steel (1.4541 base composition; H-designation requirements per ASTM/ASME).
- JIS G3463: Stainless steel boiler and heat exchanger tubes — SUS321HTB designation.
- GB/T 13296: Seamless stainless steel tubes for boiler and heat exchanger use — 06Cr18Ni11Ti designation.
- GOST 9940 / 9941: Seamless stainless steel tubes — 08X18H10T equivalent applied.
Manufacturing Standards Comparison Table:
| Standard | ASTM | EN/DIN | JIS | GB/T | GOST |
| Seamless Boiler Tube | A213 TP321H | EN 10216-5 (1.4541) | G3463 SUS321HTB | GB/T 13296 | GOST 9941 |
| Seamless Pipe | A312 TP321H | EN 10216-5 (1.4541) | G3459 SUS321H | GB/T 14976 | GOST 9940 |
| Welded Boiler Tube | A249 TP321H | EN 10217-7 (1.4541) | G3463 SUS321H | GB/T 12771 | — |
5. Primary Applications
- Boiler Superheater and Reheater Tubes: Power generation boilers operating at steam temperatures above 550°C, where creep rupture life is a design-limiting factor and ASME allowable stresses must be maximised.
- Petrochemical Reformer Furnaces: Radiation section tubes in catalytic reforming and steam reforming furnaces are subject to sustained high-temperature operation with intermittent thermal cycling.
- Thermal Cracking Units: Transfer lines in ethylene cracker furnaces where exposure exceeds the sensitisation range for extended periods and titanium stabilisation prevents weld-zone attack.
- Industrial Furnace Components: Radiant tubes, muffles, and retort components in heat treatment furnaces operating at continuous temperatures in the 700-900°C range.
- Aerospace Structural Tubes: High-temperature structural applications in aircraft engine nacelles and exhaust systems where long-duration creep loading is combined with oxidising atmospheres.