SA213 T22 vs T91 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
SA213 T22 and T91 are two widely used alloy-steel tube grades in power generation, petrochemical, and high-temperature industrial services. Engineers and procurement professionals often face a selection dilemma between the two: balancing elevated-temperature strength and long-term creep performance against weldability, cost, and fabrication ease. Typical decision contexts include choosing materials for boiler and heat-exchanger tubing, piping in steam systems, or replacement components in pressure-retaining assemblies.
The principal distinction between these grades is their alloying strategy and resulting microstructure: T22 is a lower-alloy chromium‑molybdenum steel designed for moderate high-temperature strength and good fabricability, while T91 is a martensitic, high-chromium, microalloyed steel engineered for substantially higher creep- and yield-strength at elevated temperature. This difference drives most downstream choices in design, welding practice, and lifecycle cost.
1. Standards and Designations
- ASTM/ASME:
- SA213 T22 — ASTM A213 / ASME SA213 (seamless ferritic alloy-steel boiler, superheater, and heat-exchanger tubes)
- SA213 T91 — ASTM A213 / ASME SA213 (seamless ferritic alloy-steel high-temperature service tubes)
- Other standards:
- EN/ISO equivalents are often specified under EN 10216-2 or EN 10222 (for similar alloyed steels); national standards (JIS, GB) provide comparable grades under different names.
- Classification:
- SA213 T22 — Low‑to‑moderate alloy ferritic steel (commonly called Cr–Mo alloy steel)
- SA213 T91 — High‑Cr martensitic alloy steel (tempered martensitic/HSLA-type for high‑temperature service)
2. Chemical Composition and Alloying Strategy
Table: Typical chemical composition ranges (wt%) for SA213 T22 and T91. These are representative ranges commonly referenced in industry specifications; exact limits depend on the standard and heat/product form.
| Element | SA213 T22 (typical range) | SA213 T91 (typical range) |
|---|---|---|
| C | 0.05 – 0.15 | 0.08 – 0.12 |
| Mn | 0.30 – 0.60 | 0.40 – 0.60 |
| Si | 0.10 – 0.50 | 0.20 – 0.60 |
| P | ≤ 0.025 (max) | ≤ 0.020 (max) |
| S | ≤ 0.025 (max) | ≤ 0.010 (max) |
| Cr | 1.8 – 2.3 | 8.0 – 9.5 |
| Ni | ≤ 0.40 (trace) | ≤ 0.40 (trace) |
| Mo | 0.40 – 0.70 | 0.85 – 1.05 |
| V | trace – 0.05 | 0.18 – 0.25 |
| Nb (Cb) | trace – 0.05 | 0.06 – 0.12 |
| Ti | — (typically low) | — (typically low) |
| B | — (typically not specified) | — (often controlled at very low ppm) |
| N | — (low) | 0.03 – 0.07 |
How alloying affects properties - Chromium and molybdenum: Both Cr and Mo improve strength at elevated temperature and oxidation resistance. T22's modest Cr/Mo delivers moderate creep performance; T91's high Cr and Mo, combined with microalloying, yield substantially higher creep resistance. - Carbon: Higher carbon in T91 supports martensite formation and tempering response; T22 has lower carbon to maintain ductility and weldability. - Microalloying (V, Nb): Present in T91 to stabilize carbides/nitrides, refine grain size, and improve creep-strength and resistance to softening during long-term exposure. - Silicon and manganese: Deoxidation and solid-solution strengthening; also affect hardenability and toughness.
3. Microstructure and Heat Treatment Response
- SA213 T22:
- Typical microstructure after standard heat treatment: tempered ferrite with pearlite/tempered bainitic constituents, depending on cooling rate. It is not a fully martensitic steel.
- Heat-treatment response: normalization and tempering or stress-relief can adjust toughness and strength. It is less responsive to martensitic hardening than T91; heavy quenching is not typically required or used.
- SA213 T91:
- Typical microstructure: quenched and tempered martensite (tempered lath martensite) with fine carbides and carbonitrides (V/Nb/Ti rich) after proper normalization and tempering.
- Heat-treatment response: requires controlled normalization and tempering to develop intended microstructure. Thermo‑mechanical processing and precise tempering are important to achieve grain refinement and desired creep resistance.
- Effects of processing:
- Normalizing refines prior austenite grain structure in T91 and is an essential step before tempering.
- Quenching and tempering (Q&T) for T91 yields high strength and elevated-temperature stability; over-tempering reduces strength but improves toughness.
- T22 relies more on controlled cooling and tempering to balance ductility and strength; it is less sensitive to quench rates.
4. Mechanical Properties
Table: Typical mechanical-property ranges for normalized and tempered or commonly supplied conditions. Values depend on heat treatment, product form, and specification.
| Property | SA213 T22 (typical, N&T or as-supplied) | SA213 T91 (typical, normalized & tempered) |
|---|---|---|
| Tensile strength (MPa) | 400 – 600 | 550 – 800 |
| Yield strength (0.2% offset, MPa) | 200 – 350 | 400 – 650 |
| Elongation (%) | 18 – 30 | 12 – 20 |
| Impact toughness (Charpy V, J, RT) | 20 – 60 (varies with thickness) | 30 – 100 (temper + HT dependent) |
| Hardness (HB) | ~150 – 230 | ~200 – 300 |
Interpretation - Strength: T91 is clearly the stronger grade in tensile and yield strength, especially at elevated temperatures and for long-term creep resistance, due to martensitic structure and microalloying. - Toughness and ductility: T22 usually offers more ductility and easier plastic deformation; T91 provides a strong combination of strength and adequate toughness when properly normalized and tempered but is less ductile. - Choice implications: For high-pressure, high-temperature service where creep matters, T91 is favored. For moderate temperatures, where easier fabrication and cost are priorities, T22 remains competitive.
5. Weldability
Weldability depends on carbon equivalent and hardenability; both grades require attention but T91 is more demanding.
Common welding indices: - Carbon equivalent (IIW-type): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - More comprehensive parameter: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation - SA213 T22: Lower overall hardenability and lower carbon equivalent than T91 in most cases. It is easier to weld with standard filler metals; preheat and post-weld heat treatment (PWHT) are recommended for pressure-retaining joints to reduce residual stresses and restore properties, but crack risk is lower than T91. - SA213 T91: Higher hardenability (due to higher Cr, Mo, and microalloying) leads to a greater risk of martensite formation in HAZ, hydrogen-assisted cold cracking, and brittle microstructures if not properly controlled. Welding T91 typically requires strict preheat, controlled interpass temperature, and a full PWHT per code requirements; qualified welding procedures and matching filler metals are essential. - Practical note: For mixed-metal welds (e.g., joining T91 to low-alloy steels), special transition procedures and qualified WPS/PQRs are required.
6. Corrosion and Surface Protection
- Neither SA213 T22 nor T91 is stainless; both are susceptible to general corrosion in wet environments and oxidation at elevated temperatures depending on service environment.
- General protection strategies:
- Protective coatings (painting), cladding, or linings for corrosive media.
- Hot-dip galvanizing is possible for some fabricated components but is uncommon for high-temperature tubes.
- For high-temperature oxidation resistance, alloy composition matters: higher Cr in T91 gives improved scaling resistance in oxidizing steam environments compared with lower-Cr T22, but oxidation resistance is still inferior to stainless grades.
- PREN (pitting resistance equivalent number) is not applicable to non-stainless Cr–Mo steels, but for reference the formula for stainless alloys is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ — this index does not meaningfully apply to T22 or T91 because localized corrosion resistance and passive behavior require much higher chromium and nickel content.
7. Fabrication, Machinability, and Formability
- Machinability:
- T22 machines reasonably well in annealed or normalized condition; machinability is moderate.
- T91, with higher strength and work-hardening tendency, is more difficult to machine and requires robust tooling, lower cutting speeds, and attention to heat generation.
- Formability and bending:
- T22 exhibits better cold-forming and bending characteristics due to lower yield strength and higher ductility.
- T91 is less suited to extensive cold forming; forming is typically done in controlled, often warm, conditions after appropriate heat treatment.
- Surface finish:
- Both can be finished to high tolerances, but T91 requires slower, more controlled processes to avoid hardening or introducing defects.
8. Typical Applications
Table: Typical uses for each grade
| SA213 T22 — Typical Uses | SA213 T91 — Typical Uses |
|---|---|
| Boiler and superheater tubes for moderate-temperature steam systems | High-pressure steam pipes, headers, and tubes in power plants requiring high creep strength |
| Heat-exchanger tubing and reheater components in mid-temperature ranges | Components exposed to 500–650°C where long-term strength is critical |
| General pressure tubing where moderate strength and good weldability are needed | Petrochemical cracking units, high-temperature pipelines, power-plant main steam lines |
| Economical replacement parts and service piping | Applications where design life and creep resistance justify higher initial material cost |
Selection rationale - Choose T22 for moderate-temperature service with priority on lower material cost, easier welding, and higher formability. - Choose T91 for higher-temperature, high-stress applications where long-term creep performance, higher yield strength, and better stability at elevated temperatures are required.
9. Cost and Availability
- Relative cost:
- T91 is typically more expensive per kilogram than T22 due to higher alloy content (Cr, Mo) and added microalloying elements, and because T91 often requires more rigorous heat treatment and processing controls.
- Availability:
- Both grades are widely available in major markets in tube, pipe, and bar forms, but product lead times for T91 can be longer for specialized sizes and heat-treated conditions.
- Inventory and local supply tend to be better for T22 because it has longer-established use for moderate-temperature boiler components.
10. Summary and Recommendation
Table: Quick summary
| Characteristic | SA213 T22 | SA213 T91 |
|---|---|---|
| Weldability | Good — easier pre/post-weld procedures | Demanding — strict preheat & PWHT required |
| Strength–Toughness (elevated temp) | Moderate strength; good ductility | High strength and creep resistance; engineered toughness |
| Cost | Lower initial material cost | Higher material and processing cost |
Conclusions and recommendations - Choose SA213 T22 if: - Service temperatures and stresses are moderate (design envelopes where T22 meets allowable stresses). - Fabrication speed, ease of welding, lower initial cost, and formability are priorities. - You require a widely available, economical tube for mid-temperature heat exchangers or boilers where long-term creep is not the governing design driver. - Choose SA213 T91 if: - The application demands high creep strength and high yield strength at elevated temperatures (e.g., main steam lines, headers, components operating near 550–650°C). - Long-term life, reduced thickness for weight savings, or higher allowable stress at temperature justify the higher material and processing cost. - The project can accommodate stricter welding controls, PWHT, and qualified procedures.
Final note: Material selection should always be validated by detailed stress, creep, and corrosion analyses and reviewed against applicable codes (ASME, EN, local regulations). Consult material suppliers and welding engineers early to define heat-treatment, welding procedures, and inspection criteria to match service conditions and expected lifecycle.