ASTM A213 Alloy Steel Tubes: T11, T22, T91 and T92 for High-Temperature Service

When steam temperature exceeds 450°C, carbon steel boiler tubes approach their creep limit, and alloy steel becomes necessary. ASTM A213 chrome-moly grades — from the well-established T11 and T22 to the high-performance T91 and T92 — extend the operating envelope of steam boilers into the supercritical and ultra-supercritical range. Understanding the metallurgical differences between these grades is essential for selecting the correct tube grade and specifying the heat treatment and fabrication requirements correctly.

ZC Steel Pipe supplies ASTM A213 seamless alloy steel boiler tubes in grades T11, T22, T5, T9, T91, and T92 for power generation, industrial boilers, and process heat recovery applications. This guide covers chemistry, mechanical properties, creep performance, heat treatment, and fabrication considerations for each grade.

1. Grade Overview

ASTM A213 covers multiple tube grades. The chrome-moly ferritic grades most relevant to power boilers are:

The T-prefix indicates tube product form (as opposed to P-prefix for pipe — e.g. P91 is the equivalent pipe grade under ASTM A335).

2. Chemistry Requirements

T11 (1.25Cr-0.5Mo-Si)

T11 is one of the most widely used grades in economisers and low-temperature superheaters. Its silicon range (0.50–1.00%) is higher than other chrome-moly grades and contributes to oxidation resistance.

T22 (2.25Cr-1Mo)

T22 was the dominant superheater tube grade for utility boilers for decades. It has good creep resistance up to 565°C and is still widely used in older boiler designs and for replacement tubes.

T91 (9Cr-1Mo-V)

T91 is the most technically demanding grade in the standard. The tight aluminium limit (≤0.02%) is critical — aluminium combines preferentially with nitrogen to form AlN, making nitrogen unavailable to stabilise the fine carbides and nitrides that provide T91's creep resistance. Always verify Al on the MTC.

3. Mechanical Properties

The higher hardness limit for T91 and T92 (250 HB max) reflects their martensitic microstructure, which is significantly harder than the ferritic/bainitic microstructure of T11 and T22.

For SA-192, SA-209, SA-210, and SA-213 mechanical properties and tube dimensions, see ASME Section I allowable stress tables (current edition).

To convert between MPa/ksi, mm/inches, and bar/psi, use the Unit Converter →

4. High-Temperature Creep Strength

The primary selection criterion for boiler tube grades at temperatures above 500°C is maximum allowable stress (ASME Section I). The following values are approximate for comparison — always use the current edition of ASME Section I for design:

T91 provides approximately 60–70% more allowable stress than T22 at 600°C, enabling significantly thinner walls and lighter tube banks in supercritical boilers. T92 adds a further 15–20% improvement over T91 at ultra-supercritical conditions.

Wall Thickness Comparison — T22 vs T91 at 580°C

For a superheater tube: OD = 44.5 mm, operating pressure = 10.5 MPa (105 bar), temperature = 580°C

Step 1 — ASME Section I allowable stress at 580°C (approximate, use current ASME edition):

• T22 at 580°C: S ≈ 52 MPa
• T91 at 580°C: S ≈ 90 MPa

Step 2 — Required minimum wall thickness (Lame thin-wall: t = PD / 2S):

• T22: t = (10.5 × 44.5) / (2 × 52) = 467.25 / 104 = 4.49 mm
• T91: t = (10.5 × 44.5) / (2 × 90) = 467.25 / 180 = 2.60 mm

Step 3 — Add mill tolerance (−0% minimum wall, specify to next commercial size):

• T22: specify 4.5 mm minimum wall (Sch equivalent varies by OD standard)
• T91: specify 2.8 mm minimum wall (thinner, lighter tube bank)

Step 4 — Weight comparison per metre of tube:

• T22 at 4.5 mm WT: approximately 4.6 kg/m
• T91 at 2.8 mm WT: approximately 2.9 kg/m
• Weight saving with T91: 37% per metre

Conclusion: T91 allows a 37% wall and weight reduction vs T22 at 580°C. For a superheater bank of 5,000 tube-metres, this is approximately 8.5 tonnes of tube weight saved — directly reducing thermal mass and startup time, in addition to material cost.

5. Heat Treatment Requirements

T11 and T22

Both grades are supplied in the full annealed, isothermal annealed, or normalised and tempered condition. Typical heat treatment:

• Annealing: 760–790°C, furnace cool
• Normalising: 900–955°C, air cool; followed by tempering at ≥620°C

T91 — Critical Requirements

T91 must be normalised and tempered. Incorrect heat treatment is the leading cause of T91 service failures.

Normalising: 1040–1080°C, air cool (achieves fully martensitic microstructure)

Tempering: ≥730°C. The tempering temperature must produce a tempered martensitic microstructure. Under-tempering (at temperatures below 730°C) produces brittle martensite. Over-tempering (above 780°C) can result in partial re-austenitisation during welding and a loss of creep properties.

Post-Weld Heat Treatment (PWHT): 730–760°C for a minimum hold time of 1 hour per 25 mm of thickness (minimum 1 hour). PWHT is mandatory for T91 — welding without PWHT produces a hardened HAZ that is susceptible to hydrogen embrittlement and creep damage.

T92

T92 requires normalising at 1040–1080°C and tempering at ≥730°C, similar to T91. However, T92 is more sensitive to the exact tempering temperature and cooling rate. Project specifications for T92 should include detailed PWHT requirements and hold times.

6. Welding Considerations

For T91 and T92, maintaining preheat throughout the weld, controlling interpass temperature, and not allowing the weld to cool below the preheat temperature before PWHT are critical. Consult the current AWS D1.1 and ASME Section IX requirements and the specific project weld procedure specification (WPS).

7. Specification Comparison — Tube vs Pipe

ASTM A213 covers tubes (T-grades). The equivalent pipe grades (P-grades) under ASTM A335 have the same chemistry requirements but different mechanical testing due to the larger diameter and heavier wall:

When specifying materials for a complete boiler system, ensure tube and pipe grades match to avoid galvanic or metallurgical mismatch at weld joints.

When NOT to Use T22 — And When T91 Is Required

Procurement trap — T91 aluminium content not checked on MTC:

Wrong PO: "ASTM A213 T91 seamless alloy steel tubes, 44.5 mm OD × 6.0 mm WT, normalised and tempered, MTC EN 10204 3.1."

What ships: T91 tubes with all mechanical properties within specification. The MTC shows heat analysis with Al = 0.035% — exceeding the ≤0.02% limit. The receiving inspector checks tensile (585 MPa ✓), yield (415 MPa ✓), elongation (21% ✓), hardness (228 HB ✓). All pass. Al content on the MTC is not checked. Tubes are installed. Type IV creep failure occurs at 19,000 hours.

Correct PO: "ASTM A213 T91 seamless alloy steel tubes, 44.5 mm OD × 6.0 mm WT, normalised and tempered, UNS K91560. MTC EN 10204 3.1. Heat analysis must show Al ≤0.02% — orders with Al >0.02% will be rejected. MTC reviewer must explicitly sign off Al content before acceptance."

Failure Modes

Failure Mode 1 — T91 with excess aluminium — premature Type IV creep

Mechanism: T91 tube heat contains Al = 0.035%, double the ≤0.02% limit. AlN forms in preference to MX carbonitrides. The fine MX dispersion that resists dislocation movement at 580–620°C service temperature is absent or severely degraded. Under normal operating stress, creep deformation proceeds at a rate 5–10× higher than design. Type IV cracking initiates at the heat-affected zone of tube-to-header welds at 15,000–25,000 hours — well below the expected design life of 100,000+ hours.

Diagnostic: Superheater tube failure at tube-to-header weld at service life far below design. Metallurgical analysis shows lack of MX carbonitride dispersion in the microstructure. Heat analysis retrieval from MTC confirms Al above the ≤0.02% specification limit.

Fix: Reject T91 deliveries with Al >0.02% on the heat analysis. Issue MTC review checklists that include an explicit Al sign-off line. For installed tubes that cannot be replaced, commission a detailed remaining life assessment using elevated-temperature creep testing of representative samples.

Failure Mode 2 — T91 welded without PWHT → brittle HAZ

Mechanism: T91 is welded using the correct filler (E9015-B9) and preheat (220°C). Due to schedule pressure, PWHT is deferred. The weld is pressure-tested and the system is commissioned without PWHT. The HAZ microstructure is untempered martensite — hardness 38–42 HRC. Under the combination of residual welding stress and internal pressure at operating temperature, hydrogen embrittlement or brittle fracture initiates at the HAZ within the first thermal cycle.

Diagnostic: Weld joint failure at the HAZ shortly after commissioning — typically within the first startup/shutdown cycle. Metallurgical examination shows untempered martensite in the HAZ. PWHT records confirm PWHT was not performed.

Fix: PWHT at 730–760°C is mandatory for all T91 welds — no exception for schedule. Implement a hold-and-inspect gate that prevents system pressure testing until the PWHT completion certificate is signed. For stainless-lined systems, use induction heating or furnace PWHT with calibrated temperature records.

Failure Mode 3 — T11/T22 grade mix from unlabelled bundles

Mechanism: A project receives both T11 and T22 tubes. During unloading, bundle tags are lost or damaged. Visually, T11 and T22 are identical — same OD, wall, and surface finish. Without heat-number tracing, the bundles are mixed. T11 tubes (max temp 565°C) are installed in a reheater circuit operating at 580°C — a service condition requiring T22. ASME Section I allowable stress for T11 at 580°C is approximately 38 MPa vs 55 MPa for T22 — a 31% reduction. The T11 tubes are underdesigned for the operating pressure.

Diagnostic: ASME code compliance review during plant commissioning finds tubes in the 580°C reheater circuit cannot be positively identified as T22. Heat number traceability check fails — bundles were mixed. Hardness testing cannot distinguish T11 from T22 (both have similar hardness limits). Chemistry testing (portable XRF) on installed tubes shows Cr ~1.25% (T11) not ~2.25% (T22).

Fix: All A213 alloy tube deliveries must maintain bundle-level heat number traceability from receipt through installation. Require heat number stencilling or paint-marking on individual tube ends in addition to bundle tags. Specify in the PO that mixed-grade bundles will be rejected on delivery.

8. Procurement Checklist

1. Standard: ASTM A213 (latest edition)
2. Grade: T11 / T22 / T91 / T92 (specify grade explicitly)
3. OD and minimum wall thickness
4. Heat treatment condition: per standard or state specific requirement
5. Hydrostatic test: per standard, or specify NDET alternative
6. NDE: specify UT body scan or eddy current if required by design code or project specification
7. Mill test certificate: EN 10204 3.1 — must include heat analysis with Al content verified for T91/T92
8. Marking: per standard, including grade designation
9. PWHT: state requirements for site welding if fittings are pre-attached