ASTM A213 Alloy Tubes: T11, T22 and T91 Grades, Heat Treatment and Ordering

Quick Answer

ASTM A213 covers seamless ferritic and austenitic alloy-steel tubes for boilers, superheaters and heat exchangers. Its "T" grades are tubes, sized by outside diameter and wall thickness: usually 3.2 mm ID to 127 mm (5 in) OD and 0.4–12.7 mm wall, supplied to minimum wall unless the order calls for average wall. The matching "P" grades of ASTM A335 are pipe, sized by NPS and schedule. T11 (1.00–1.50% Cr, 0.44–0.65% Mo) and T22 (1.90–2.60% Cr, 0.87–1.13% Mo) are annealed or normalised and tempered, with 415 MPa minimum tensile strength, 205 MPa minimum yield and 163 HBW maximum hardness. T91 (8.0–9.5% Cr, 0.85–1.05% Mo, plus vanadium, niobium and nitrogen) is a creep-strength-enhanced ferritic steel, normalised at 1040–1080 °C and tempered at 730–800 °C, with 585 MPa tensile, 415 MPa yield and a hardness window of 190–250 HBW. Its strength depends on a tempered martensite structure that wrong heat treatment or post-weld heat treatment can destroy. The austenitic H grades, such as TP304H and TP347H, hold carbon at 0.04–0.10% for higher creep-rupture strength. The standard calls for tension, hardness, flattening and flaring tests, and a hydrostatic or nondestructive electric test of each tube.

A typical enquiry reads: "T22 tube, 2 inch, 4 mm, 10 tonnes." Each item has two readings. "2 inch" may be a tube of 50.8 mm OD or NPS 2 pipe of 60.3 mm OD. "4 mm" may be minimum or average wall. And "T22" may have been copied from a drawing that meant P22 pipe. Each wrong guess gives a tube that does not fit the header, fails the pressure calculation or arrives under the wrong specification.

Superheater and reheater tubes are among the thinnest and hottest pressure parts in a boiler. This guide covers how the A213 grades differ, why T91 needs more care than T11 and T22, and how to order tubes that match the design.

What ASTM A213 covers

ASTM A213 covers seamless ferritic and austenitic steel boiler, superheater and heat-exchanger tubes, designated T5, TP304 and so on. The current edition is A213/A213M-25, and ASME adopts it as SA-213. Its main points:

• Manufacture: seamless, hot finished or cold finished; TP347HFG must be cold finished
• Sizes: usually 3.2 mm (1/8 in) ID to 127 mm (5 in) OD, with walls of 0.4–12.7 mm (0.015–0.500 in); other diameters are allowed if the tubes meet every other requirement
• Wall basis: minimum wall, or average wall if the order says so
• Heat treatment: a separate treatment in addition to any heating for hot forming; ferritic grades are reheated, austenitic grades are furnished heat treated
• H grades: grades with an H in their name have different requirements that give higher creep-rupture strength
• General requirements: tolerances and test methods come from ASTM A1016/A1016M

For the wider choice of tube materials, including copper alloys and titanium, see our heat exchanger tube materials guide. Pipe for the other end of the temperature range is covered in our guide to ASTM A333 low-temperature pipe.

T grades and P grades: tube versus pipe

The alloy families line up: T11 with P11, T22 with P22, T91 with P91, so a P22 header is often paired with T22 tubes, though each tube's grade is set by its own metal temperature. They are still different specifications, with their own sizing, tests and tolerances. A P-grade pipe certificate does not cover a tube, and a tube certificate does not cover pipe.

Size is the most common confusion. A 2 in tube is 50.8 mm OD; NPS 2 pipe is 60.3 mm OD. The 9.5 mm difference means the tube will not fit the hole in the header, or the swaged end will not match.

The ferritic grades and their chemistry

Chemistry is from A213/A213M-19; check the edition on your order. The same alloy families come as plate under ASTM A387; see our guide to chrome-moly plate.

What the elements do. Chromium raises resistance to oxidation and steam-side scaling; molybdenum raises creep strength, the resistance to slow deformation under sustained stress at temperature. In T91 and T92, vanadium and niobium form fine carbonitride precipitates that hold the tempered martensite structure together at temperature, which is where their extra creep strength comes from.

Properties of T11, T22 and T91

T11 and T22 have the same room-temperature minimums, so the table hides their real difference. With more chromium and molybdenum, T22 keeps more strength and resists oxidation better at temperature. That difference shows up in the allowable stresses for the design temperature in ASME Section II, Part D, which is where grades should be compared.

T91 is in another class: its minimum yield is about double that of T11 and T22, and it has both a maximum and a minimum hardness. The minimum is the more telling of the two.

T91: why heat treatment and PWHT decide its life

T91 is a creep-strength-enhanced ferritic (CSEF) steel. Normalising at 1040–1080 °C and cooling produces martensite; tempering at 730–800 °C softens it and precipitates the fine carbides and carbonitrides that carry the creep strength. Lose that structure and the tube keeps its chemistry but not its strength. US research on these steels has reported numerous failures of CSEF components after only a few years in service, partly where fabrication or repair failed to produce the required microstructure.

The structure can be lost in three ways:

1. Wrong normalising or tempering. The tube comes out soft and weak in creep. Hardness is the quick check: a T91 tube below 190 HBW is a warning that it has not reached the intended structure or has been over-tempered. Published reviews link incorrect heat treatment of Grade 91 to poor creep performance.
2. Crossing the lower critical temperature (Ac1). If tempering, post-weld heat treatment (PWHT) or local heating goes above Ac1, new austenite forms and turns into untempered martensite on cooling, and material heated between Ac1 and Ac3 has shown poor creep strength. A thermodynamic calculation by Oak Ridge National Laboratory put the equilibrium lower critical temperature (A1) across the Grade 91 composition range (calculated for the plate specification) anywhere from about 766 °C to 856 °C, so a heat at the low end has no margin over the top of the 730–800 °C tempering range. Nickel and manganese are austenite stabilisers, and T91 Type 2 holds them tighter than Type 1: nickel 0.20% maximum against 0.40%, manganese 0.30–0.50% against 0.30–0.60%.
3. Type IV cracking in welds. This creep cracking forms in the fine-grained or intercritical zone at the outer edge of the weld heat-affected zone. Studies show the creep strength of such joints can fall by as much as about 50%, and failures typically appear after 6–10 years of service, some as early as 35,000 hours. Early cracking has been seen in heats with low nitrogen-to-aluminium ratios; Type 2 sets a minimum N/Al ratio of 4.0, along with limits on copper, tin, antimony and arsenic.

PWHT. Grade 91 welds are post-weld heat treated within a temperature range set by the construction code; check the current edition of ASME Section I or B31.1 for the range and any exemptions. Use welding procedures qualified for Grade 91, not borrowed from T22.

What the buyer controls: heat-treatment charts and hardness results for each lot; positive material identification (PMI); the T91 type where the project specifies one; and a rule that no tube is hot formed or locally heated without an approved heat-treatment step.

Austenitic grades in brief: TP304H and TP347H

Where metal temperatures or steam-side oxidation are beyond the ferritic grades, the hottest superheater and reheater sections use austenitic stainless tubes:

• TP304H (S30409): 18.0–20.0% Cr, 8.0–11.0% Ni, carbon 0.04–0.10%
• TP347H (S34709): 17.0–19.0% Cr, 9.0–13.0% Ni, carbon 0.04–0.10%, niobium from 8 × C up to 1.10%

The carbon range is part of what makes them H grades, with the higher creep-rupture strength the standard describes. The fine-grained TP347HFG must be cold finished. Austenitic tubes expand more than ferritic ones and join them through dissimilar-metal welds, which need their own procedures. For the general grades see our stainless steel pipe guide, and for oxidation-resistant grades, our guide to heat-resistant stainless steels.

Minimum wall versus average wall

A213 tubes are supplied to minimum wall unless the order calls for average wall:

• Minimum wall: no point on the tube may be thinner than the figure ordered, so all the tolerance is on the plus side
• Average wall: the wall varies above and below the figure ordered, within the A1016/A1016M tolerances

This matters for design. The ASME Section I tube formula gives a minimum required thickness. On a minimum-wall order that figure can be ordered as it stands; on an average-wall order the permitted under-tolerance has to be added. It also matters for price: a minimum-wall tube averages thicker than its nominal figure, so it weighs more per metre than a calculation on the minimum wall suggests.

Tests and inspection

A213 calls for tension, hardness, flattening and flaring tests, plus a hydrostatic test or a nondestructive electric test on each tube. The flattening and flaring tests check that the tube can deform without cracking, the property that bending, expanding and swaging rely on. Hardness confirms the heat treatment; for T91 it has to fall inside a window, not just under a ceiling.

Buyers often add PMI of every tube, because T11, T22 and T91 look identical; heat-treatment records; third-party inspection; and EN 10204 3.1 or 3.2 certificates (see our guide to material test certificates).

Worked example: superheater tube wall and weight

A typical check: a superheater tube of 50.8 mm (2 in) OD, strength-welded to its headers, at a design pressure of 17.0 MPa (170 bar, about 2,470 psi). The proposal is 5.0 mm minimum wall.

ASME Section I (paragraph PG-27.2.1) gives the thickness of tubes up to and including 125 mm (5 in) OD as:

t = PD ÷ (2Sw + P) + 0.005D + e

Here S is the allowable stress at the design metal temperature, from Section II, Part D, w is the weld joint strength reduction factor (1.0 for seamless tubes), and e is the thickness factor for expanded tube ends: 0.04 in (about 1 mm) for expanded tubes and zero for strength-welded tubes. Check the formula and its notes in the current edition.

Step 1: the allowable stress 5.0 mm needs. With w = 1.0 for a seamless tube, rearranged, S = P(D − t′) ÷ (2t′), with t′ = t − 0.005D − e.

• t′ = 5.0 − 0.005 × 50.8 − 0 = 5.0 − 0.254 = 4.746 mm
• S = 17.0 × (50.8 − 4.746) ÷ (2 × 4.746) = 17.0 × 46.054 ÷ 9.492 = 82.5 MPa (12.0 ksi)

The grade's allowable stress at the design metal temperature must be at least 82.5 MPa. Look up each candidate grade at that temperature in Section II, Part D.

Step 2: the wall if the allowable stress is half. With S = 41.2 MPa:

• t = 17.0 × 50.8 ÷ (2 × 41.2 + 17.0) + 0.254 = 863.6 ÷ 99.4 + 0.254 = 8.94 mm

Halving the allowable stress raises the required wall by 79%.

Step 3: weight. Mass per metre = 0.02466 × (D − t) × t, for steel at 7.85 kg/dm³:

• 5.0 mm wall: 0.02466 × 45.8 × 5.0 = 5.65 kg/m (3.8 lb/ft)
• 8.94 mm wall: 0.02466 × 41.86 × 8.94 = 9.23 kg/m, 63% heavier
• For 4,000 m of tubing: 22.6 t against 36.9 t

What it shows. A higher allowable stress at temperature buys a thinner wall: less steel, less weight, and a smaller temperature difference through the wall for the same heat flux. That is the case for creep-strength-enhanced grades in the hottest sections, provided their heat treatment is right. Because A213 tubes default to minimum wall, the 5.0 mm result can be ordered directly as "5.0 mm minimum wall".

Common Mistakes

1. Ordering "2 inch" without saying OD. A 2 in tube is 50.8 mm OD; NPS 2 pipe is 60.3 mm OD.

   Prevention: Give tube sizes as OD × wall, in millimetres or inches.

2. Ordering average wall when the design gives a minimum. Parts of the delivered wall may be thinner than the calculation requires.

   Prevention: Order minimum wall, or add the A1016 under-tolerance to an average-wall figure.

3. Treating T and P grades as interchangeable. A213 tube and A335 pipe differ in sizing, tests and tolerances.

   Prevention: Order tubes to A213 and pipe to A335, even when the alloy is the same.

4. Accepting T91 without heat-treatment records and hardness results. A soft tube can pass a visual check and still lack creep strength.

   Prevention: Require heat-treatment charts and hardness within 190–250 HBW for each lot.

5. Overheating Grade 91 in PWHT or repair. Crossing Ac1 forms untempered martensite.

   Prevention: Keep to the code PWHT range, place thermocouples carefully, and treat any local heating as a heat-treatment step.

6. Mixing grades in the store. T11, T22 and T91 tubes look identical.

   Prevention: PMI every tube on receipt and keep heat numbers traceable.

7. Welding T91 like T22. Preheat, filler metal and PWHT requirements differ.

   Prevention: Use procedures qualified for Grade 91.

8. Leaving the T91 type unstated. Type 2 tightens residual elements and adds a minimum nitrogen-to-aluminium ratio.

   Prevention: State Type 1 or Type 2 where the project or code requires it.

Ordering checklist

• Specification: ASTM A213 or A213M; ASME SA-213 where the code calls for it
• Grade: and, for T91, the type
• Size: OD and wall, stating minimum or average wall
• Finish: hot finished or cold finished
• Lengths and quantity: plus any bends, fins or other fabrication
• Design data: pressure and metal temperature, so the wall can be checked
• Tests: hydrostatic or nondestructive electric test, hardness limits, PMI, supplementary requirements
• Documents: heat-treatment records, EN 10204 3.1 or 3.2 certificates, third-party inspection
• Delivery: end protection, marking, packing, destination and Incoterm

Supply from Kasko Makine

Kasko Makine supplies ASTM A213 / ASME SA213 seamless alloy tubes:

• T5, T5b, T9, T11, T12, T22 and T91 — seamless, hot or cold finished as specified, heat treated separately from hot forming; tension, hardness, flattening and flaring tests, with a nondestructive electric or hydrostatic test on each tube
• Sizes: the product pages give the A213 range of 3.2 mm (1/8 in) ID to 127 mm (5 in) OD and 0.4–12.7 mm wall, with other sizes on request. The T5 page lists a typical supply range of 5/8 in to 1-1/2 in OD, 0.049–0.135 in wall and 6–60 ft lengths, with bending and finning on request
• Delivery condition: annealed, on the T9, T11 and T22 pages
• T91 tube applications listed: power generation, boilers and superheaters, heat exchangers, oil and gas processing, and fertiliser and chemical plants
• T91 seamless pipes — A335 P5, P9, P11, P22 and P91 pipe from 1/2 in NB to 36 in NB, and A213 T5, T9, T12, T22, T91 and T92 tubes; the page notes supply from stock or at short delivery, depending on size and grade
• ASTM A213 alloy tubes — seamless ferritic and austenitic alloy-steel tubes for boilers, superheaters and heat exchangers

Engineering support: Send the tube schedule (OD, wall basis, grade, lengths and quantities) with the design pressure and metal temperature. We will check grade and wall against your data and quote with the tests and documents you need.

Send the tube OD, wall, grade, length and quantity, with the design pressure and metal temperature, for a quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Alloy Steel Pipe (ASTM A335): Chrome-Moly Grades P11, P22 and P91
• Heat Exchanger Tube Materials: Selection Guide
• Industrial Boilers and Steam Systems: Types and Selection
• WPS, PQR and Welder Qualification: ASME Section IX
• Stainless Steel Pipe: 304, 316 and ASTM A312
• Material Test Certificates: EN 10204 3.1 vs 3.2

Frequently Asked Questions

Q: What is ASTM A213?
A: ASTM A213 is the specification for seamless ferritic and austenitic alloy-steel tubes for boilers, superheaters and heat exchangers. Grades are designated T5, T11, T22, T91, TP304H and so on. Tubes are usually 3.2 mm inside diameter to 127 mm outside diameter, with walls of 0.4 to 12.7 mm, and are supplied to minimum wall unless the order specifies average wall. ASME adopts the specification as SA-213.

Q: What is the difference between A213 T22 and A335 P22?
A: Both are 2.25 percent chromium, 1 percent molybdenum steels, but T22 is a tube made to ASTM A213 and sized by outside diameter and wall thickness, usually minimum wall. P22 is a pipe made to ASTM A335 and sized by NPS and schedule. They have separate tests and tolerances, so a pipe certificate does not cover tube. A 2 inch tube is 50.8 mm OD; NPS 2 pipe is 60.3 mm OD.

Q: What is the difference between T11, T22 and T91 tubes?
A: T11 has about 1.25 percent chromium and 0.5 percent molybdenum, and T22 about 2.25 percent chromium and 1 percent molybdenum; both have a 415 MPa minimum tensile strength and 163 HBW maximum hardness. T91 is a 9 percent chromium, 1 percent molybdenum steel with vanadium, niobium and nitrogen. It is normalised and tempered, with 585 MPa tensile strength, and offers much higher creep strength at superheater temperatures.

Q: Why is heat treatment so important for T91 tubes?
A: T91 gets its creep strength from tempered martensite with fine carbide and carbonitride precipitates, produced by normalising at 1040 to 1080 °C and tempering at 730 to 800 °C. Wrong tempering leaves it soft, and overheating above its lower critical temperature during tempering, PWHT or repair forms untempered martensite. A213 limits delivered tubes to 190 to 250 HBW, catching mill faults; after PWHT or repair, a hardness check against the code or project limit does the same job.

Q: What does minimum wall mean for A213 tubes?
A: Minimum wall means no point on the tube may be thinner than the wall ordered, so all the thickness tolerance is on the plus side. A213 tubes are supplied this way unless the order calls for average wall, where the wall can vary above and below the figure. The ASME Section I formula gives a minimum required thickness, which can be ordered directly as minimum wall.

Q: What tests does ASTM A213 require?
A: ASTM A213 requires tension, hardness, flattening and flaring tests, and each tube must pass either a hydrostatic test or a nondestructive electric test. Flattening and flaring show that the tube deforms without cracking, and hardness confirms the heat treatment. Buyers often add positive material identification of every tube, heat-treatment records, third-party inspection and EN 10204 3.1 or 3.2 certificates.

Q: What do the H grades such as TP304H and TP347H mean?
A: The H grades are austenitic stainless steels with requirements that give higher creep-rupture strength than the matching standard grades, including a carbon range of 0.04 to 0.10 percent. TP304H is an 18 percent chromium, 8 percent nickel steel; TP347H adds niobium. They are used in the hottest superheater and reheater sections, where ferritic grades are no longer suitable.