Selecting the wrong boiler tube grade is not a procurement inconvenience — it is an engineering failure waiting to happen. Carbon steel tubes used above their creep limit will fail by microstructural degradation over years of service. Alloy steel tubes specified without proper PWHT can crack in the weld HAZ within months. And substituting one grade for a "similar" grade without checking the allowable stress tables in ASME BPVC Section II can void a pressure vessel certification.
ZC Steel Pipe manufactures seamless boiler tubes and heat exchanger tubes to ASTM A192, A210, A213, and ASME SA-106, in carbon steel and chrome-moly alloy grades. We supply boiler fabricators, power plant contractors, and petrochemical plant builders across Africa, the Middle East, and South America. This guide covers what each grade is, what it can and cannot do, and how to select between them for your application.
CONTENTS
Overview of Boiler Tube Standards
Carbon Steel Boiler Tubes — A192 and A210
Alloy Steel Boiler Tubes — ASTM A213
A213 T11 — 1¼Cr-½Mo
A213 T22 — 2¼Cr-1Mo
A213 T91 — 9Cr-1Mo-V (Modified)
SA-106 Grade B — Seamless Carbon Steel Pipe for Boiler Service
Full Grade Comparison Table
Boiler Zone Selection Guide
PWHT Requirements for Alloy Grades
FAQ
1. Overview of Boiler Tube Standards
KEY STANDARDS — BOILER TUBEASTM A192: Seamless carbon steel tubes for high-pressure boiler service — minimum wall, low carbon.
ASTM A210: Seamless medium-carbon steel boiler and superheater tubes — Grades A-1 and C.
ASTM A213: Seamless ferritic and austenitic alloy steel boiler, superheater, and heat exchanger tubes — covers T11, T22, T91, and many more grades.
ASME SA-106: Seamless carbon steel pipe for high-temperature service — Grades A, B, and C. The ASME BPVC equivalent of ASTM A106.
All four standards are seamless — boiler tube for heat transfer service is virtually never welded, as the weld seam represents a stress concentration and a potential corrosion initiation site in a cyclic thermal environment. The standards differ in alloy content, strength level, and the maximum temperature at which the grade retains adequate creep strength to be safe in sustained pressure service.
The dividing line in boiler tube selection is usually temperature. Carbon steel grades (A192, A210, SA-106) are appropriate for service up to approximately 450°C. Above that, the creep rate of carbon steel becomes unacceptably high and low-alloy grades (A213 T11, T22) must be used. Above approximately 580°C, advanced 9Cr grades (A213 T91) are required for modern supercritical and ultra-supercritical boiler designs.
2. Carbon Steel Boiler Tubes — A192 and A210
2.1 ASTM A192 — Minimum Wall Carbon Steel
ASTM A192 is the simplest and most economical boiler tube grade. It is a minimum-wall seamless carbon steel tube with a maximum carbon content of 0.18% and a minimum tensile strength of only 325 MPa (47 ksi). The low carbon content improves weldability and reduces the risk of carbide precipitation at grain boundaries during thermal cycling.
A192 is used for water tube boiler drums, mud drums, and lower-temperature water wall tubes in utility and industrial boilers operating below approximately 425°C. Its low strength ceiling means it is not appropriate for superheater or reheater service in modern high-pressure boilers.
2.2 ASTM A210 — Medium Carbon Steel Boiler Tube
ASTM A210 provides greater strength than A192 through slightly higher carbon and manganese content. It is produced in two grades:
Grade A-1: Minimum tensile 415 MPa (60 ksi), minimum yield 255 MPa (37 ksi). The more common grade for water wall, economiser, and lower superheater service.
Grade C: Minimum tensile 485 MPa (70 ksi), minimum yield 275 MPa (40 ksi). Higher strength for more demanding carbon steel boiler applications. Less commonly specified than A-1.
Field Note — A192 vs A210 in ProcurementIn practice, A192 and A210 Grade A-1 are often used interchangeably for economiser and lower water wall tube circuits in industrial boilers below 400°C. The difference in tensile strength means A210 can achieve the required wall thickness at a lower wall thickness than A192 — useful where tube weight matters. However, some older boiler designs specify A192 by name and substitution requires engineering approval and a code calculation. Never substitute grades on a boiler without verifying the allowable stress in ASME BPVC Section II-D and obtaining the required documentation.
3. Alloy Steel Boiler Tubes — ASTM A213
ASTM A213 covers a wide range of seamless ferritic (chromium-molybdenum) and austenitic (stainless) alloy steel tubes for boiler, superheater, and heat exchanger service. The ferritic grades are designated with a T prefix (T11, T22, T91), and the austenitic grades with TP (TP304, TP316, TP321 etc.).
The chrome-moly (CrMo) alloy system is the workhorse of high-temperature boiler tube metallurgy. Adding chromium improves oxidation resistance and high-temperature strength. Adding molybdenum improves creep strength. Increasing both — from T11 (1¼Cr-½Mo) to T22 (2¼Cr-1Mo) to T91 (9Cr-1Mo-V modified) — progressively extends the temperature ceiling and improves long-term creep life.
4. A213 T11 — 1¼Cr-½Mo
ASTM A213 T11
Alloy: 1¼Cr – ½Mo
Min tensile: 415 MPa (60 ksi)
Min yield: 205 MPa (30 ksi)
Max service temp: ~570°C continuous
ASME BPVC equiv: SA-213 T11
P-number (ASME): P4
T11 is the entry-level alloy grade for boiler tube service above the carbon steel temperature limit. It provides good oxidation resistance up to approximately 570°C and improved creep strength versus carbon steel in the 450–570°C range. T11 is commonly used in superheater outlet tubes, reheater inlet sections, and lower-temperature superheater tube circuits in subcritical and supercritical boilers.
T11 is generally weldable without preheat for thin walls but requires PWHT above certain thickness thresholds per ASME BPVC. Its 1¼Cr content makes it susceptible to temper embrittlement if cooled too slowly through the 370–565°C range during PWHT — furnace cooling rates must be controlled.
5. A213 T22 — 2¼Cr-1Mo
ASTM A213 T22
Alloy: 2¼Cr – 1Mo
Min tensile: 415 MPa (60 ksi)
Min yield: 205 MPa (30 ksi)
Max service temp: ~580°C continuous
ASME BPVC equiv: SA-213 T22
P-number (ASME): P5A
T22 (2¼Cr-1Mo) has been the workhorse alloy for high-temperature boiler superheater and reheater service for decades. Its higher chromium and molybdenum content versus T11 provides better creep resistance in the 540–580°C range and improved oxidation resistance for steam-side and fire-side exposure. T22 is used for high-temperature superheater outlet tubes, reheater outlet headers, and any boiler tube circuit operating in the upper subcritical temperature range.
Engineering Insight — T22 vs T11: When to UpgradeThe practical decision point between T11 and T22 is typically the tube metal temperature, not just the steam temperature. In a boiler superheater, tube metal temperature can be 30–80°C higher than the steam temperature inside, depending on heat flux and tube OD. If your steam outlet temperature is 540°C, tube metal may reach 600°C or above at peak load — placing it above T11's reliable creep range. The conservative approach is to use T22 for any superheater or reheater tube with an expected tube metal temperature above 540°C, not just those where steam temperatures exceed that threshold.
6. A213 T91 — 9Cr-1Mo-V (Modified)
ASTM A213 T91
Alloy: 9Cr – 1Mo – V (modified)
Min tensile: 585 MPa (85 ksi)
Min yield: 415 MPa (60 ksi)
Max service temp: ~650°C continuous
ASME BPVC equiv: SA-213 T91
P-number (ASME): P91
T91 is a martensitic 9Cr-1Mo alloy modified with vanadium, niobium, and nitrogen to dramatically improve its creep strength compared to the unmodified 9Cr-1Mo grade (T9). Its minimum tensile strength of 585 MPa and minimum yield of 415 MPa — both significantly higher than T11 or T22 — allow thinner tube walls for the same pressure rating, reducing thermal cycling stresses and improving heat transfer efficiency.
T91 is the standard material for advanced supercritical (SC) and ultra-supercritical (USC) boiler designs where steam temperatures exceed 580°C. It is used in high-temperature superheater outlet tubes, hot reheat piping, and high-pressure steam headers in modern power plant boilers.
Critical Engineering Point — T91 PWHT Is Non-NegotiableT91 must be heat-treated to a specific normalised-and-tempered condition both at the mill (to meet mechanical properties) and after any field or shop welding. The required PWHT is a normalise at 1040–1080°C followed by a temper at 730–800°C. Without this PWHT, the weld heat-affected zone will contain untempered martensite — a very hard, brittle microstructure that is susceptible to hydrogen cracking and stress corrosion. Field failures of T91 welds are almost always traceable to incorrect or omitted PWHT. This is the single most critical quality control point in T91 tube fabrication — it cannot be skipped or abbreviated regardless of schedule pressure.
7. SA-106 Grade B — Seamless Carbon Steel for Boiler Service
ASME SA-106 (equivalent to ASTM A106) is a seamless carbon steel pipe specification for high-temperature service. It is not a tube specification — it covers pipe, not tubes — and is used primarily for steam headers, boiler connecting piping, feedwater lines, and steam distribution mains rather than for heat transfer surface tubes. The three grades are:
Grade A: Minimum tensile 330 MPa (48 ksi). Rarely specified — Grade B is the standard choice.
Grade B: Minimum tensile 415 MPa (60 ksi), minimum yield 240 MPa (35 ksi). The dominant grade for boiler and pressure vessel piping.
Grade C: Minimum tensile 485 MPa (70 ksi). Used where higher pressure at ambient to moderate temperature is needed.
SA-106 Grade B is limited to approximately 425–450°C for sustained high-stress service due to carbon steel creep limitations. Above this temperature, the equivalent alloy grade (SA-335 P11 or P22 pipe) must be used.
8. Full Grade Comparison Table
| Grade | Standard | Alloy System | Min Tensile (MPa) | Min Yield (MPa) | Max Service Temp (°C) | PWHT Required? | P-Number |
| A192 | ASTM A192 | Carbon steel (low C) | 325 | 180 | ~425 | Thickness-dependent | P1 |
| A210 A-1 | ASTM A210 | Medium carbon steel | 415 | 255 | ~450 | Thickness-dependent | P1 |
| A210 C | ASTM A210 | Medium carbon steel | 485 | 275 | ~450 | Thickness-dependent | P1 |
| SA-106 B | ASME SA-106 | Carbon steel pipe | 415 | 240 | ~450 | Thickness-dependent | P1 |
| T11 | ASTM A213 | 1¼Cr-½Mo | 415 | 205 | ~570 | Above threshold WT | P4 |
| T22 | ASTM A213 | 2¼Cr-1Mo | 415 | 205 | ~580 | Yes — always recommended | P5A |
| T91 | ASTM A213 | 9Cr-1Mo-V (mod.) | 585 | 415 | ~650 | Yes — mandatory always | P91 |
9. Boiler Zone Selection Guide
A power boiler has multiple heat transfer zones, each with different tube metal temperatures and therefore different grade requirements. The table below gives typical grade allocations for a subcritical and a supercritical boiler design:
| Boiler Zone | Tube Metal Temp Range | Subcritical Boiler Grade | Supercritical/USC Grade |
| Economiser | 150 – 350°C | A192 / A210 A-1 | A210 A-1 |
| Water walls | 300 – 450°C | A192 / A210 A-1 | A210 A-1 / T11 |
| Primary superheater | 400 – 520°C | A210 A-1 / T11 | T11 / T22 |
| Secondary superheater | 500 – 580°C | T11 / T22 | T22 / T91 |
| Reheater (hot leg) | 520 – 620°C | T22 | T91 |
| Reheater outlet / headers | 540 – 650°C | T22 / T91 | T91 |
Procurement Note — Buying Boiler Tube from a Chinese MillChinese mills produce large volumes of A192, A210, and A213 boiler tube to full ASTM/ASME specifications. The practical procurement requirement is the same as for any pressure-critical material: demand a full material test certificate (MTC) per EN 10204 3.1 or 3.2 (depending on project specification), verify that the heat number on the MTC matches the markings on the tube, and confirm that PWHT records are included for alloy grades. For T91 specifically, always request the PWHT furnace chart — not just the PWHT confirmation on the MTC — as evidence that the full normalise-and-temper cycle was executed correctly. Third-party inspection at the mill (by SGS, Bureau Veritas, or TÜV) is standard practice for critical boiler tube procurement.
10. PWHT Requirements for Alloy Grades
| Grade | Preheat (Welding) | PWHT Required? | PWHT Temperature | Key Risk if Omitted |
| A192 / A210 | Not typically required (thin wall) | Per code — wall thickness dependent | 595–650°C (stress relief) | Residual stress, HAZ hardness |
| T11 | 150°C min for WT >13 mm | Per code — wall thickness dependent | 675–750°C | Temper embrittlement, HAZ cracking |
| T22 | 200°C min | Strongly recommended always | 700–750°C | High HAZ hardness, hydrogen cracking |
| T91 | 200°C min — mandatory | Mandatory — always | Normalise 1040–1080°C + Temper 730–800°C | Untempered martensite — brittle fracture in service |
11. Frequently Asked Questions
What is the difference between ASTM A192 and A210 boiler tubes?
ASTM A192 is a minimum-wall, low-carbon seamless tube with a minimum tensile of 325 MPa — the most basic carbon steel boiler tube grade, used for lower-pressure drums and water wall service below 425°C. ASTM A210 is a medium-carbon grade available in two strengths: Grade A-1 (min 415 MPa tensile) and Grade C (min 485 MPa tensile). A210 provides greater strength than A192 and is used for higher-pressure superheater and water wall tube circuits where the additional strength enables thinner walls. Both grades share the same approximate maximum service temperature ceiling of 425–450°C.
What is ASTM A213 T91 boiler tube?
ASTM A213 T91 is a 9Cr-1Mo-V modified martensitic alloy steel seamless tube used in high-temperature superheater and reheater applications in supercritical and ultra-supercritical boilers. With a minimum tensile of 585 MPa and a maximum continuous service temperature of approximately 650°C, T91 outperforms T11 and T22 in both strength and creep resistance at elevated temperatures. It requires mandatory PWHT after welding — a normalise and temper cycle — and failure to perform this correctly is the primary cause of T91 weld failures in service.
What is the maximum temperature for carbon steel boiler tubes?
Carbon steel boiler tubes (ASTM A192, A210, SA-106) should not be used for sustained pressure service above approximately 425–450°C. Above this range, carbon steel undergoes accelerated creep and is susceptible to graphitisation — a process where carbides dissolve and graphite forms at grain boundaries, causing embrittlement. For service between 450°C and 570°C, ASTM A213 T11 should be specified. For 570–580°C, T22 is appropriate. For above 580°C, T91 is required.
What is the difference between ASME SA and ASTM A designations for boiler tubes?
ASME SA and ASTM A designations refer to technically identical material specifications. ASME adopts ASTM standards into its Boiler and Pressure Vessel Code (BPVC) Section II and republishes them as SA equivalents with an additional ASME certification requirement. SA-213 T91 and ASTM A213 T91 have the same chemistry, mechanical properties, and heat treatment requirements — the SA designation means the material was produced with ASME BPVC certification documentation. Boiler fabricators working to ASME BPVC Section I or VIII must specify SA materials, not ASTM materials, to maintain code compliance.
Can ASTM A106 pipe be used as boiler tube?
ASTM A106 (or its ASME equivalent SA-106) is a pipe specification, not a tube specification, and is used for boiler connecting piping, steam headers, and feedwater lines — not for heat transfer surface tubes (water walls, superheaters, reheaters, economisers). For heat transfer surfaces, dedicated tube specifications (A192, A210, A213) with tighter dimensional tolerances and appropriate OD/WT combinations are required. SA-106 Grade B can be used for boiler piping service up to approximately 450°C within the allowable stresses defined in ASME BPVC Section II-D.
What is PWHT and when is it required for alloy boiler tubes?
PWHT (Post Weld Heat Treatment) is a controlled thermal cycle applied after welding to relieve residual stress and restore toughness in the heat-affected zone. For T11, PWHT may be required depending on wall thickness — typically above 13 mm. For T22, PWHT is strongly recommended after all welding regardless of thickness. For T91, PWHT is always mandatory — a normalise at 1040–1080°C followed by a temper at 730–800°C. Omitting or incorrectly performing T91 PWHT leaves untempered martensite in the weld HAZ, a microstructure that is brittle and prone to cracking in thermal cycling service.
Source Boiler Tubes from ZC Steel Pipe
ZC Steel Pipe manufactures and exports seamless boiler tubes and heat exchanger tubes to ASTM A192, A210, A213 (T11, T22, T91), and ASME SA-106. Full MTC documentation to EN 10204 3.1 and 3.2, third-party inspection (SGS, Bureau Veritas), and custom OD/WT combinations available. We supply boiler fabricators, EPC contractors, and power plant projects across Africa, the Middle East, and South America.
mandy.w@zcsteelpipe.com
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