Boiler and Heat Exchanger Tubes: From A192 to T91

Why tubes, not pipe

Boiler and heat-exchanger tubes are tubes in the strict sense: ordered by exact outside diameter and wall thickness (often by BWG gauge — 16 BWG is 1.65 mm), because they must roll or weld into tube sheets, bend into U-shapes, and hold pressure at metal temperatures where steel slowly creeps. Nominal pipe sizes have no place here.

The temperature ladder

Grade selection follows service temperature, because creep strength collapses at different points for different alloys. Indicative ceilings, which always depend on stress and code:

Family	Examples	Roughly, metal temperature
Carbon steel	ASTM A192, A210 A-1, EN P235GH/P265GH	Up to about 450 °C
C-Mo and 1.25Cr	A213 T11 (≈ EN 13CrMo4-5), 16Mo3	About 500–550 °C
2.25Cr-1Mo	A213 T22 (≈ EN 10CrMo9-10)	About 550–580 °C
9Cr creep-strength-enhanced	A213 T91 (EN X10CrMoVNb9-1)	About 600 °C and above
Austenitic stainless	A213 TP304H, TP347H	Highest temperatures and corrosion duty

The key specifications

• ASTM A192 — seamless carbon steel boiler tubes for high-pressure service: the standard superheater/waterwall carbon grade, with controlled hardness for tube rolling.
• ASTM A179 — cold-drawn low-carbon tubes for heat exchangers and condensers, where pressure and temperature are moderate.
• ASTM A213 — seamless ferritic and austenitic alloy tubes for boilers, superheaters and exchangers: the T-grades (T5, T9, T11, T22, T91) and TP stainless grades live here.
• EN 10216-2 — the European counterpart, with P235GH/P265GH carbon grades and alloy grades 16Mo3, 13CrMo4-5, 10CrMo9-10, X10CrMoVNb9-1.
• ASME SA-192/SA-213 are the code-adopted versions of the ASTM standards, mandatory for ASME-stamped construction.

T91 deserves a caution

T91's creep strength comes from a precise chemistry and a normalizing-plus-tempering cycle that produces tempered martensite. It is unforgiving in fabrication: welding needs controlled preheat, interpass and post-weld heat treatment, and hardness verification. Bending, hardness testing and PWHT records are part of the purchase, not paperwork extras. Lower-alloy grades are far more tolerant of site conditions — one reason T11 and T22 remain widely specified even where T91 would be more compact.

Testing and documentation

Pressure-part tubes carry a heavier inspection burden than ordinary pipe. Product standards call for flattening, flaring or bending tests as ductility proof, hydrostatic testing or an eddy-current/ultrasonic equivalent per tube, and hardness limits on grades like A192 that will be expanded into tube sheets. For alloy and creep grades, positive material identification (PMI) at receipt is standard practice in critical plants: a single T11 tube mixed into a T22 bank, or a carbon tube into an alloy bank, may not fail until it has been in hot service. Traceability by heat number, backed by EN 10204 3.1 certificates, is what makes that check possible.

Exchanger specifics

For shell-and-tube exchangers, also specify: tube length and tolerance, U-bend requirements and their solution annealing, surface condition, eddy-current or hydrostatic testing scope, and EN 10204 3.1 certification. A tube that meets material but fails on bend condition or test scope is still the wrong tube.