ASTM A213 T91 (ASME SA213 T91) is a commonly used ferritic alloy seamless steel pipe containing 8.0% to 9.5% Cr, 0.85% to 1.05% Mo, and other microalloying elements.
These alloying additions provide T91 steel tubes with excellent high-temperature strength, creep resistance, and oxidation resistance, making them widely used in boilers, superheaters, and heat exchangers operating under high-temperature and high-pressure conditions.
UNS number: K90901.
T91 steel pipes can be classified into Type 1 and Type 2, with the main difference being slight adjustments in chemical composition.
Type 2 has stricter requirements for chemical elements; for example, the S content is reduced from a max of 0.010% in Type 1 to a max of 0.005%, and the upper and lower limits of other elements are also adjusted.
Type 2 is mainly intended for more demanding high-temperature or corrosive environments, providing improved toughness and creep resistance.
Next, let's take a closer look at the chemical composition requirements for Type 1 and Type 2 in the product analysis.
| Composition, % | ASTM A213 T91 Type 1 | ASTM A213 T91 Type 2 |
| C | 0.07 ~ 0.14 | 0.07 ~ 0.13 |
| Mn | 0.30 ~ 0.60 | 0.30 ~ 0.50 |
| P | 0.020 max | |
| S | 0.010 max | 0.005 max |
| Si | 0.20 ~ 0.50 | 0.20 ~ 0.40 |
| Ni | 0.40 max | 0.20 max |
| Cr | 8.0 ~ 9.5 | |
| Mo | 0.85 ~ 1.05 | 0.80 ~ 1.05 |
| V | 0.18 ~ 0.25 | 0.16 ~ 0.27 |
| B | — | 0.001 max |
| Nb | 0.06 ~ 0.10 | 0.05 ~ 0.11 |
| N | 0.030 ~ 0.070 | 0.035 ~ 0.070 |
| Al | 0.02 max | 0.020 max |
| W | — | 0.05 max |
| Ti | 0.01 max | |
| Zr | 0.01 max | |
| Other Elements | — | Cu: 0.10 max Sb: 0.003 max Sn: 0.010 max As: 0.010 max N/Al: 4.0 min |
T91 Type 1 and 2 have slight differences in chemical composition, but they share the same requirements for mechanical properties and heat treatment.
Tensile Properties
| Grade | Tensile Strength | Yield Strength | Elongation in 2 in. or 50 mm |
| T91 Type 1 and 2 | 85 ksi [585 MPa] min | 60 ksi [415 MPa] min | 20 % min |
Hardness Properties
| Grade | Brinell / Vickers | Rockwell |
| T91 Type 1 and 2 | 190 to 250 HBW
196 to 265 HV |
90 HRB to 25 HRC |
Flattening Test
The testing method shall comply with the relevant requirements of Clause 19 of ASTM A1016.
One flattening test shall be made on specimens from each end of one finished tube, not the one used for the flaring test, from each lot.
Flaring Test
The testing method shall comply with the relevant requirements of Clause 22 of ASTM A1016.
One flaring test shall be made on specimens from each end of one finished tube, not the one used for the flattening test, from each lot.
Manufacturer and Condition
ASTM A213 T91 tubes shall be made by the seamless process and shall be either hot-finished or cold-finished, as required.
Seamless steel pipes, with their continuous and weld-free structure, distribute stress more evenly under high temperature, high pressure, and complex loading conditions, providing superior strength, toughness, and fatigue resistance.
Heat Treatment
All T91 steel pipes shall be reheated and heat-treated in accordance with the requirements specified in the table.
Heat treatment shall be carried out separately and in addition to heating for hot forming.
| Grade | Heat treat type | Austenitizing / Solution Treatment | Subcritical Annealing or Temperature |
| T91 Type 1 and 2 | normalize and temper | 1900 - 1975 ℉ [1040 - 1080 ℃] | 1350 - 1470 ℉ [730 - 800 ℃] |
For Grade T91 Type 2 material, the heat treatment shall ensure that following austenitizing the rate of cooling from 1650 °F to 900 °F [900 °C to 480 °C] is no slower than 9 °F/min [5 °C/ min].
T91 tubing sizes and wall thicknesses are usually furnished with inside diameters ranging from 3.2 mm to outside diameters of 127 mm, and minimum wall thicknesses from 0.4 mm to 12.7 mm.
Other sizes of T91 steel pipes can also be supplied, provided that all other requirements of ASTM A213 are met.
The dimensional tolerances of T91 are the same as those of T11. For details, you can refer to the T11 Dimensions and Tolerances.
| UNS | ASME | ASTM | EN | GB |
| K90901 | ASME SA213 T91 | ASTM A335 P91 | EN 10216-2 X10CrMoVNb9-1 | GB/T 5310 10Cr9Mo1VNbN |
Product: ASTM A213 T91 Type 1 and Type 2 seamless alloy steel pipes and fittings;
Size: 1/8" to 24", or customized according to your requirements;
Length: Random length or cut to order;
Packaging: Black coating, beveled ends, pipe end protectors, wooden crates, etc.
Support: IBR certification, TPI inspection, MTC, cutting, processing, and customization;
MOQ: 1 m;
Payment Terms: T/T or L/C;
Price: Contact us for the latest T91 steel pipe prices.
