ASTM A335 P9 Seamless Alloy Steel Pipe (9Cr-1Mo) for Refinery and High-Temperature Service
ASTM A335 P9 is a 9Cr-1Mo seamless alloy steel pipe designed for high-temperature and high-pressure service. With excellent oxidation resistance and creep strength, it is widely used in refinery heater tubes, power plant superheater and reheater systems, hot process piping, and high-temperature steam lines. This guide covers P9 chemical composition, mechanical properties, dimensions, heat treatment, testing requirements, applications, and key differences from other chrome-moly grades.
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ASTM A335 P9 is a seamless ferritic alloy-steel pipe in the 9Cr-1Mo (9% chromium, 1% molybdenum) family, built for demanding high-temperature service. It's a step up the chrome-moly ladder from P11 and P22, and refineries rely on it for heater tubes, superheater and reheater circuits, and hot process lines where lower alloys run out of oxidation resistance. People in the trade often just call it "9 chrome."
Let's clear up a common misconception: A335 is a seamless specification - the standard requires pipe to be manufactured by a seamless process (hot finished or cold drawn). There is no such product as an "A335 spiral welded pipe" or any A335 welded pipe. If your project needs welded 9Cr-1Mo construction, the correct product is ASTM A691 Grade 9 pipe, made from ASTM A387 Grade 9 plate by electric fusion welding - a different product family we can also discuss. This page covers genuine A335 P9 seamless pipe.
ASTM A335 P9 Features
The high chromium content (8.00–10.00%) is the headline. Compared with P22 (2.25Cr-1Mo), P9 holds meaningfully better oxidation and scaling resistance at elevated temperature, plus better resistance to certain hot process environments - including the sulfur-bearing atmospheres common in refineries. Molybdenum adds the creep strength that keeps the pipe from slowly stretching under sustained load at temperature.
One honest note for buyers: P9 is a ferritic alloy steel, not stainless steel. Whether it suits a corrosive service depends on the actual fluid, temperature, and corrosion mechanism - evaluate it against your process conditions, and ask us when in doubt.
Specifications
| Item | Specification |
|---|---|
| Standard | ASTM A335 / ASME SA335 (Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service) |
| Grade | P9 (9Cr-1Mo, "9 Chrome") |
| UNS number | S50400 |
| Manufacturing process | Seamless - hot finished or cold finished (welded construction is not permitted) |
| Equivalent grades | EN 10216-2 X11CrMo9-1 (1.7386); JIS G3458 STPA26 / JIS G3462 STBA26 |
| Size range | NPS 1/2"–24"; schedules STD through XXH, custom wall thickness on request |
| Dimensions | Per ASME B36.10M |
| Length | 5.8–12 m (SRL/DRL or fixed lengths per drawing) |
| End finish | Plain (PE), beveled (BE), or threaded |
| Heat treatment | Full or isothermal anneal, or normalize and temper per ASTM A335 |
| Related products | A234 WP9 fittings, A182 F9 flanges, A387 Grade 9 plate, A213 T9 boiler tube (minimum wall) |
| Third-party certs | EN 10204 3.1 / 3.2 mill certificates; inspection by SGS, BV, TÜV on request |
Chemical Composition (ASTM A335, Table 1)
| Element | Composition (%) |
|---|---|
| Carbon (C) | 0.15 max |
| Manganese (Mn) | 0.30 – 0.60 |
| Phosphorus (P) | 0.025 max |
| Sulfur (S) | 0.025 max |
| Silicon (Si) | 0.25 – 1.00 |
| Chromium (Cr) | 8.00 – 10.00 |
| Molybdenum (Mo) | 0.90 – 1.10 |
A practical note for refinery buyers: vanadium is often restricted to 0.03% max in heater-tube service because higher V levels have been linked to creep cracking. Tell us if your spec carries this restriction and we'll confirm the heat analysis.
Mechanical Properties
| Property | Value (minimum) |
|---|---|
| Tensile strength | 415 MPa (60 ksi) |
| Yield strength | 205 MPa (30 ksi) |
| Elongation (in 2 in. / 50 mm, longitudinal) | 30% |
These are room-temperature values. For high-temperature piping design, use the allowable-stress tables in the applicable ASME code - don't scale cold tensile numbers.
Available Sizes (ASME B36.10M)
The table below lists standard outside diameters for commonly ordered P9 sizes. Heater tubes - one of the biggest uses of P9 - typically run 4"–8" NPS from STD through XXH wall; larger sizes are rolled or drawn to order.
| NPS | OD (mm) | OD (in) | Common schedules |
|---|---|---|---|
| 1/2 | 21.3 | 0.840 | 80 / 160 / XXH |
| 3/4 | 26.7 | 1.050 | 80 / 160 / XXH |
| 1 | 33.4 | 1.315 | 80 / 160 / XXH |
| 2 | 60.3 | 2.375 | 40 / 80 / 160 / XXH |
| 3 | 88.9 | 3.500 | 40 / 80 / 160 / XXH |
| 4 | 114.3 | 4.500 | STD / 40 / 80 / 160 / XXH |
| 6 | 168.3 | 6.625 | STD / 40 / 80 / 120 |
| 8 | 219.1 | 8.625 | STD / 40 / 80 / 120 |
| 10 | 273.0 | 10.750 | STD / 40 / 60 / 80 |
| 12 | 323.8 | 12.750 | STD / 40 / 60 / 80 |
| 16 | 406.4 | 16.000 | STD / 40 / 60 |
| 20 | 508.0 | 20.000 | STD / 40 |
| 24 | 609.6 | 24.000 | STD / 40 |
OD and wall-thickness tolerances follow ASME B36.10M and ASTM A530/A999. Minimum-wall (rather than nominal-wall) P9 tube to ASTM A213 Grade T9 is also available for heater designs that call for it.
P5 vs P9 vs P22 vs P91 - Picking the Right Chrome-Moly
| Grade | Nominal composition | Tensile (min, MPa) | Yield (min, MPa) | Where it fits |
|---|---|---|---|---|
| A335 P5 | 5Cr–0.5Mo | 415 | 205 | Refinery service with sulfur concern, moderate temperature |
| A335 P22 | 2.25Cr–1Mo | 415 | 205 | Headers, main steam - the general-purpose workhorse |
| A335 P9 | 9Cr–1Mo | 415 | 205 | Heater tubes, hotter refinery and steam service |
| A335 P91 | 9Cr–1Mo–V | 585 | 415 | Modern high-efficiency plants - highest strength |
Rule of thumb: the more chromium, the better the oxidation resistance and the higher the service temperature you can run - at higher cost and more demanding welding. P9 occupies the sweet spot between P22 and P91. P91 adds vanadium and niobium for dramatically higher strength, but it's far less forgiving in fabrication. Share your design temperature, pressure, and fluid with us and we'll help you land on the right grade.
Where A335 P9 Is Actually Used
Refinery heater tubes - furnace coils in crude, vacuum and reformer units, where tube skin temperatures climb well past what low alloys tolerate
Superheater and reheater circuits in power and industrial boilers
Hot process piping in refineries and petrochemical plants, including hydrocracker and hydrotreating service
Heat exchanger and reformer tubing in high-temperature units
High-pressure steam lines where P22's oxidation resistance isn't enough
Not on the list: water pipelines, drainage, piling, and general structural work. Those are carbon steel (A53, A252) or spiral-welded line pipe (API 5L) jobs - cheaper grades do them better. If that's what your project actually needs, tell us and we'll quote the right product.
Heat Treatment and Delivery Condition
A335 requires P9 pipe to be delivered heat-treated. Common conditions are full or isothermal anneal or normalize and temper. The treatment controls the microstructure and hardness so the pipe performs in long-term hot service and remains workable in the shop. The actual delivery condition is stated on the material test certificate, and every pipe is traceable to its heat number.
Testing and Inspection
Chemical analysis per ASTM A751, heat-by-heat traceability
Hydrostatic test per ASTM A999 on every pipe
Tensile and flattening tests per ASTM A370
Hardness testing - typically limited to around 179–250 HBW depending on the heat-treatment condition ordered
Optional: ultrasonic or eddy-current testing, PMI/XRF grade verification
Mill certificate EN 10204 3.1; third-party 3.2 inspection (SGS, BV, TÜV) available
Get a Quote for A335 P9 Seamless Pipe
Send your OD, wall thickness, length, quantity, and any testing or element-restriction requirements - pricing and availability come back within hours. Free samples, third-party inspection, and custom sizes are all available. Use the contact page on this site and mention "A335 P9" so it reaches our alloy-pipe team directly.
FAQ
Is A335 P9 a welded or a seamless pipe?
Seamless. ASTM A335 covers seamless ferritic alloy-steel pipe only - hot finished or cold drawn. Spiral-welded or any other welded construction is not permitted under A335. If you need welded 9Cr-1Mo pipe, the correct specification is ASTM A691 Grade 9, made from A387 Grade 9 plate.
What is the difference between A335 P9 and A213 T9?
Same chemistry (9Cr-1Mo), different product form and wall basis. A335 P9 is pipe with nominal wall thickness for pressure piping systems; A213 T9 is tube with minimum wall thickness, typically ordered for boiler and heater designs. Some heater specs require the minimum-wall basis - check which one your design calls for.
What is A335 P9 used for?
Its main uses are refinery heater tubes, boiler superheater and reheater circuits, hot refinery process piping (including hydrocracker service), and high-temperature steam lines. It is not a general-purpose grade for water lines, drainage, or structural work.
What is the difference between P9 and P91?
Both are 9Cr-1Mo steels, but P91 adds vanadium and niobium for a much stronger microstructure: minimum tensile 585 MPa vs 415 MPa for P9. P91 serves modern high-efficiency power plants; P9 is easier to fabricate and widely used in refinery service. P91's welding and heat-treatment requirements are also far stricter.
What are the equivalent grades of A335 P9?
The closest equivalents are EN 10216-2 X11CrMo9-1 (material no. 1.7386) and JIS G3458 STPA26 (tube form: JIS G3462 STBA26). The UNS number for P9 is S50400.
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