ASTM A335 P11 VS P22
Oct 28, 2026
What Is A335 P11 Pipe?
ASTM A335/A335M is the specification for seamless ferritic alloy-steel pipe for high-temperature service - pressure pipe, seamless only, for steam, hot process and high-temperature hydrogen. P11 is the grade most orders name: 1.25 % chromium with 0.5 % molybdenum, UNS K11597.
The alloy content is modest on purpose: enough to hold tensile strength in the creep range and keep a protective oxide scale on the bore, still weldable with a routine preheat and post-weld heat treatment. That balance made P11 the default for main steam, reheat, boiler feed and hot condensate service below about 540 °C - and the grade most likely to be in stock in every schedule.
| Item | A335 P11 |
|---|---|
| Grade designation | P11 - 1.25Cr-0.5Mo |
| UNS number | K11597 |
| Carbon | 0.05 – 0.15 % |
| Manganese | 0.30 – 0.60 % |
| Phosphorus | 0.025 % max |
| Sulphur | 0.025 % max |
| Silicon | 0.50 – 1.00 % |
| Chromium | 1.00 – 1.50 % |
| Molybdenum | 0.44 – 0.65 % |
| Tensile strength, min | 415 MPa (60 ksi) |
| Yield strength, min | 205 MPa (30 ksi) |
| Elongation, min (2 in, longitudinal) | 30 % |
| Delivery condition | Annealed / isothermal anneal / N+T |
| Product form | Seamless only |
| Where it is usually specified | Steam, feed and hot process lines up to about 540 °C |
Delivery condition is at the manufacturer's option - annealed, isothermally annealed or normalised and tempered - and all three meet the same minimums. A heat arriving at 550 MPa tensile is fully compliant: above-minimum strength is neither a rejection criterion nor a benefit.
What Is A335 P22 Pipe?
P22 is the same specification and the same product form one alloy step up: 2.25 % chromium with 1 % molybdenum, UNS K21590. Doubling chromium and molybdenum roughly doubles the alloy content, and what it buys is allowable stress in the creep range - the property that made 2.25Cr-1Mo the standard material for main steam and hot reheat headers, and for hot hydrogen-bearing process lines.
It also brings the obligations of 2.25Cr-1Mo: a higher minimum tempering temperature, a far greater susceptibility to temper embrittlement in the 350–550 °C band, and filler metals of its own. Those are dealt with below, after the numbers.
| Item | A335 P22 |
|---|---|
| Grade designation | P22 - 2.25Cr-1Mo |
| UNS number | K21590 |
| Carbon | 0.05 – 0.15 % |
| Manganese | 0.30 – 0.60 % |
| Phosphorus | 0.025 % max |
| Sulphur | 0.025 % max |
| Silicon | 0.50 % max |
| Chromium | 1.90 – 2.60 % |
| Molybdenum | 0.87 – 1.13 % |
| Tensile strength, min | 415 MPa (60 ksi) |
| Yield strength, min | 205 MPa (30 ksi) |
| Elongation, min (2 in, longitudinal) | 30 % |
| Delivery condition | Annealed / isothermal anneal / N+T |
| Product form | Seamless only |
| Where it is usually specified | Main steam and hot reheat headers, hot hydrogen process lines |
Read the two sheets together: four of the seven elements share a window, the three mechanical minimums are identical, two elements rise and one falls. The rest of this page is about which of those differences actually decides anything.
A335 P11 vs P22
Every property that separates a P11 order from a P22 order, in one view. The sections that follow explain the mechanisms; this table is the comparison itself.
| Item | A335 P11 | A335 P22 | What it changes |
|---|---|---|---|
| Base alloy | 1.25Cr-0.5Mo | 2.25Cr-1Mo | Alloy cost and everything below |
| Tensile / yield / elongation, min | 415 / 205 MPa · 30 % | 415 / 205 MPa · 30 % | Nothing at room temperature |
| Allowable stress at 550 °C | 46 MPa | 55 MPa (+19.6 %) | Wall thickness |
| Oxidation limit | ≈ 570 °C | ≈ 570 °C | Neither grade above it |
| Temper embrittlement risk | Moderate | High | J-factor and X̄ limits on the order |
| API 941 hydrogen envelope | Lower | 50 – 100 °C higher | Design review, never substitution |
| Minimum tempering | 649 °C | 677 °C | Furnace scheduling |
| Filler metal | ER80S-B2 / E8018-B2 | ER90S-B3 / E9018-B3 | Separate WPS |
| Relative cost, against carbon steel | ≈ 1.5× | ≈ 1.8× | Crossover near 550 °C |
The second row is the one to read twice. Tensile, yield and elongation are the same numbers in both grades, and no amount of alloy content changes them, because A335 sets them as acceptance minimums for the delivered pipe rather than as design values.
tensile strength, min 415 MPa (60 ksi) = 415 MPa (60 ksi)
yield strength, min 205 MPa (30 ksi) = 205 MPa (30 ksi)
elongation, min (2 in) 30 % = 30 %
This is where most comparisons stop, and where they go wrong: identical values are then used to argue that P22 is the stronger grade. These three numbers are a mill acceptance test - they say whether a heat passes inspection, not what the pipe carries at 540 °C, where creep rupture and oxidation govern the wall.
Not in either data sheet
A335 sets no hardness ceiling for either grade; the widely quoted 163 HBW comes from other product standards. A335 P22 hardness is a welding acceptance item, checked after PWHT - not a mill property.
Composition: Three Elements Carry the Difference
Carbon, manganese, phosphorus and sulphur occupy the same windows in both grades. Of the three that differ, two rise and one falls - and the one that falls is the interesting one.
| Element, % | P11 | P22 | Effect of the difference |
|---|---|---|---|
| Carbon | 0.05 – 0.15 | 0.05 – 0.15 | None |
| Manganese | 0.30 – 0.60 | 0.30 – 0.60 | None |
| Phosphorus | 0.025 max | 0.025 max | None |
| Sulphur | 0.025 max | 0.025 max | None |
| Silicon | 0.50 – 1.00 | 0.50 max | Opposite direction - see below |
| Chromium | 1.00 – 1.50 | 1.90 – 2.60 | Oxidation and creep resistance |
| Molybdenum | 0.44 – 0.65 | 0.87 – 1.13 | Creep strength, carbide stability |
Density decides nothing: A335 P22 density and P11 density are both about 7.85 g/cm³ (0.283 lb/in³). P22 carries roughly twice the chromium and molybdenum, but that says what changes, not how much - the allowable stress table says how much, and it is less than a doubling.
The silicon row runs the other way
P11 sets a minimum silicon of 0.50 %; P22 sets only a maximum of 0.50 %. Same element, opposite requirement, adjacent rows, one trade-off:
P11 needs 0.50 % or above. At 1.00–1.50 % chromium it cannot hold the oxide scale alone; silicon forms a thin silica-rich layer at the metal/oxide interface that slows further oxidation.
P22 must stay at or below 0.50 %. Silicon sits in the numerator of the J-factor, J = (Si + Mn) × (P + Sn) × 10⁴, and P22 is already the more embrittlement-prone of the two. Its 2.25 % chromium means it does not need silicon for oxidation.
So "more silicon for oxidation resistance" is sound for P11 and a mistake for P22 - and it is why the two do not share a welding procedure despite sharing a P-number.
Equivalent grades and matching parts
The A335 P11 equivalent material and A335 P22 equivalent material in other systems are close and not interchangeable, because the windows only partly overlap.
| System | For P11 | For P22 | Where it does not line up |
|---|---|---|---|
| A335 (pipe) | P11 · UNS K11597 | P22 · UNS K21590 | - |
| Forging (A182) | F11 | F22 | Same UNS; different form and heat-treatment rules |
| Fitting (A234) | WP11 | WP22 | Class 1 / 2 / 3 conditions are not in A335 |
| Heat exchanger tube (A213) | T11 | T22 | Tubes, not pipe - different tolerances |
| Plate (A387) | Gr 11 | Gr 22 | Plate has its own heat-treatment table |
| China (GB/T 5310) | 15CrMoG | 12Cr2MoG | 15CrMoG runs about 0.80–1.10 % Cr, below the A335 P11 window |
The grade that gets misassigned
12Cr1MoVG is routinely written as "the Chinese equivalent of P11". It is not: it carries vanadium (about 0.15–0.30 %) and only about 1 % chromium, matching neither grade. The A335 P11 equivalent material in GB/T 5310 is 15CrMoG, and even that overlaps only part of the chromium window. The certificate chemistry is what gets accepted, not the grade name.
Allowable Stress: Where the Grades Separate
The P11 P22 temperature range is not a property of either grade. It is whatever the construction code's allowable stress table says at your metal temperature. Below the creep range the two are the same material for design; above it they are not.
| Metal temp. | °C | Allowable stress | MPa |
|---|---|---|---|
| ≤ 100 °F | ≤ 38 | 20.0 ksi | 138 |
| 600 °F | 316 | 15.7 ksi | 108 |
| 700 °F | 371 | 15.1 ksi | 104 |
| 800 °F | 427 | 14.4 ksi | 99 |
| 900 °F | 482 | 13.6 ksi | 94 |
| 950 °F | 510 | 9.3 ksi | 64 |
| 1000 °F | 538 | 6.3 ksi | 43 |
| 1050 °F | 566 | 4.2 ksi | 29 |
| 1100 °F | 593 | 2.8 ksi | 19 |
One step matters more than the rest: between 900 °F and 950 °F the allowable stress falls 31.6 %, from 13.6 to 9.3 ksi, as the basis switches from yield to creep rupture. That cliff - not a fixed percentage - is why "maximum temperatures" for P11 read 510, 540 or 550 °C: they are different points on it.
P22 shares those values to about 450 °C, then separates: 8.9 % higher at 500 °C (86 against 79 MPa in the metric edition of that table) and 19.6 % higher at 550 °C (55 against 46 MPa). That ratio is the whole of the P11 P22 creep strength difference - there is no fixed 30 % advantage, and none below the creep range.
Why P22 is quoted as a ratio
Secondary sources differ by 3–8 % on absolute allowable stress between code editions and between B31.1 and B31.3. The ratio is stable across them; the absolute is not. Check both against the edition named in your project specification before sizing.
Substitution and Welding: Which Direction Works
Can I substitute P11 for P22? In one direction only. P11 cannot replace P22 where the design relies on the higher allowable stress above 500 °C: every wall grows, and above 570 °C the oxidation limit is reached regardless of thickness. P22 can replace P11 - stronger at every temperature - at three costs:
The design is re-run. A thinner wall changes line flexibility, support spacing, and the ratings of the flanges and fittings sized against the original.
The welding procedure is void. Different filler metal, different heat-treatment window.
You import the embrittlement risk into the 350–550 °C band where P11 normally runs - and pay for strength the design did not ask for.
A335 P11 and P22 pipe factory
How to Specify
What to put on the order so that what arrives is what you designed:
Standard and edition: ASTM A335/A335M, with the edition named, plus grade, size (NPS × schedule or OD × wall), quantity and length.
Design metal temperature, not fluid temperature - the two differ by 30–50 °C in fired service, and the allowable stress table is indexed on metal.
J-factor and/or X̄ limit - for P22 in the 350–550 °C band this is not optional in practice.
Sn, Sb and As analysis - request it explicitly; it is not on a routine certificate.
Inspection and documentation - third-party witness, EN 10204 3.1 or 3.2.
FAQ
Is A335 P11 carbon steel or stainless steel?
Neither. P11 is a low-alloy ferritic steel: total alloy content is well under 5 %, and chromium sits at 1.00–1.50 % against the roughly 11 % a stainless steel needs to stay passive - so the shorthand "A335 P11 is CS or SS" gets the same answer, no. A carbon steel welding procedure is not qualified for it either: preheat and post-weld heat treatment are mandatory, not optional.
What does the P in P11 and P22 stand for?
The letter denotes product form, not alloy or strength: P for pipe in A335, T for heat exchanger tube in A213, F for forging in A182, WP for wrought fitting in A234. The number is a grade sequence inside the specification. That is why one 1.25Cr-0.5Mo chemistry appears as P11, T11, F11 and WP11 - one alloy, four product standards, four sets of tolerances and heat-treatment rules.
Can P11 pipe be welded to P22 pipe?
Yes, as a dissimilar Cr-Mo joint rather than a routine one. Filler metal follows the higher-alloy side - ER90S-B3 or E9018-B3 - and post-weld heat treatment is set by the more restrictive grade, so P22's 677 °C minimum applies. Because both base metals must be covered, the joint normally needs its own WPS listing both grades, and usually its own procedure qualification.
Should I order A213 T22 or A335 P22?
Match the standard to the item on the drawing. A213 covers seamless heat exchanger and boiler tube - smaller outside diameters, tighter wall tolerances, eddy-current or hydrostatic testing as standard; A335 covers pipe. A tube certificate is not a pipe certificate, so tube-sheet and boiler-bank items go to A213 and run pipe to A335.
Does ASTM A335 set a maximum service temperature?
No. A335 is a product specification: it fixes composition, heat treatment and room-temperature mechanical requirements, and states no service temperature limit. Every maximum temperature attached to P11 or P22 comes from outside it - the code's allowable stress table, the oxidation behaviour of the alloy, or the purchaser's specification. That is why the numbers differ by project.
Is A335 pipe available welded?
A335 covers seamless pipe only - seamless is in the scope, not an option. For welded construction in the same alloy family the reference is ASTM A691, electric-fusion-welded pipe with 1.25Cr and 2.25Cr grades and its own filler metal and heat-treatment qualification. Large-diameter 1.25Cr or 2.25Cr lines are usually A691, and the two are not interchangeable at the same nominal size.
