A335 P22 vs P11: choosing alloy pipe for main steam lines
Short answer: P11 (1¼Cr-½Mo) covers main steam and hot reheat up to about 540 °C; P22 (2¼Cr-1Mo) adds creep strength and is used to about 565 °C. Above that the 9 % Cr grades take over — P91 up to about 600 °C, P92 beyond. The choice is made on design temperature first, then on wall thickness, welding discipline and availability.
The two grades, side by side
| Property | A335 P11 | A335 P22 |
|---|---|---|
| Chemistry (typical) | 1¼Cr-½Mo | 2¼Cr-1Mo |
| Typical service ceiling | ~540 °C | ~565 °C |
| Creep strength | Baseline | Higher — thinner wall at the same pressure and temperature |
| Typical duty | Main steam and hot reheat at lower parameters, feedwater, process steam | Higher-temperature main steam and hot reheat, headers, refinery high-temperature lines |
| Welding & heat treatment | Well-established, relatively forgiving | Chemistry controlled against temper embrittlement; PWHT and hardness limits part of acceptance |
| Availability | Widely rolled | Widely rolled, but heavier walls are more mill-dependent |
| Relative price | Baseline | Higher |
How the decision is actually made
1. Start with the design temperature, not the price
Round the design temperature up to the next grade boundary and check the allowable stress in the code the project uses (ASME B31.1 for power piping, EN 13480 in Europe). A line designed at 545 °C is a P22 conversation, whatever the current price difference looks like.
2. Then look at the wall thickness
If P11 needs a wall 20–30 % heavier than P22 for the same pressure, the saving in material turns into heavier supports, larger expansion loops and more welding hours — and more thermal stress at start-up. On thick-wall main steam, P22 is often the cheaper system even when the pipe is dearer per tonne.
3. Then check the welding route
P22 requires controlled chemistry against temper embrittlement and a documented post-weld heat treatment with hardness checks. If the fabrication yard does not hold a qualified P22 procedure and PWHT capacity, that cost appears late in the project — better to raise it at enquiry stage.
4. If the temperature is above 565 °C, neither
That is P91 territory (to about 600 °C) and P92 beyond it. We cover that decision in P91 vs P92: which heat-resistant steel for a supercritical boiler.
What we hold and quote
| Grade | Standard | Typical OD (mm) | Typical wall (mm) | Service |
|---|---|---|---|---|
| A335 P11 | ASTM A335 / ASME SA335 | 168.3 – 610 | 7 – 60 | Up to ~540 °C |
| A335 P22 | ASTM A335 / ASME SA335 | 168.3 – 610 | 8 – 70 | Up to ~565 °C |
| A213 T11 / T22 | ASTM A213 | 25.4 – 168.3 | 2.1 – 22 | Superheater, reheater, steam line |
| 13CrMo4-5 | EN 10216-2 | 168.3 – 508 | 4.5 – 60 | Boiler and steam service |
Normalised and tempered as standard, with the heat treatment stated on the certificate the pipe actually received.
EN 10204 3.1 with heat numbers matching the marking; 3.2 and third-party release to SGS / BV / TÜV on request.
PMI on every heat, ultrasonic testing, hydrostatic test on request, hardness survey for the grades that require it.
Mistakes we see in enquiries
Buying on price per tonne and comparing P11 against P22 without comparing the wall thickness each one needs.
Substituting P11 for P22 on a line already designed for the higher temperature — the wall, supports and expansion calculations all change.
Leaving post-weld heat treatment and hardness limits out of the enquiry, then finding the fabricator cannot satisfy them.
Not stating the design code (ASME B31.1 / EN 13480) — allowable stresses and the wall calculation follow from it.
Treating A213 T22 tube and A335 P22 pipe as one line item: different standard, tolerances and testing.
Assuming P22 is the ceiling. Above 565 °C the answer is P91 / P92, not a thicker P22.
Questions we answer every week
Can P11 be used instead of P22?
Only if the design temperature allows it. P11 is normally used to about 540 °C, P22 to about 565 °C. Substituting downward means a heavier wall for the same pressure, and that changes supports, expansion loops and start-up thermal stress — an engineering decision, not a purchasing one.
Why does P22 need more careful welding?
P22 is susceptible to temper embrittlement: the chemistry is controlled against residual elements (the X-bar factor) and the post-weld heat treatment has to be specified, performed and recorded, with hardness limits verified afterwards. P11 is more forgiving.
Which grade for a 600 °C main steam line?
Neither — that is 9 % Cr territory: A335 P91 to about 600 °C, P92 above it. See our P91 vs P92 note for the selection rules.
Are P11 and P22 available as both pipe and tube?
Yes. A335 covers pipe (larger OD and wall); A213 covers tube for superheaters, reheaters and heat exchangers — T11 and T22. They are the same steel families under different standards with different tolerances and testing.
Send the line parameters — we will confirm the grade and quote it
Medium, design pressure, design temperature, size and quantity are enough. We reply with the recommended grade and wall thickness, the heat treatment condition and a price, normally the same working day.