P91 vs P92: which heat-resistant steel for a supercritical boiler?
Short answer: both are 9 % chromium martensitic steels to ASTM A335 / ASME SA335. P91 (9Cr-1Mo-V-Nb) is the proven choice up to about 600 °C. P92 (9Cr-2W-V-Nb-N) carries roughly 25–30 % more creep strength at the same temperature, so main steam and hot reheat lines above 600 °C — and thick-wall headers where wall thickness has to come down — are normally built in P92.
The two grades, side by side
| Property | A335 P91 | A335 P92 |
|---|---|---|
| Chemistry (typical) | 9Cr-1Mo-V-Nb | 9Cr-1.8W-Mo-V-Nb-N |
| Creep strength at 600 °C | Baseline | ≈ 25–30 % higher |
| Typical service ceiling | ~600 °C | 600–620 °C and above |
| Wall thickness for the same duty | Thicker | Thinner — lighter pipe, lower thermal stress |
| Welding & heat treatment | Controlled, well-established procedures | Tighter window; Type IV cracking must be managed |
| Mill availability & lead time | Wide, shorter | Fewer mills, longer lead time |
| Relative material price | Baseline | Higher |
How the choice is actually made
Main steam, hot reheat and feedwater up to about 600 °C
P91. It has decades of operating history, published creep data, a wide supply base and a welding procedure most fabrication shops already hold. Below 600 °C the extra strength of P92 buys very little and costs more per tonne.
Ultra-supercritical units, 600 °C and above
P92. Above 600 °C the allowable stress of P91 drops fast, so the wall thickness required to hold the same pressure climbs — and with it the thermal stress during start-up, the weight on the hangers and the welding time. P92 keeps the wall thinner, which is the reason USC projects specify Grade 92.
Thick-wall headers and thick pipe
The heavier the wall, the more P92 pays for itself: a 30 % saving in wall thickness is a large saving in weight, in welding hours and in heat-treatment time, not only in material price.
Retrofits and life extension
Matching the existing grade matters more than upgrading. A mixed P91 / P92 system needs dissimilar joints throughout, and each one has to be qualified, welded and heat treated separately.
What we hold and quote
| Product | Standard | OD range (mm) | Wall range (mm) | Service |
|---|---|---|---|---|
| A335 P91 pipe | ASTM A335 / ASME SA335 | 168.3 – 820 | 12 – 120 | Up to ~600 °C |
| A335 P92 pipe | ASTM A335 / ASME SA335 | 168.3 – 820 | 14 – 130 | 600 °C and above |
| A213 T91 tube | ASTM A213 | 31.8 – 168.3 | 3.5 – 25 | Header, superheater outlet |
| A213 T92 tube | ASTM A213 | 31.8 – 168.3 | 4.0 – 28 | Ultra-supercritical boiler |
EN 10204 3.1 mill certificate for every heat; the heat number on the certificate matches the marking on the pipe.
PMI on each heat, ultrasonic testing of the body, hydrostatic test and hardness survey on request.
SGS, BV or TÜV inspection arranged before shipment, with the release note in the document package.
Mistakes we see in enquiries
Assuming P92 is always the better pipe. Below 600 °C it mainly adds cost, price and lead time.
Quoting P92 without checking mill availability — Grade 92 in heavy wall is not an off-the-shelf item.
Treating T92 tube and P92 pipe as the same line item: different standard, different tolerances, different testing.
Leaving post-weld heat treatment out of the enquiry. For both grades the PWHT record is part of the acceptance package.
Not stating the design temperature. Grade selection for a 580 °C line and a 620 °C line is not the same decision.
Mixing grades in one weld without a qualified dissimilar-joint procedure.
Questions we answer every week
Can P91 and P92 be welded to each other?
Yes — with a qualified procedure and matching filler. Treat it as a dissimilar joint: its own WPS, its own preheat and post-weld heat treatment plan, and the heat-treatment chart in the document package.
Is P92 more expensive than P91?
Yes, both in material price and in lead time, because fewer mills roll Grade 92 in the heavier walls. Below 600 °C service that premium rarely pays back.
A335 P91 pipe or A213 T91 tube?
The steel family is the same; the standard decides the product. A335 covers pipe (larger OD and wall), A213 covers tube for superheaters, reheaters and heat exchangers — so tolerances, tests and marking follow the tube standard.
Which grade for a 620 °C main steam line?
P92 / Grade 92. At that temperature P91 needs a much heavier wall for the same pressure, which works against start-up thermal stress and support design.
Send the line parameters, get the grade and the price
Tell us the medium, design pressure, design temperature and size — or send the line list or isometric — and we will recommend the grade and wall thickness and quote it, normally the same working day.