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Why replace P22 with P91?
Short answer
P91 has much higher allowable stresses than P22 at main-steam temperatures, so the same pressure can be carried with walls roughly half as thick. That cuts weight, thermal stress and support loads in supercritical plants. The trade-off is far stricter control of heat treatment, welding and PWHT.
P22 vs P91 at a glance
| Property | P22 | P91 |
|---|---|---|
| UNS | — | — |
| EN | — | — |
| Family | Chromium-molybdenum alloy steel | Chromium-molybdenum alloy steel |
| C (%) | 0.05 – 0.15 | 0.08 – 0.12 |
| Cr (%) | 1.90 – 2.60 | 8.00 – 9.50 |
| Ni (%) | — | 0.40 max |
| Mo (%) | 0.87 – 1.13 | 0.85 – 1.05 |
| N (%) | — | 0.030 – 0.070 |
| Nb (%) | — | 0.06 – 0.10 |
| V (%) | — | 0.18 – 0.25 |
| Al (%) | — | 0.02 max |
| Tensile strength | 415 MPa (60 ksi) | 585 MPa (85 ksi) |
| Yield strength | 205 MPa (30 ksi) | 415 MPa (60 ksi) |
| PREN | — | — |
| Density | 7.85 g/cm³ | 7.77 g/cm³ |
| Magnetic | Magnetic (ferritic/martensitic). | Magnetic (ferritic/martensitic). |
| Typical use | Main steam and hot-reheat piping | Main steam and hot-reheat piping |
Which should you choose?
There's no universal "winner": the right choice depends on service conditions, codes and cost. Share your conditions with us for a recommendation.
Thinner walls, new risks
Thinner P91 walls heat and cool faster, which reduces thermal fatigue in cycling plants. But P91's creep strength depends on correct tempering, so a fabrication error can halve its life, as described in five P91 fabrication pitfalls.
Dissimilar welds
Where P91 meets P22 or austenitic stainless, dissimilar-metal welds need careful filler and PWHT selection. Projects define these joints specifically in the welding procedures.
Products in both grades
Related guides, comparisons and articles
Frequently asked questions
Is P91 more expensive than P22?
Per kilogram yes, but thinner walls often reduce the total material cost for high-temperature lines.
Which specification covers P91 pipe?
ASTM A335 P91 (or ASME SA335), with Type 1 and Type 2 chemistry in recent editions.
What is the main difference between P22 and P91?
P91 is a creep-strength-enhanced 9Cr steel with much higher allowable stresses than P22, enabling thinner walls in supercritical plants. It needs far stricter heat-treatment and welding control.
When should I choose P22?
Subcritical plants, hydroprocessing reactors.
When should I choose P91?
Supercritical main steam and hot reheat.
Can P22 replace P91?
Only when the design, code and specification allow it. Strength, corrosion resistance and temperature limits differ, so substitution must be approved by the design authority.