P5 vs P11 vs P91 Alloy Steel Pipes: Which to Choose?
Quick answer: Choose P11 (1¼Cr-½Mo) for general steam and process lines up to about 538 °C at the lowest cost; P5 (5Cr-½Mo) where sulphidation and oxidation resistance matter, mainly in refineries; and P91 (9Cr-1Mo-V-Nb) for high-temperature, high-pressure power piping – it is about 40% stronger (585 vs 415 MPa tensile) with much higher creep strength, but needs strict welding and heat treatment.
See ASTM A335 P5 pipes, P11 pipes and P91 pipes.
P5 vs P11 vs P91 – Comparison Table
| Property | P11 | P5 | P91 |
|---|---|---|---|
| Nominal alloy | 1¼Cr-½Mo | 5Cr-½Mo | 9Cr-1Mo-V-Nb |
| UNS | K11597 | K41545 | K90901 |
| Tensile strength, min | 415 MPa (60 ksi) | 415 MPa (60 ksi) | 585 MPa (85 ksi) |
| Yield strength, min | 205 MPa (30 ksi) | 205 MPa (30 ksi) | 415 MPa (60 ksi) |
| Elongation, min (long.) | 30% | 30% | 20% |
| Approx. max service temperature* | ≈ 538 °C | ≈ 593 °C | ≈ 593 – 620 °C |
| Creep strength | Moderate | Moderate | Very high |
| Sulphidation / oxidation resistance | Fair | Good | Very good |
| Heat treatment (supplied) | Annealed, or N&T (temper ≥ 650 °C) | Annealed, or N&T (temper ≥ 675 °C) | Normalized 1,040–1,080 °C + tempered 730–800 °C |
| Welding | Preheat; PWHT by thickness/code | Preheat + PWHT | Strict preheat, interpass control and PWHT |
| Relative cost | Lowest | Medium | Highest |
| Tube equivalent (A213) | T11 | T5 | T91 |
*Typical guidance only – the allowable temperature depends on the design code (ASME B31.1 / B31.3), pressure and wall thickness.
Chemical Composition – ASTM A335 P5, P11, P91 (wt %)
| Element | P11 | P5 | P91 |
|---|---|---|---|
| Carbon (C) | 0.05 – 0.15 | 0.15 max | 0.08 – 0.12 |
| Manganese (Mn) | 0.30 – 0.60 | 0.30 – 0.60 | 0.30 – 0.60 |
| Silicon (Si) | 0.50 – 1.00 | 0.50 max | 0.20 – 0.50 |
| Chromium (Cr) | 1.00 – 1.50 | 4.00 – 6.00 | 8.00 – 9.50 |
| Molybdenum (Mo) | 0.44 – 0.65 | 0.45 – 0.65 | 0.85 – 1.05 |
| Vanadium (V) | – | – | 0.18 – 0.25 |
| Niobium (Nb) | – | – | 0.06 – 0.10 |
| Nitrogen (N) | – | – | 0.030 – 0.070 |
| Phosphorus (P), max | 0.025 | 0.025 | 0.020 |
| Sulphur (S), max | 0.025 | 0.025 | 0.010 |
Vanadium, niobium and nitrogen are what make P91 different: they form fine carbonitrides that give P91 its high creep strength after normalizing and tempering.
ASTM A335 P11 – the General-Purpose Grade
P11 pipe (1¼Cr-½Mo) is the most widely used chrome-moly pipe for steam lines, headers and refinery piping up to about 538 °C. It is the cheapest of the three and the easiest to weld, though preheat and post-weld heat treatment are still needed above certain wall thicknesses.
ASTM A335 P5 – for Refinery Corrosion Service
P5 pipe (5Cr-½Mo) has the same minimum strength as P11, but its higher chromium gives much better resistance to high-temperature sulphidation and oxidation. That makes it the standard choice for refinery heater tubes, crude and vacuum-unit transfer lines and hot hydrocarbon service. P5b and P5c are variants with extra silicon or titanium.
ASTM A335 P91 – for High-Temperature Power Piping
P91 pipe (9Cr-1Mo-V-Nb) is a modified 9-chrome steel with about 40% higher tensile strength and far higher creep strength than P5 or P11. It allows thinner walls for main steam and hot-reheat piping in supercritical and combined-cycle power plants. It must be normalized and tempered correctly, and welding needs strict preheat, interpass control and PWHT.
Which Should You Choose?
- Choose P11 for steam, process and refinery lines up to about 538 °C where cost and easy fabrication matter.
- Choose P5 for refinery heaters and hot sulphur-bearing hydrocarbon service up to about 593 °C.
- Choose P91 for high-pressure main steam and reheat piping above about 540 °C, where its strength lets you reduce wall thickness and weight.
- Also consider P22 (2¼Cr-1Mo) between P11 and P91 for power-plant piping, P9 for refinery corrosion service, and P92 for even higher temperatures than P91.
Frequently Asked Questions
What is the difference between P5, P11 and P91 pipe?
All three are ASTM A335 chrome-moly seamless pipes. P11 is 1¼Cr-½Mo, P5 is 5Cr-½Mo and P91 is 9Cr-1Mo with vanadium and niobium. P5 and P11 have the same minimum strength (415 MPa tensile / 205 MPa yield); P91 is much stronger (585 / 415 MPa) and has far better creep strength.
Which is better for high temperature – P11 or P91?
P91. Its creep strength above about 540 °C is much higher than P11, so walls can be thinner at the same pressure. P11 is cheaper and easier to weld and is used for lower-temperature steam and refinery lines.
Why is P5 used in refineries?
Its 4–6% chromium gives P5 much better resistance to high-temperature sulphidation and oxidation than P11 or P22, which is why it is common in refinery heater tubes, crude and vacuum-unit piping and hot hydrocarbon service.
Does P91 need post-weld heat treatment?
Yes. P91 needs controlled preheat, interpass temperature control and post-weld heat treatment (typically around 750–770 °C) to restore weld toughness. P5 and P11 also need preheat and PWHT depending on wall thickness and the design code.
What are the tube equivalents of P5, P11 and P91?
For boiler and heat-exchanger tubes under ASTM A213 the equivalents are T5, T11 and T91.
Where do P9 and P22 fit?
P22 (2¼Cr-1Mo) sits between P11 and P5 for creep strength and is widely used in power plants. P9 (9Cr-1Mo, without V and Nb) is used mainly in refineries for corrosion resistance; P91 replaced it in high-temperature power piping.
Buy ASTM A335 Chrome-Moly Pipes from ASC LLP
ASC LLP, Mumbai, supplies ASTM A335 / ASME SA335 seamless pipes in P5, P9, P11, P22, P91 and P92, plus matching T11 and T91 tubes – with EN 10204 3.1 certificates, IBR on request and third-party inspection. See the full range of alloy steel seamless pipes, or send your requirement to sales@amardeepsteel.com / 022-67439130.
Related reading: P5 pipes for high-temperature service · Alloying elements in P91 · P11 heat treatment
