Seamless ferritic alloy steel pipe to ASTM A335, grades P5, P9, P11, P12, P22, P91 and P92, for high temperature and high pressure service in power and petrochemical plants.
- Standard: ASTM A335, ASME B36.10, ASME B36.19
- Grades: P1, P2, P5, P9, P11, P12, P22, P23, P91, P92
Specifications
| Outer diameter | 1/8" - 36" |
|---|---|
| Wall thickness | SCH 10 - SCH XXS |
| Length | SRL, DRL, 20 FT, 40 FT, or custom |
| Ends | Plain, beveled, threaded |
| Surface treatment | Black painting, varnish |
| Manufacturing process | Hot finished seamless, cold drawn seamless |
| Pressure rating | High temperature and high pressure service |
What Chrome Moly Pipe Is and Why It Is Used
ASTM A335 pipe is a ferritic alloy steel containing chromium and molybdenum as the principal alloying additions. Chromium improves resistance to oxidation and to high temperature hydrogen attack, while molybdenum raises the creep strength at elevated temperature. Plain carbon steel loses significant strength above roughly 425 degrees C and is susceptible to graphitization and hydrogen attack, so alloy pipe takes over where steam temperatures and pressures climb. The grades form a ladder of increasing alloy content and temperature capability: P11 (1.25 Cr, 0.5 Mo) is the workhorse for main steam up to about 565 degrees C, P22 (2.25 Cr, 1 Mo) extends the range, and P91 (9 Cr, 1 Mo, V, Nb) is used in supercritical and ultra-supercritical units.
Specification and Welding Considerations
Chrome moly pipe requires preheat and post-weld heat treatment to avoid hydrogen cracking and to restore the heat affected zone properties. Preheat temperature typically ranges from 150 degrees C for P11 up to 200 to 250 degrees C for P91. After welding, post-weld heat treatment at a controlled temperature and hold time is mandatory. Hardness limits are usually imposed on the finished weld, for example a maximum of 350 HV for P91. For this reason chrome moly pipe is normally supplied with a full heat treatment record and, where specified, hardness survey data. When ordering, state the grade, size, schedule, quantity, end finish and whether a hardness survey or supplemental NDE is required.
What is the difference between P11 and P22 pipe?
P11 contains 1.25 percent chromium and 0.5 percent molybdenum; P22 contains 2.25 percent chromium and 1 percent molybdenum. The higher alloy content in P22 gives it greater creep strength and better high temperature performance, so it can be used at higher steam temperatures, typically up to about 580 to 600 degrees C against roughly 565 degrees C for P11. P22 is also more expensive and requires tighter welding control, so the choice is normally driven by the design steam temperature and pressure.
Why is P91 pipe specified for supercritical boilers?
P91 is a 9 percent chromium, 1 percent molybdenum alloy with vanadium and niobium additions that produce a tempered martensitic microstructure with very high creep strength. This allows much thinner walls than P22 for the same pressure and temperature, which reduces thermal stress during startup and shutdown and lowers the weight of the piping system. Its low thermal expansion coefficient also improves fatigue performance in cyclic operation. The trade-off is that P91 demands stringent welding and heat treatment control, and hardness must be verified on every weld.
Materials
- Chrome-moly alloy steel
- Low alloy steel
Typical applications
- Power plant main steam and reheat lines
- Petrochemical furnace and reformer piping
- Hydrogen service and hydroprocessing units
- High temperature heat exchangers
- Boiler superheater and reheater tubing
Inspection & testing
- 100% ultrasonic testing
- Hydrostatic pressure test
- Tensile, yield and elongation testing
- Hardness testing across the wall
- Chemical analysis including chromium and molybdenum
- Third-party inspection (SGS, BV, TUV)
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