What Is ASTM A335 P1 Pipe?
ASTM A335 is the standard specification for seamless ferritic alloy-steel pipe intended for high-temperature service. Grade P1 is the carbon-molybdenum grade: it is plain carbon steel strengthened with a molybdenum addition of roughly 0.44-0.65%. The molybdenum raises the creep resistance of the steel so that it can carry load at temperatures where carbon steel would soften, which made P1 one of the earliest alloy grades used in power and refinery piping.
Chemical Composition
| Element | Requirement (%) |
|---|---|
| Carbon | 0.10-0.20 |
| Manganese | 0.30-0.80 |
| Phosphorus | 0.025 max |
| Sulfur | 0.025 max |
| Silicon | 0.10-0.50 |
| Molybdenum | 0.44-0.65 |
The specification also limits residual elements such as chromium, copper and nickel to low levels, since P1 is not an alloyed grade beyond the molybdenum addition.
Mechanical Properties
| Property | Requirement |
|---|---|
| Tensile strength (min) | 55,000 psi (380 MPa) |
| Yield strength (min) | 30,000 psi (205 MPa) |
| Elongation in 2 in. (min) | 30% |
Compared with A106 Grade B, P1 has a similar room-temperature strength but holds its strength better at elevated temperature, which is the reason it is specified for steam and process lines above about 425 degrees C.
Manufacturing and Heat Treatment
P1 pipe is produced by the same seamless routes as other A335 grades, from pierced billets, and is normally supplied in the annealed or normalized condition. The heat treatment is recorded on the mill certificate. For the best elevated-temperature properties, normalized product is preferred over annealed where the design takes advantage of creep strength.
Size Range and Supply Forms
A335 P1 is available in the full range of seamless pipe sizes, from small-bore NPS 1/4 up to large diameters above 24 in., with wall thicknesses from Schedule 40 through Schedule 160 and heavier. Ends are supplied plain, beveled or threaded, and lengths are single random, double random or cut to order.
Applications
P1 pipe is found in refinery heater and transfer lines, power plant steam and feedwater piping where temperatures exceed carbon steel limits, boiler external piping, and chemical plant process lines carrying hot hydrocarbons. It is also used for flanges and fittings in the same systems, which are covered by ASTM A234 WP1 for wrought fittings and A182 F1 for forged flanges.
Welding and Fabrication
C-Mo steels need controlled welding. Preheat is generally required for heavier walls, and post-weld heat treatment is commonly specified to temper the weld heat-affected zone and avoid cracking. Matching filler metals for the C-Mo family, such as E7018-A1 electrodes or ER70S-A1 wire, keep the weld strength aligned with the base metal. Procedure qualification per ASME Section IX is required for code work.
P1 vs P11 vs P22 vs P91
| Grade | Alloying | Relative Creep Strength | Typical Use |
|---|---|---|---|
| P1 | C-Mo | Low | Older plants, moderate temperature |
| P11 | 1.25Cr-0.5Mo | Medium | Refinery and power piping |
| P22 | 2.25Cr-1Mo | Medium-high | Main steam and hot reheat |
| P91 | 9Cr-1Mo-V | High | Ultra-supercritical steam |
Modern designs favor P22 and P91, but P1 remains in service in many existing plants and is still ordered for replacement and maintenance work.
Inspection and Certification
Standard requirements include hydrostatic testing, tensile testing per heat, and flattening or bend tests. The mill certificate must show chemistry, mechanical results, heat treatment, heat numbers and test pressures. Additional ultrasonic or eddy current examination can be specified by agreement.
FAQ
Is A335 P1 a stainless steel?
No. It is a ferritic low-alloy carbon-molybdenum steel. It is selected for high-temperature strength, not corrosion resistance.
What does the P in P1 stand for?
P is the prefix used in ASTM A335 for pipe grades, following the pattern P1, P5, P9, P11, P22, P91 and so on. The number or combination identifies the alloy family.
At what temperature is P1 useful?
P1 is typically used up to about 480-540 degrees C depending on the design code and allowable stress. Above that range, chromium-molybdenum grades such as P11 or P22 are needed.
Can P1 pipe be welded with ordinary carbon steel electrodes?
No. Use low-hydrogen C-Mo consumables such as E7018-A1 or ER70S-A1, and follow the qualified procedure with preheat and PWHT as required.
What is the difference between P1 and P2?
P2 is the low-carbon variant with a carbon limit of 0.05-0.15%, compared with 0.10-0.20% for P1. P2 welds slightly more easily but has lower strength; P1 is far more common in practice.
Is P1 pipe available with impact testing?
Charpy impact testing is not a default requirement for A335 P1, but it can be added by agreement for low-temperature or critical service.
