What ASTM A335 P2 Pipe Is
ASTM A335 P2 is a seamless ferritic alloy steel pipe grade within the A335 specification, which covers seamless pipe for high-temperature service. The grade sits at the entry point of the chromium-molybdenum family, with roughly 0.5% chromium and 0.5% molybdenum, and it is followed in the same specification by the better known P11, P12, P22 and P91 grades. Pipe is supplied in the hot-finished seamless condition without a longitudinal weld seam.
Molybdenum is the element that provides creep strength, while the modest chromium addition improves resistance to oxidation and to high-sulphur process streams. The combination gives a performance window above that of plain carbon steel while keeping the alloy content - and therefore the price, weldability and heat treatment burden - well below the higher Cr-Mo grades.
Chemical Composition and Mechanical Requirements
The table summarises the heat analysis limits and tensile requirements that ASTM A335 applies to Grade P2. Tolerance and rounding rules are governed by the base specification A335/A335M.
| Item | Requirement |
|---|---|
| Carbon | 0.10-0.20% |
| Manganese | 0.30-0.61% |
| Phosphorus, max | 0.025% |
| Sulphur, max | 0.025% |
| Silicon | 0.10-0.30% |
| Chromium | 0.50-0.81% |
| Molybdenum | 0.44-0.65% |
| Tensile strength, min | 380 MPa |
| Yield strength, min | 205 MPa |
| Hardness | Not specified for this grade |
| Testing | Tensile, bend, flattening and hydrostatic tests apply to seamless pipe of this grade |
Because the specification fixes only minimum tensile and yield values, producers aim at a controlled working range above those minimums, and mill certificates should be read as actual results rather than as design values. Hardness figures for this grade are frequently quoted in commercial literature but are not part of the A335 requirement, so a hardness limit is only meaningful when it is agreed separately as a supplementary acceptance criterion.
Service Range and Typical Applications
Boiler pressure parts: superheater and reheater tubes, economiser sections and steam drum internals.
Steam and hot water piping where the design temperature exceeds the practical carbon steel limit but does not justify a 1-1/4Cr or 2-1/4Cr grade.
Heat exchanger shells, channels and nozzles, and intermediate-temperature pressure vessel components.
Process piping in refining and chemical plants handling hot oils, gases and vapours below the temperature range of P11 and P22.
Replacement and repair spools in ageing plants, where matching the installed grade simplifies weld procedure qualification.
Allowable stress values for the grade are tabulated in recognised pressure equipment design codes. Where the design temperature falls outside the tabulated range, a higher-chromium grade is selected rather than extending this grade beyond its published data.
Fabrication, Welding and Heat Treatment
P2 is the most forgiving grade in the Cr-Mo family to fabricate, but standard alloy steel discipline still applies.
Welding is performed to a qualified procedure with a matching Cr-Mo consumable or a low-hydrogen consumable selected for the joint strength requirement.
Preheat removes moisture and slows the cooling rate of the heat-affected zone; the minimum preheat temperature increases with wall thickness.
Post-weld heat treatment is required for heavier sections and for services where residual stress and heat-affected zone hardness must be controlled; soak temperature and time follow the construction code.
Cold bending suits moderate radii, while heavy walls are usually bent warm; formed areas are checked for wall thinning and ovality after bending.
Thermal cutting leaves a hardened layer that is removed by grinding before the joint is welded.
Ordering and Inspection Points
Match the grade and heat number on the mill certificate to the drawing or purchase order, and read the heat analysis rather than a nominal composition.
Verify outside diameter, wall thickness, length and ovality against the order; seamless pipe tolerances are broader than those of cold-drawn mechanical tubing.
Confirm that the hydrostatic test and the applicable non-destructive examination have been performed and recorded.
Store pipe clear of the ground with end caps in place, and keep low-hydrogen electrodes dry before use.
For repair spools, record the original grade, wall thickness and heat number so that the replacement matches the installed line.
FAQ
Q: What is ASTM A335 P2 pipe?
It is a seamless ferritic alloy steel pipe grade containing about 0.5% chromium and 0.5% molybdenum, produced under the A335 specification for high-temperature service.
Q: How does P2 differ from carbon steel pipe?
The molybdenum addition raises creep strength and the chromium addition improves oxidation resistance, so the grade can be used at temperatures where carbon steel would deform under sustained load.
Q: Where is P2 pipe normally used?
In boiler pressure parts, steam piping, heat exchangers and refinery process lines operating above the carbon steel range but below the range where P11 or P22 become necessary.
Q: Is P2 easy to weld?
It is the most weldable of the chromium-molybdenum grades, but preheat, controlled interpass temperature and post-weld heat treatment for heavy sections are still required.
Q: What is the difference between P2 and P11?
P11 contains about 1-1/4% chromium and offers better oxidation and corrosion resistance together with a higher useful temperature ceiling than the 0.5Cr grade.
Q: How should the material be ordered?
Specify the specification and grade, the delivery condition, outside diameter and wall thickness, the length, the test requirements and the form of mill certification required.
