What Is ASTM A335 Alloy Steel Pipe
ASTM A335 is the specification for seamless ferritic alloy steel pipe intended for high-temperature service in power plants, refineries and petrochemical units. The grades are defined by their chromium and molybdenum content, which raises creep strength and oxidation resistance so that the pipe can carry steam or hot process fluid well above the temperature range of carbon steel.
Because the pipe works under creep conditions, the specification combines chemical composition, mechanical property and heat treatment requirements rather than strength alone. Grade P91, for example, contains vanadium and niobium and is supplied normalised and tempered to a controlled hardness range.
Grades and High-Temperature Mechanical Properties
| Grade | Nominal composition | Yield, MPa | Tensile, MPa | Elongation |
|---|---|---|---|---|
| P5 | 5Cr - 0.5Mo | 205 min | 415 min | 30% min |
| P9 | 9Cr - 1Mo | 205 min | 415 min | 30% min |
| P11 | 1.25Cr - 0.5Mo | 205 min | 415 min | 20% min |
| P12 | 1Cr - 0.5Mo | 205 min | 415 min | 30% min |
| P22 | 2.25Cr - 1Mo | 205 min | 415 min | 30% min |
| P91 | 9Cr - 1Mo - V | 415 min | 585 min | 20% min |
The lower-alloy grades, P11 and P22, are the workhorses of refinery and power piping, while P91 is chosen where metal temperatures and steam conditions are most demanding. Hardness is controlled as well, for instance a maximum for the low-chromium grades and a controlled range for P91 to preserve both toughness and creep strength.
Dimensional Range and Wall Thickness Tolerances
A335 pipe is normally supplied from 1/2 in. to 24 in. in schedules 40, STD, 80, XS and 160, in lengths of 6 m, 12 m or cut to order. Wall thickness tolerance is applied as a percentage of nominal and depends on the size and on the ratio of wall thickness to outside diameter:
| Size condition | Tolerance |
|---|---|
| NPS 1/8 to NPS 2.5 | +20.0% / -12.5% |
| NPS above 2.5, wall to diameter ratio 5% or less | +22.5% / -12.5% |
| NPS above 2.5, wall to diameter ratio above 5% | +15.0% / -12.5% |
Outside diameter tolerance is set for the size range in the ordering specification, with a fixed allowance applied to small sizes and a larger allowance for heavy-wall and large-diameter pipe. The minimum wall, not the nominal wall, governs the pressure design calculation, so the minus tolerance always has to be taken into account when the thickness is selected.
Heat Treatment and Welding Considerations
Grades P11, P12 and P22 are normally supplied in the normalised and tempered or fully annealed condition, P5 and P9 in the annealed or normalised and tempered condition, and P91 in the normalised and tempered condition with a controlled cooling rate and a strict tempering temperature. Heat treatment is performed by the producing mill and recorded on the test report.
Welding requires qualified procedures and matched or matching filler metal: low-alloy electrodes or filler wires for the chromium-molybdenum grades and a matching consumable for P91. Preheat and interpass temperature are controlled, and post-weld heat treatment is specified for the section thicknesses and grades where the code requires it. Because the grades harden readily, uncontrolled cooling of a weld or flame-cut edge should be avoided.
Applications and Matching Fittings
Typical services are main steam and hot reheat piping, boiler headers and superheater tubes, refinery and petrochemical process piping, hydrogen and hydrocracker circuits, and high-temperature steam distribution. Fittings for the same systems are ordered to ASTM A234 in the matching WP grades, such as WP5, WP9, WP11, WP22 and WP91, and forged components to ASTM A182 in F5, F9, F11 and F22 so that the whole line has consistent metallurgy and heat treatment.
FAQ
Q: What does the P in P91 stand for?
P designates pipe in the ASTM A335 series; the same steel as a forging is designated F91 and as a fitting WP91.
Q: Which grade should be used for main steam piping?
P22 is widely used for conventional steam conditions and P91 for the highest temperature and pressure service.
Q: Is P11 magnetic?
Yes, all grades in the series are ferritic steels and are magnetic.
Q: Why is P91 hardness controlled?
A controlled hardness range balances creep strength against toughness and weldability.
Q: Can A335 pipe be welded without post-weld heat treatment?
Heavy sections and the higher-alloy grades normally require post-weld heat treatment as set out by the applicable code and the qualified procedure.
Q: What lengths are standard?
Six metre, twelve metre and cut lengths to the project drawing are customary.
