Scope of ASTM A335
ASTM A335 is the specification for seamless ferrous alloy steel pipe intended for high-temperature service. It is the standard reference for boiler and superheater piping, process piping and high-temperature headers, and is often written as ASME SA335 when the pipe is supplied for code-stamped construction. The specification covers pipe with nominal wall and with minimum wall thickness, and pipe furnished to it is suitable for bending, flanging and similar forming operations as well as for fusion welding.
The common grades are P5, P9, P11, P22 and P91. Service temperature for the family is typically quoted over the range 540 ℃ to 750 ℃, with the grade chosen according to temperature, pressure and the required creep strength. Because the same material is used for fittings and flanges, the grades are referenced in ASTM A234 WP5, WP9, WP11, WP22 and WP91 for wrought fittings, and in ASTM A182 F5, F9 and F11 for forged flanges, whose compositions and strength levels are aligned with the matching pipe grade.
Supply Range and Features
| Item | Range or description |
|---|---|
| Standards and grades | ASTM A335 Grades P5, P9, P11, P22, P91 |
| Material type | Chromium-molybdenum alloy steel pipe |
| Fabrication | Seamless, hot rolled or cold drawn |
| Size range | 1/2 in to 24 in |
| Wall thickness | SCH 40, SCH STD, SCH 80, SCH XS, SCH 160 |
| Length | 6 m, 12 m or cut to order |
Chromium and molybdenum together give this family its high-temperature character. Chromium provides oxidation and corrosion resistance at temperature, while molybdenum raises elevated-temperature strength and hardness and resists softening, so the material keeps its shape and load-bearing capacity during long exposure in the creep range. Molybdenum also restricts grain growth and reduces the risk of embrittlement during service.
Grade-by-Grade Overview
P5: 5 % chromium, 0.5 % molybdenum grade for elevated-temperature piping with good resistance to high-temperature hydrogen attack.
P9: 9 % chromium, 1 % molybdenum grade with higher oxidation resistance than P5 at equivalent temperature.
P11: 1.25 % chromium, 0.5 % molybdenum grade, the standard choice for moderately elevated temperature boiler and process piping.
P22: 2.25 % chromium, 1 % molybdenum grade giving higher creep strength than P11 and widely used for main steam and hot reheat lines.
P91: 9 % chromium, 1 % molybdenum, vanadium and niobium modified grade with markedly higher elevated-temperature strength than P22, allowing thinner walls for the same duty.
Chemical Composition
| Grade | C | Mn | Si | Cr | Mo |
|---|---|---|---|---|---|
| P5 | 0.15 max | 0.30 – 0.60 | 0.50 max | 4.00 – 6.00 | 0.45 – 0.65 |
| P9 | 0.15 max | 0.30 – 0.60 | 0.25 – 1.00 | 8.00 – 10.00 | 0.90 – 1.10 |
| P11 | 0.05 – 0.15 | 0.30 – 0.60 | 0.50 – 1.00 | 1.00 – 1.50 | 0.44 – 0.65 |
| P22 | 0.05 – 0.15 | 0.30 – 0.60 | 0.50 max | 1.90 – 2.60 | 0.87 – 1.13 |
| P91 | 0.08 – 0.12 | 0.30 – 0.60 | 0.20 – 0.50 | 8.00 – 9.50 | 0.85 – 1.05 |
Figures are percentages by mass. Phosphorus and sulfur are restricted to 0.025 % in the chromium-molybdenum grades and to 0.020 % and 0.010 % respectively in P91, where residual control is critical to creep performance. Grade P91 also carries deliberate vanadium, niobium and nitrogen additions that produce its strengthened tempered martensitic structure.
Mechanical Properties
| Grade | Yield strength MPa [ksi] | Tensile strength MPa [ksi] | Elongation % | Hardness |
|---|---|---|---|---|
| P5 | 205 [30] | 415 [60] | 30 | 207 HB max |
| P9 | 205 [30] | 415 [60] | 30 | Not specified |
| P11 | 205 [30] | 415 [60] | 20 | Not specified |
| P12 | 205 [30] | 415 [60] | 30 | 174 HB max |
| P22 | 205 [30] | 415 [60] | 30 | Not specified |
| P91 type 1 and 2 | 415 [60] | 585 [85] | 20 | 190 – 250 HBW |
P91 hardness is controlled within a range rather than a single ceiling because the grade depends on a tempered martensitic structure. Too soft a structure loses creep strength and too hard a structure loses toughness, so the acceptance band of 190 to 250 HBW, equivalent to 196 to 265 HV or 91 HRBW to 25 HRC, is specified for both type 1 and type 2 material.
Dimension Tolerances
| NPS [DN] designator | Over + % | Under − % |
|---|---|---|
| 1/8 to 2 1/2 [6 to 65], all t/D ratios | 20.0 | 12.5 |
| Above 2 1/2 [65], t/D 5 % and below | 22.5 | 12.5 |
| Above 2 1/2 [65], t/D above 5 % | 15.0 | 12.5 |
Wall thickness tolerances are expressed as a percentage of the ordered wall, where t is wall thickness and D is outside diameter. Outside diameter tolerances follow the size bands below.
| NPS [DN] designator | Over + mm | Under − mm |
|---|---|---|
| 1/8 to 1 1/2 [6 to 40] | 0.40 | 0.40 |
| Above 1 1/2 to 4 [40 to 100] | 0.79 | 0.79 |
| Above 4 to 8 [100 to 200] | 1.59 | 0.79 |
| Above 8 to 12 [200 to 300] | 2.38 | 0.79 |
| Above 12 [300] | ±1 % of the specified outside diameter | |
FAQ
Q: What is ASTM A335 pipe used for?
It is seamless alloy steel pipe for high-temperature service, used in boiler and superheater tubing, main steam and hot reheat lines, process piping and high-temperature headers, generally over the 540 ℃ to 750 ℃ range.
Q: What is the difference between P11 and P22?
P11 contains 1.00 to 1.50 % chromium and P22 contains 1.90 to 2.60 % chromium with about 1 % molybdenum. P22 therefore offers higher creep strength and is preferred for hotter or more heavily loaded lines.
Q: Why does P91 have much higher strength than P22?
P91 is a modified 9 % chromium grade with controlled vanadium, niobium and nitrogen. Its tempered martensitic structure gives a minimum yield strength of 415 MPa against 205 MPa for P22, so thinner walls can be used for the same pressure.
Q: Are fittings and flanges available in matching grades?
Yes. Wrought fittings are covered by ASTM A234 WP5, WP9, WP11, WP22 and WP91, and forged flanges by ASTM A182 F5, F9 and F11, with compositions and properties aligned to the matching pipe grade.
Q: How is P91 hardness controlled?
P91 is accepted within a hardness band of 190 to 250 HBW, equivalent to 196 to 265 HV or 91 HRBW to 25 HRC, for both type 1 and type 2 material, so that creep strength and toughness are both retained.
Q: What sizes and walls are normally produced?
A335 pipe is supplied from 1/2 in to 24 in in SCH 40, STD, SCH 80, XS and SCH 160 walls, in 6 m or 12 m lengths or cut to order.