ASTM A335 — Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service
Covers seamless chrome-moly alloy steel pipes in grades P5, P9, P11, P22, P91, P92 for refinery, power plant, and high-temperature process piping.
ASTM A335 (formally ASTM A335/A335M) is the ASTM standard specification for seamless ferritic alloy-steel pipe for high-temperature service. It covers chrome-moly (chromium-molybdenum) alloy steel pipes used in refineries, power plants, petrochemical plants, and process piping systems where operating temperatures exceed the capability of carbon steel. The standard is the pipe counterpart of ASTM A213 (which covers tubes); both specifications share the same alloy chemistry systems but differ in product form — A335 pipes are ordered by NPS and schedule, while A213 tubes are ordered by OD and wall thickness. The grade system uses the prefix "P" (for pipe) followed by a numeric identifier that corresponds to the alloy composition. The most commonly procured grades in India are: P5 (5Cr-0.5Mo) for refinery heater tubes and transfer lines exposed to high-temperature sulfidation; P9 (9Cr-1Mo) for similar refinery applications requiring higher chromium for enhanced sulfidation resistance; P11 (1.25Cr-0.5Mo) the entry-level chrome-moly grade for boiler piping, headers, and steam lines operating up to approximately 550°C; P22 (2.25Cr-1Mo) the workhorse grade for superheater headers, main steam piping, and hot-reheat piping in subcritical power plants operating up to approximately 580°C; P91 (9Cr-1Mo-V-Nb, modified) the advanced creep-strength enhanced ferritic steel for supercritical and ultra-supercritical power plants operating up to approximately 620°C; and P92 (9Cr-2W-Mo-V-Nb) the tungsten-enhanced variant of P91 for the most demanding ultra-supercritical applications up to approximately 630°C. The chrome-moly composition rationale is straightforward: chromium provides oxidation and sulfidation resistance at elevated temperatures, while molybdenum enhances creep strength — the resistance to slow deformation under sustained stress at high temperature. As operating temperature increases, higher chromium and molybdenum content is required. P11 (1.25Cr-0.5Mo) is the baseline; P22 (2.25Cr-1Mo) doubles the chromium; P91 jumps to 9Cr-1Mo with micro-alloying additions of vanadium and niobium that create a tempered martensite microstructure with exceptionally high creep rupture strength. Temperature limits per grade are a critical selection parameter. P5 and P9 are rated for continuous service up to approximately 600°C in refinery applications (where sulfidation resistance is the primary requirement). P11 is limited to approximately 550°C for long-term creep service in power plants. P22 extends capability to approximately 580°C. P91 is the standard choice for 580–620°C service, offering 2–3× the allowable stress of P22 at these temperatures, which permits thinner walls, lighter weight, and reduced thermal fatigue. P92 pushes the envelope to approximately 630°C for the most advanced ultra-supercritical designs. Heat treatment is mandatory for all A335 grades and is critical to achieving the required microstructure and mechanical properties. P11 and P22 are normalized at 900–950°C and tempered at 650–730°C to produce a ferrite-bainite microstructure. P91 requires a precise austenitising treatment at 1040–1080°C followed by air cooling (or accelerated cooling) to produce full martensite, then tempering at 730–780°C (minimum 1 hour per 25 mm of thickness) to produce tempered martensite with a fine distribution of MX (vanadium/niobium carbonitrides) precipitates. Incorrect heat treatment of P91 — particularly under-tempering or over-tempering — can reduce creep life by orders of magnitude and is a significant procurement risk. In the Indian context, the Indian Boiler Regulations (IBR) govern the use of alloy steel pipe in boiler and steam-line applications. ASTM A335 is approved under IBR through its ASME counterpart: SA-335 (ASME Boiler and Pressure Vessel Code, Section II, Part A). The distinction is identical to A106/SA-106: ASTM A335 and ASME SA-335 have the same chemistry and mechanical requirements, but SA-335 carries the ASME code mark required for ASME-coded pressure vessels and boiler piping. For IBR documentation, always specify "SA-335" to ensure Form III-C references the correct ASME code. IBR certification requires manufacturing under the supervision of an IBR-recognized Inspecting Authority (Lloyd's Register, TÜV, Bureau Veritas, DNV, or Indian BID). Procurement of A335/SA-335 pipe in India requires careful specification: grade (P11, P22, P91, etc.), NPS size, schedule (typically Sch 60, 80, 120, 160 or XXS for high-pressure service), length (SRL or DRL), heat treatment condition (normalized and tempered for P11/P22; quenched and tempered for P91), documentation (MTC 3.1 or 3.2 per EN 10204, IBR Form III-C for boiler service), and any supplementary requirements (hardness verification, impact testing, PWHT simulation). For P91 specifically, buyers should request verification of the Ac1 temperature, hardness mapping (200–260 HV typical), and creep test data from the mill. RP Sales supplies ASTM A335/SA-335 pipes in all grades with full IBR and ASME documentation for power plant and refinery projects.
Scope and grade system
ASTM A335 covers seamless ferritic alloy-steel pipe from NPS 1/8" through NPS 30" and larger. The grade system uses "P" designations tied to chromium-molybdenum composition: P5 (5Cr-0.5Mo), P9 (9Cr-1Mo), P11 (1.25Cr-0.5Mo), P22 (2.25Cr-1Mo), P91 (9Cr-1Mo-V-Nb modified), and P92 (9Cr-2W-Mo-V-Nb). Additional grades P1, P2, P12, P15, P36, and others exist for specialised applications but represent a small fraction of Indian procurement volume.
Grade selection is driven by the maximum design metal temperature and the required creep rupture life. For temperatures below 550°C, P11 is the standard choice. Between 550°C and 580°C, P22 is specified. For 580–620°C, P91 offers significantly higher allowable stress and thinner walls. P92 extends capability to 630°C for ultra-supercritical designs. The refinery grades P5 and P9 are selected primarily for sulfidation resistance rather than creep, and are used in heater tubes, transfer lines, and catalyst piping.
Temperature and creep resistance
Creep — the slow, time-dependent deformation of steel under sustained stress at elevated temperature — is the governing failure mechanism for high-temperature piping. ASTM A335 grades are designed to resist creep through alloying: chromium provides thermal stability and oxidation resistance, while molybdenum (and vanadium/niobium in P91/P92) enhances creep rupture strength by forming stable precipitates that pin dislocations and resist microstructural degradation.
The allowable stress advantage of P91 over P22 is dramatic: at 550°C, P91 allows approximately 117 MPa vs P22's approximately 103 MPa. At 600°C, the gap widens — P91 allows approximately 80 MPa vs P22's approximately 40 MPa. This means P91 piping can be significantly thinner and lighter than P22 piping for the same service conditions, reducing dead weight, support loads, and thermal stresses during transient operations.
Temperature limits are not absolute boundaries but engineering judgements based on 100,000-hour creep rupture data. Operating above the recommended maximum temperature accelerates creep damage and shortens component life. The design engineer references ASME BPVC Section II, Part D for allowable stress tables by temperature for each grade.
Heat treatment requirements
All ASTM A335 grades require mandatory heat treatment to achieve the specified microstructure and mechanical properties. Incorrect heat treatment is the single largest cause of premature creep failure in alloy steel piping — particularly for P91, where the tempered martensite microstructure is critical.
P11 and P22: Normalize at 900–950°C (air cool) followed by tempering at 650–730°C. The resulting microstructure is ferrite-bainite with dispersed carbides. Hardness after heat treatment is typically 130–190 HBW for P11 and 135–200 HBW for P22.
P91: Austenitise at 1040–1080°C (air cool or accelerated cool to achieve full martensite transformation — cooling rate must be sufficient to avoid ferrite formation). Temper at 730–780°C for a minimum of 1 hour per 25 mm of thickness. The resulting microstructure is tempered martensite with M₂₃C₆ carbides at prior austenite grain boundaries and MX (V/Nb carbonitrides) within the matrix. Hardness must be 200–260 HV. Under-tempering (hardness >270 HV) increases cracking risk; over-tempering (hardness <180 HV) degrades creep strength.
P92: Similar to P91 but with austenitising at 1040–1080°C and tempering at 730–780°C. The tungsten addition modifies the carbide distribution and increases long-term creep stability.
Indian procurement and IBR context
In India, ASTM A335 pipes are critical materials for coal-fired power plants (P22 for main steam and hot-reheat piping in subcritical units; P91 for supercritical and ultra-supercritical units), refineries (P5 and P9 for heater tubes and transfer lines; P11/P22 for high-temperature process piping), and fertilizer plants (P11 for reformer piping). The demand for P91 has grown significantly with India's push toward supercritical thermal power generation.
ASTM A335 is approved under IBR through its ASME counterpart SA-335. For boiler and steam-line service under IBR jurisdiction, always specify "SA-335" and require Form III-C certification from an IBR-recognized Inspecting Authority. Standard EN 10204 MTC 3.1 certificates alone are not sufficient for IBR-controlled installations. The Form III-C must reference the SA-335 grade, heat number, chemical composition, mechanical properties, and heat treatment details.
Procurement checklist for A335/SA-335: grade (P11, P22, P91), NPS size, schedule (Sch 60, 80, 120, 160, XXS), length (SRL or DRL), heat treatment condition (state "normalized and tempered" for P11/P22, "normalized/quenched and tempered" for P91), end finish (plain end or beveled for welding), documentation (MTC 3.1/3.2, IBR Form III-C), and supplementary requirements (hardness survey, impact testing at design minimum temperature, PWHT simulation if applicable). For P91, additionally request Ac1 verification, hardness mapping across the wall, and confirmation that the tempering temperature was at least 20°C below the measured Ac1.
| Grade | Cr % | Mo % | Max temp (°C) | UTS (MPa) | Yield (MPa) | Key application |
|---|---|---|---|---|---|---|
| P5 | 4.0–6.0 | 0.45–0.65 | ~600 | 415 min | 205 min | Refinery heater tubes, sulfidation resistance |
| P9 | 8.0–10.0 | 0.90–1.10 | ~600 | 415 min | 205 min | High-chrome refinery service |
| P11 | 1.0–1.5 | 0.44–0.65 | ~550 | 415 min | 205 min | Boiler piping, headers, steam lines |
| P22 | 1.9–2.6 | 0.87–1.13 | ~580 | 415 min | 205 min | Superheater headers, main steam piping |
| P91 | 8.0–9.5 | 0.85–1.05 | ~620 | 585 min | 415 min | Supercritical power plant piping |
| P92 | 8.5–9.5 | 0.30–0.60 | ~630 | 620 min | 440 min | Ultra-supercritical, W-enhanced |
| Size range | NPS 1/8" (DN 6) to NPS 30" (DN 750) and larger |
|---|---|
| Wall thickness | Schedule 40, 60, 80, 120, 160, XXS |
| Standard | ASTM A335/A335M (ASME SA-335) |
| Grades | P5, P9, P11, P22, P91, P92 (and others) |
| Process | Seamless — hot finished or cold drawn |
| Heat treatment | Normalized + tempered (P11/P22); Quenched + tempered (P91/P92) |
| Length | SRL 4–7 m, DRL 7–14 m; cut-to-length available |
| End finish | Plain end (PE) or beveled (BW) for welding |
| Surface | Black (as-heat-treated); machined bore on request |
| Documentation | MTC 3.1/3.2 per EN 10204; IBR Form III-C for boiler service |
- ASTM A335/A335M — Standard Specification for Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service — ASTM International
- ASME BPVC Section II Part A — SA-335 — Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service (American Society of Mechanical Engineers)
- Indian Boiler Regulations, 1950 — Form III-C — Boiler tube material certification — Central Boilers Board, Government of India
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