ASTM A335 A335M Seamless Alloy Steel Tube Buyer's Guide: P11, P22 and P91 for High-Temperature Power, Boiler and Petrochemical Service

Why ASTM A335 A335M Is the Default Alloy Tube for Power, Boiler and Petrochemical Service Above 540 °C

Inside the boiler island of a 600 MW supercritical unit, the primary superheater outlet sits around 580 °C, and the high-temperature reheat line runs to 600 °C and beyond. Plain carbon steel pipe is no longer enough in that band — the pipe the engineer calls out for those sections is ASTM A335 A335M steel tube, the ASME/ANSI specification for seamless ferritic alloy-steel pipe intended for high-temperature service. A335 covers NPS 1/8 to NPS 48 and a full family of Cr-Mo and Cr-Mo-V grades, the most procured being P11, P22, P91 and P92.

What separates A335 from a generic "alloy pipe" is the combination of a controlled Cr-Mo or Cr-Mo-V chemistry, a mandatory normalize-and-treat heat treatment, and high-temperature tensile and creep data that the spec writer can plug directly into an ASME B31.1 or B31.3 design calculation. The corresponding ASME designation is SA335; the two specifications are identical. For international buyers, A335 occupies the same slot in the U.S. system that EN 10216-2 (16Mo3, 13CrMo4-5, 10CrMo9-10) and GB/T 5310 (15CrMoG, 12Cr1MoVG, 12Cr2MoWVTiB) occupy in the European and Chinese systems. The chemistry overlaps, but the testing regime, the MTC format, and the high-temperature allowable-stress tables all differ, and that is where most cross-standard procurement errors happen.

The A335 Grade Family: Which P-Number Fits Which Service

A335 is a family, not a single material. The standard defines more than 20 P-numbered grades — P1, P2, P5, P5b, P5c, P9, P11, P12, P15, P21, P22, P23, P24, P36, P91 (Type 1 and Type 2), P92, P93, P115, P122, P128, P911, and P921. Most project orders concentrate on four grades that map cleanly to project service.

Chemical Composition: Reading the MTC and Spotting Substitutions

The MTC that accompanies an A335 delivery has to be read against the standard, not against a generic alloy-steel expectation. Three elements deserve close attention for the four primary grades.

Carbon (C). P11 and P22 cap C at 0.05-0.15 %; P91 Type 1 tightens to 0.08-0.12 %. A mill running at the upper end of the range will produce a tube that is harder to weld and demands a more stringent preheat and PWHT. If the welding procedure is qualified to a specific carbon-equivalent ceiling, write that ceiling into the purchase order — for P91 the typical CEV ceiling is 0.45 % and for P22 around 0.50 %.

Chromium (Cr) and molybdenum (Mo). The Cr-Mo envelope gives the A335 grades their high-temperature strength and oxidation resistance. P11 has 1.00-1.50 % Cr and 0.44-0.65 % Mo; P22 doubles Cr to 1.90-2.60 % and lifts Mo to 0.87-1.13 %; P91 jumps to 8.00-9.50 % Cr with V (0.18-0.25 %), Nb (0.06-0.10 %) and N (0.03-0.07 %) for dispersion strengthening. Mills offering P91 without reporting V, Nb, N have slipped out of the P91 envelope and the creep margin is compromised.

Residual elements and trace alloys. A335 caps sulfur and phosphorus tightly (typically ≤ 0.025 % by heat) because S and P segregate to grain boundaries and shorten creep life. For sour-service applications with hydrogen sulfide, the S and P caps tighten further and the mill should be asked for HIC / SSC test data per NACE TM0170 / TM0284, which are not automatically part of an A335 delivery.

Mechanical Properties and Heat Treatment: The Difference Between a Pipe That Passes and a Pipe That Lasts

A335 mechanical properties are tied directly to the heat-treatment condition. The standard mandates full / isothermal anneal, normalize-and-temper, or subcritical anneal depending on the grade, and the tempering temperature is the single most important variable for creep life.

Two practical consequences follow. First, the tempering temperature for P91 is unusually narrow and high compared with P11 / P22, and a mill that runs P11 and P22 on the same line cannot trivially switch to P91 without retuning the furnace control — the as-tempered hardness range, the V-Nb-N dispersion, and the creep-rupture margin all depend on getting the temper exactly right. Second, the tempering temperature of P91 sits inside the standard temper-embrittlement window for some Mn / Si / P combinations, so the MTC has to report these elements with sufficient resolution to confirm the steel is not embrittled.

A335 vs A213 vs A691 vs EN 10216-2 vs GB 5310: Where Each Standard Sits

A335 sits inside a wider family of high-temperature pipe and tube standards, and on most real tenders two or three of them appear in the same datasheet. The table below shows how A335 lines up against the standards it is most often compared to.

The mechanical targets and chemistry for A335 P22 and EN 10216-2 10CrMo9-10 are close but not identical — the allowable-stress tables, the impact-test temperatures, and the MTC format all differ. Substituting one for the other on a cross-continental project should always be cross-checked against the project piping code (ASME B31.1, B31.3, EN 13480, or Chinese GB/T 16507) and approved by the engineer of record. For the same reason, an A335 tube should not be substituted for an A213 tube without confirming the OD / wall / tolerance envelope. For sustained metal temperatures above ~ 600 °C, the engineer should also consider moving up from A335 to ASME B16.34 alloy-steel industrial valves and matching A182 alloy-steel pipe fittings (F11, F22, F91) to keep the same creep envelope across the flanged joint.

Dimensional Range and Bundled Package Supply

A335 is manufactured as seamless pipe in the size range 1/8" to 48" (DN 6 to DN 1200) per ASME B36.10, in schedules SCH 20 to SCH 160 and XXS, in lengths of 6 m, 12 m, or random. For P22 and P91 in the heavy-wall section, the seamless production route (hot-pierce, hot-extrude, or hot-pilger) is what makes the heavy wall economically viable — welded alternatives (A671, A672, A691) cannot match the seamless ductility and creep margin. Surface treatment typically includes shot blasting and primer, FBE / 2PE / 3PE anti-corrosion coating for buried service, or hot-dip galvanizing for structure works and atmospheric service.

Most project buyers do not want just the line pipe. For a 600 MW USC unit, a refinery hydrocracker, or an ethylene cracker the package typically includes the A335 A335M steel tube, the matching butt-weld fittings in P11 / P22 / P91 chemistry, the forged pipe flanges in A182 F11 / F22 / F91, the gasket, stud bolt and nut set, and the alloy-steel industrial valves. Buying the package from one mill-direct source — all on the same Bill of Lading and all with the same MTC chain — cuts the coordination overhead of a multi-supplier tender and gives the operator a single audit trail for the whole pressure-containing assembly.

Testing, Inspection, and the Mill Test Certificate

An A335 delivery is not complete without the inspection chain the standard expects. At minimum, the mill should provide:

For P91, the MTC must also report the actual tempering temperature and the as-tempered hardness, since the creep-rupture margin is set by the temper, not the room-temperature tensile result. For sour-service (H₂S partial pressure above 0.0003 MPa), request HIC and SSC test data per NACE TM0170 and TM0284, which are not automatically part of an A335 delivery. For power plants & aerospace and nuclear service, the MTC must reference the manufacturer's NBBI / ASME III N-stamp.

Typical Applications: Where A335 Is the Right Tube and Where It Is Not

A335 is the right call when the service envelope matches its scope. The four application areas where it dominates are: (1) power-plant superheater, reheater, and header piping — A335 P11 / P22 for the lower-temperature sections, A335 P91 for the high-temperature USC headers; (2) refinery and petrochemical facilities high-temperature process pipe — hydrocracker reactor effluent lines, ethylene cracker quench lines, sulfur-recovery unit process piping; (3) combined-cycle HRSG high-pressure sections — A335 P11 and P22 paired with heat efficiency tubes in the economizer and U bend tubes in the HP evaporator; and (4) power plants & aerospace high-temperature auxiliary piping — supercritical CO₂ cycle piping, gas-turbine exhaust, aerospace test-stand headers, all served by A335 P91 and P92.

Where A335 is the wrong call: low-temperature service below –28.9 °C (move to A333 Grade 6 or LTCS), corrosion service where the Cr-Mo envelope is not enough (move to stainless 300-series or copper & nickel alloy for seawater / acid), low-pressure process pipe where carbon & carbon alloy steel is sufficient (move to A106), and pipeline transportation of large volumes of oil and gas over long distances (move to API 5L linepipe).

Procurement Pitfalls: Five Mistakes We See on A335 Orders

Five pitfalls come up most often on A335 tenders. They are easy to avoid once you know they exist.

Pitfall 1 — Quoting P91 on a mill that runs P11 / P22 only. P91 demands a tighter normalize-and-temper window, a more accurate furnace control, and a documented V-Nb-N chemistry. A mill that runs P11 and P22 on the same line cannot trivially switch to P91 without retuning the heat-treatment cycle and qualifying the procedure. Confirm the mill has a documented P91 production record and a recent creep-rupture test before accepting the order.

Pitfall 2 — Treating "alloy steel pipe" as a single category. A335 P11 and P22 share a basic Cr-Mo envelope, but the tempering temperature, the creep strength, and the MTC reporting all differ. A mill that supplies P22 in the boiler envelope cannot automatically be trusted to supply P91 in the USC envelope.

Pitfall 3 — Skipping the EN 10204 certificate type. "MTC" on its own is ambiguous. A 2.1 or 2.2 certificate is mill-issued without independent validation; a 3.1 certificate is mill-issued and traceable to a specific heat; a 3.2 certificate is mill-issued and independently validated. For high-temperature or high-pressure service, the minimum should be EN 10204 3.1; for nuclear or safety-related service, 3.2.

Pitfall 4 — Substituting A335 for A213 on a boiler-tube call-out. A335 is the pipe standard; A213 is the boiler-tube standard for smaller OD and tighter wall-thickness tolerances. The mechanical targets overlap, but the dimensional envelope, the surface finish, and the eddy-current inspection thresholds all differ. If the datasheet says A213, the MTC must say A213.

How to Source A335 A335M from a Mill-Direct Manufacturer

The A335 seamless tube in the export market is dominated by a small group of integrated mills that run hot-pierce + hot-extrude + cold-draw lines with online NDT, hydrostatic test beds, and ASME / ISO 9001 / CE-PED certified quality systems. Four questions separate a true mill-direct supplier from a stockist or trading house.

1. Can the mill show dual A335 / SA335 certification for the production line that will run your order? A 3.1 MTC referencing the heat is non-negotiable for any high-temperature or high-pressure service.
2. Does the mill have a documented heat-treatment cycle for the specific grade — P11, P22, P91, or P92 — with the tempering temperature actually reported on the MTC? This is the difference between a tube that passes the room-temperature tensile test and one that survives 100,000 hours of creep at 600 °C.
3. Can the mill deliver bundled with the matching pipe fittings, pipe flanges, and industrial valves on the same vessel? Single-shipment package supply cuts months off the construction schedule and gives you one MTC chain for the whole pressure-containing assembly.
4. What is the mill's annual capacity for A335 P22 and P91 in the diameter and wall-thickness range you need? A multi-thousand-tonne tender against a mill with sub-1,000 t/year capacity for P91 is a delivery-risk signal.

EZ Steel Industrial, established in 1994, operates three manufacturing locations across China — Cangzhou for alloy steel tube inventory and custom fittings, Yangzhou for large-scale carbon & carbon alloy steel and alloy steel tube production including A335 P11, P22, and P91 in hot-finish and cold-drawn conditions, and Lishui for stainless and copper-nickel corrosion-resistant materials. The Yangzhou facility is configured to run A335 / SA335 dual certification with online ultrasonic and eddy-current inspection, full-body hydrostatic testing, and a documented normalize-and-temper cycle for the P11, P22, and P91 grades. Orders are supported with mill-direct pricing, MTCs traceable to heat number under EN 10204 3.1, and bundled project supply including the matching pipe fittings, flanges, and industrial valves specified in the same tender package.

A335 Procurement FAQ

A335, the Rest of the Package, and the Mill-Direct Advantage

For a power-plant USC header, a refinery hydrocracker, or an ethylene cracker, the engineer usually wants the A335 A335M steel tube in P11 / P22 / P91, the matching ASME B16.9 butt-weld fittings in the same chemistry, the forged pipe flanges in A182 F11 / F22 / F91, the alloy-steel industrial valves, the gasket, stud bolt and nut set, and the heat efficiency tubes and U bend tubes in the HP evaporator — all on the same Bill of Lading, all with the same MTC chain, and all with the same heat-traceability record. Buying that package from one mill-direct source cuts the coordination overhead of a multi-supplier tender, eliminates the document-traceability gaps that show up during the operator's audit, and gets the package onto the project site on a single shipping window.