ASTM A335 A335M Steel Tube Buyer’s Guide: P11, P22, P91, P92 Grades for High-Temperature Power, Petrochemical and Refinery Service

Why ASTM A335 A335M Still Matters in 2026

If your plant runs a superheater, a hydrocracker reactor loop, a steam line above 540 °C, or a delayed coker fractionator, the pipe in that service has a name: A335 A335M steel tube. It is the American standard for seamless ferritic alloy-steel pipe intended for high-temperature service, and after more than eighty years of revisions it is still the most common B2B specification called out on RFQs crossing the Pacific.

The 2021 edition (A335/A335M-21) tightened the heat-treatment tables for P92, P122 and P23, the workhorse alloy steel tube grades for ultra-supercritical (USC) boilers, and clarified the wording on supplementary requirements S1 through S7. None of that changes the procurement reality: a buyer who orders P11 or P22 today still receives the same chemistry and the same tensile envelope, but the paperwork and the marking have to be exact if the pipe is going to be accepted at site.

This guide walks through what the standard actually says, the P-grade decision tree, the cross-reference to EN 10216-2 and JIS G3462, and the practical RFQ checklist that an experienced mill-direct supplier will ask you to fill in before quoting. If you are also buying boiler tubing, heat exchanger tube or pipe fittings for the same unit, the rules below still apply, because A335 sits inside a wider piping-package decision.

What ASTM A335 A335M Actually Covers

The title gives away the scope: "Standard Specification for Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service." Three clauses are worth remembering before you write the PO.

First, "ferritic" here means a low- or medium-alloy steel with chromium content up to 10 percent by weight. That family includes the chrome-moly (Cr-Mo) grades such as P11 (1.25Cr-0.5Mo), P22 (2.25Cr-1Mo), P5 (5Cr-0.5Mo) and P9 (9Cr-1Mo), plus the modern 9-12 percent Cr martensitic steels such as P91, P92, P122 and P911. Austenitic stainless steels (TP304H, TP316H) are deliberately excluded; they live in A312/A213 instead.

Second, the pipe is supplied in the heat-treated condition. Every grade has its own mandatory heat-treatment cycle defined in Table 1, and the lab report has to show the actual austenitising and tempering temperatures that were used. For P92 and P122 the standard requires a minimum 1040 °C normalize followed by a minimum 730 °C temper, and the hardness cap is set at 250 HB / 265 HV / 25 HRC. A pipe that misses the hardness cap is, by definition, not in compliance.

Third, the forming clause: A335 pipe is suitable for bending, flanging, and similar forming operations plus fusion welding. That sentence is short but it is the basis on which fabricators can buy A335 pipe in straight random lengths and bend it into U-tube sections for heat exchanger tube bundles, or roll it into cold-formed headers.

The 11-Item Order Specification

A335 Section 4 lists the eleven items that a purchase order must contain for the manufacturer to accept the enquiry. Skip any one of them and the mill will call you back to clarify, which adds a week to lead time. Here is the same list, in the order a Chinese mill expects to see it.

• Quantity, expressed in feet, metres, or number of lengths.
• Material name, written as "seamless alloy-steel pipe" or "seamless ferritic alloy-steel pipe".
• Grade, from the standard's 16-grade table (P1, P2, P5, P5b, P5c, P9, P11, P12, P15, P21, P22, P23, P36, P91, P92, P122, P911).
• Manufacture route, either hot-finished or cold-drawn, with the final heat treatment called out.
• Size designation, which can be NPS plus schedule number, OD plus nominal wall, OD plus minimum wall, ID plus nominal wall, or ID plus minimum wall.
• Length, either single-random, double-random, or cut-to-length with tolerance.
• End finish, usually plain-end square cut, or bevelled to ASME B16.25 for butt-welding.
• Special requirements, such as hydrostatic test pressure, weight tolerance, or low-temperature impact test at −46 °C.
• Test report format, referenced to EN 10204 type 3.1 or 3.2.
• Standard number, with the year of edition (A335/A335M-21 is current).
• Any supplementary requirements, S1 through S7, only if the buyer actually needs them.

A good practice on multi-line POs is to repeat items 5, 6, 8 and 11 on every line, because most mills process each line independently through their production planning system.

P-Grade Family and Selection by Service Temperature

The 16 grades in the A335 table are not equally common. Roughly 85 percent of global demand is concentrated in P11, P22 and P91, with P92 growing fast as USC units ｒｅｐｌａｃｅ P22 in the 600-620 °C range. The table below is the practical selection reference that an alloy steel tube buyer can carry into a technical meeting.

A common selection shortcut is to match allowable stress, not just chemistry. ASME B31.1 Power Piping and B31.3 Process Piping publish the maximum allowable stress (SMYS-dependent) for each P-grade, and the data table in Section II Part D is the legal reference. If the design temperature is below 540 °C and the section is straightforward, P22 is the workhorse. If the design temperature is between 540 °C and 600 °C, the conversation shifts to P91. Above 600 °C, P92 or P122 enters the picture, with corresponding increases in preheat, interpass and PWHT control.

Cross-Reference: A335 vs EN 10216-2 vs JIS G3462 vs GB/T 5310

Most global EPCs write one specification and then ask the mill to dual-certify. The four-way mapping below is the version that works in 2026 and lines up with the EN 10216-2:2024 changes to the TC1/TC2 test categories. Note that "equivalent" here means chemistry and tensile envelope, not creep rupture life, which is why long-term insurance and warranty contracts always list the ASME designation first.

• P11 (1.25Cr-0.5Mo) maps to EN 10216-2 13CrMo4-5, JIS G3462 STPA23, GB/T 5310 12CrMoG.
• P22 (2.25Cr-1Mo) maps to EN 10216-2 10CrMo9-10, JIS G3462 STPA24, GB/T 5310 12Cr2MoG.
• P91 (9Cr-1Mo-V-Nb) maps to EN 10216-2 X11CrMo9-1, JIS G3462 STPA28, GB/T 5310 10Cr9Mo1VNbN.
• P92 (9Cr-2W-V-Nb) maps to EN 10216-2 X11CrMoWVNb9-1-1, GB/T 5310 10Cr9MoW2VNbBN.
• P5 (5Cr-0.5Mo) maps to EN 10216-2 X12CrMo5, with no direct JIS or GB/T twin.
• P12 (1Cr-0.5Mo) maps to EN 10216-2 13CrMo4-5 (lower-tensile variant), JIS G3462 STPA22.

If your PO calls for "ASTM A335 P22 or EN 10216-2 10CrMo9-10", the mill will run one heat and dual-stamp the pipe. The supplementary requirements then have to be combined, which is where most RFQ errors creep in — EN 10216-2 uses test category TC1/TC2 instead of A450 S1-S7, and the impact-test temperature is set at 0 °C by default rather than at room temperature as in A335.

Mechanical and Dimensional Requirements Buyers Always Ask About

Tensile and Hardness

The tensile values in the table above are minimums. Elongation depends on the test-piece geometry: a strip specimen from a P11 pipe must reach 30 percent longitudinal / 20 percent transverse, while a round specimen drops to 22 percent longitudinal / 14 percent transverse. The P91 and P92 grades have their own minimums at 30 percent and 20 percent respectively. A 38 mm wall P91 pipe must also pass the hardness cap of 250 HB / 265 HV / 25 HRC; if it does not, the heat-treatment batch is re-processed or scrapped.

Diameter Tolerance

A335 diameter tolerance depends on how the pipe was ordered. For NPS-ordered pipe, the tolerance is ±0.79 mm for sizes between 1.5 and 4 inches, +1.59 /−0.79 mm for 4-8 inches, +2.38 /−0.79 mm for 8-12 inches, and ±1 percent for sizes above NPS 12. For OD-ordered pipe the tolerance is ±1 percent across the range. For ID-ordered pipe the tolerance is ±1 percent of the specified inside diameter. Mills that routinely supply power plants & aerospace projects typically cut to half of the standard tolerance for U-bend and header applications, but the buyer has to ask for it.

Wall Tolerance

A335 does not define a unique wall-thickness tolerance; it inherits the A530 table. For Schedule 40 and heavier, the tolerance is −12.5 percent of nominal wall. For extra-thin schedules used in finned-tube panels, mills routinely agree to ±10 percent, but only if the PO asks for it under "special requirements". A request for "no negative wall tolerance" is a red flag for any responsible mill — it implies hot-finishing only, no welding, and no cold-drawing, which is not how the standard is actually produced.

Test and Inspection Package

A buyer who is paying for A335 pipe is paying for a documented test package as much as for the metal. The standard gives the manufacturer the option to use hydrostatic testing or NDT (or both), and the supplementary requirements S1 through S7 add individual test obligations. Below is the package that an EPC inspector will look for on a 100-tonne P22 order for a refinery petrochemical facilities unit.

• Heat-treatment chart showing actual furnace temperatures and hold times, traceable to heat number.
• Chemical composition report, product analysis per S1 if invoked, with all 12+ elements reported.
• Tensile test per heat-treatment batch, including yield, ultimate, elongation and reduction of area.
• Hardness survey for P91 / P92 / P122, with cap documented at 250 HB / 265 HV / 25 HRC.
• Flattening or bend test, depending on NPS-to-wall ratio, performed on the first and last pipe of each batch.
• Hydrostatic test at the manufacturer-selected pressure (or as agreed on the PO), with hold time recorded.
• NDT per E213 (UT), E309 (eddy current) or E570 (flux leakage), with reference-block sensitivity documented.
• Dimensional report, including OD, wall and straightness, with sample frequency per batch.
• Visual and surface-finish report, with any 1.6 mm-deep mechanical mark or any 5 percent-of-wall defect flagged for repair.
• Marking photograph, showing the colour-code and the "S" stamps for any supplementary requirements invoked.

Six Procurement Pitfalls and How to Avoid Them

1. Confusing A335 with A312

A312 is the austenitic stainless pipe standard. P91 and TP304H are different alloys with different heat-treatment cycles, different welding procedures and different price points. A buyer who writes "A335 TP304H" gets either a rejected RFQ or a pipe made to the wrong standard. Keep the family names straight: A335 for ferritic alloy, A312 for austenitic stainless, A213 for ferritic and austenitic tubes (smaller OD), A106 for carbon steel.

2. Forgetting the Year of Edition

Specifying "A335" without a year tells the mill to use the latest version, which today is A335/A335M-21. Some EPCs still have a 2018 or 2015 reference on their datasheets. The differences between editions are small but real, especially around P92 and P122 heat treatment. A 2015 edition call-out for P92 may not require the 730 °C minimum temper, which is why some 2015-stamped P92 pipe has hardness readings above 265 HV and fails the 2021 specification.

3. Not Invoking Supplementary Requirements

S1 through S7 are not applied by default. If you need 100 percent product analysis (S1), 100 percent transverse tension testing (S2), 100 percent flattening (S3), macro-etch on every pipe (S4), or 100 x metallographic on the first heat of a new size (S5), you must say so on the PO. A missing S2 is the most common cause of a delay at the receiving warehouse.

4. Treating P91 Like P22

P91 is a martensitic 9Cr steel that needs preheat at 200-250 °C, interpass temperature control at 300 °C max, and a PWHT at 760 ±15 °C for at least 2 hours. A fabricator used to P22 will run P91 like P22 and produce hard, brittle weldments that fail the ASME B31.1 hardness walk-down. P91 and P92 must be welded, inspected, and tracked with a separate procedure.

5. Mixing NPS and DN Without Conversion

A335 is an inch-pound standard. EN 10216-2 is metric. If the datasheet is in DN and the standard reference is A335, the mill needs an explicit conversion. NPS 8 is DN 200, but NPS 8 sch 40 has a 8.18 mm wall, while DN 200 in EN series is typically 6.3 mm or 8.0 mm depending on the pressure rating. Locking in both designation and conversion before the PO is faster than catching the mismatch on the receiving dock.

6. Skipping the Bundling Plan

A335 pipe is rarely delivered alone. The same RFQ usually includes pipe fittings, flanges, gaskets and stud bolts that match the P-grade chemistry. If those items are sourced from a different supplier with a different MTC format, the receiving inspector will spend a day reconciling certificates. Asking one mill-direct supplier to bundle the entire piping package under a single 3.1 or 3.2 certificate is the cheapest insurance against that delay.

A 7-Question RFQ Checklist

Before you send the enquiry, run the seven questions below. If any answer is missing, mark the line "TBD" and flag it in the cover note — the mill will respond with a clarifications list anyway, and saving one round-trip is usually worth 5-7 days of lead time.

• Grade and edition: ASTM A335/A335M-21, P22 (or whichever grade matches your design code).
• Size and length: NPS 6 sch 80, single-random 5-7 m, or cut-to-length 12.0 m +/- 50 mm.
• Quantity: 200 lengths, or 85 tonnes, or 4,200 metres — one of the three, not all three.
• Test report: EN 10204 3.1 mill-issued, or 3.2 with third-party witnessing (which third party: SGS, BV, TUV, LR?).
• End finish: plain-end square cut, ASME B16.25 bevelled 30°, or threaded to ASME B1.20.1.
• Supplementary requirements: S1 product analysis, S2 transverse tension, S3 flatten, plus any project-specific extras.
• Destination and Incoterm: CIF, FOB, DDP, with port name and any import documentation needed.

Five FAQs From Real RFQs

Why a Mill-Direct Source Matters for A335 Pipe

A335 pipe has three structural cost drivers: the steel itself (about 55-60 percent of the price), the heat-treatment energy (15-20 percent), and the inspection / certification cost (10-15 percent). A mill that owns the melt shop, the hot-piercing line, the heat-treatment furnace and the in-house NDT lab captures all three. A trading company adds margin at each hand-off and rarely owns the heat-treatment chart for the lot you actually receive.

For projects that are bound to a single 3.2 certificate for the whole piping package, mill-direct sourcing is the only way to keep the chain of custody intact. For projects that are bound to ASTM A335 alone, a mill-direct supplier can still bundle the matching pipe fittings, flanges (per ASME B16.5 or B16.47), stud bolts (per ASTM A193 B16) and gaskets (spiral-wound or RTJ) under the same MTC, which collapses three receiving inspections into one.

Mill-direct also means the engineer can dial in the supplementary requirements without paying the trading-house surcharge. S2 transverse tension on every pipe, S4 macro-etch on every pipe, and S5 micro-photo on the first heat of a new size are all standard options at a mill; the same requirements at a stockist typically double the unit price.