What are the different grades of A335 A335M steel tube and their chemical compositions?

ASTM A335/A335M is the specification most procurement engineers, EPC contractors, and boiler fabricators turn to when they need seamless ferritic alloy-steel pipe for elevated-temperature service. First issued by ASTM International and updated periodically (the current revision is A335/A335M-24b), this standard covers pipe intended for use in power plants, refineries, petrochemical facilities, and other high-temperature process systems where ordinary carbon steel simply cannot survive the operating environment.

But A335 is not a single material. The specification defines a whole family of grades — from the long-established P11 and P22 workhorses to the modern creep-resistant P91 and P92 — and each grade has its own chemical composition, mechanical properties, and heat-treatment requirements. Choosing the wrong grade can mean premature creep failure, unexpected cost, or a project that simply will not meet design code. This guide walks through every commonly used A335/A335M grade, the chemistry behind it, the mechanical properties that follow, and how to match the grade to your service conditions.

If you are sourcing seamless ferritic alloy-steel pipe for power, petrochemical, or boiler service, EZ Steel Industrial manufactures and supplies A335/A335M pipe in all common grades, with full MTC documentation, third-party inspection, and the heat-treatment options required by the standard.

What Is ASTM A335/A335M?

ASTM A335/A335M is titled "Standard Specification for Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service." A few points make it distinct from other common pipe specifications:

Seamless only. A335 does not cover welded pipe. The seamless manufacturing route is required because welds are typically the weak point in elevated-temperature creep service.
Ferritic alloy steel. The pipe is made from chromium-molybdenum and chromium-molybdenum-vanadium steels. The chromium content gives oxidation and corrosion resistance, while molybdenum and vanadium deliver creep strength at high temperatures.
High-temperature service. A335 pipe is generally used above 425 °C (about 800 °F) and is designed to resist creep, graphitization, and oxidation over long operating lives.
Multi-grade specification. A335 covers grades P1, P2, P5, P5b, P5c, P9, P11, P12, P15, P21, P22, P23, P24, P36, P91, P92, and P122, each with its own chemistry and heat treatment.

For a wider view of how A335 fits into a complete alloy pipe program — together with GB5310 alloy steel tubes and the alloy steel structure pipe line — see EZ Steel Industrial's full product range.

List of A335/A335M Grades and Their UNS Designations

The current A335/A335M-24b edition defines 17 grades. Some are rarely ordered, but the following table covers the grades that show up most often in real project specifications.

Grade	UNS Designation	Typical Role
P1	K11522	Lower-temperature Cr-Mo service, legacy applications
P2	K11547	Refinery piping, lower pressure
P5	K41545	5% Cr steel for refinery heaters and reactor piping
P5b	K51545	5% Cr variant, slightly higher Cr for corrosion resistance
P5c	K41245	4% Cr variant for moderate-temperature service
P9	S50400	9% Cr steel for high-temperature refinery service
P11	K11597	1.25% Cr – 0.5% Mo, the classic low-alloy pipe grade
P12	K11562	1% Cr – 0.5% Mo, used in boilers and process piping
P15	K11578	Legacy low-alloy grade
P21	K31545	3% Cr – 1% Mo, intermediate creep strength
P22	K21590	2.25% Cr – 1% Mo, the most widely used A335 grade
P23	K41650	2.25% Cr with W-V-Nb, advanced boiler tubes
P24	K30736	Variant of P23 with modified Mo-V chemistry
P36	K21001	Higher-Cr P22-style steel for special creep applications
P91	K91560	9% Cr – 1% Mo – V-Nb, the modern high-temperature workhorse
P92	K92460	9% Cr with W addition, higher creep strength than P91
P122	K91261	11% Cr – 2% W – Cu, ultra-supercritical boiler piping

A335 pipe in P5 through P122 is in regular production at EZ Steel Industrial in sizes from 1/2" to 26", with custom OD, wall thickness, and heat-treatment options available. The most common A335 alloy steel pipe program is built around P11, P22, and P91.

Chemical Composition of Each Common Grade

Composition is the foundation of every property in A335. Chromium gives corrosion and oxidation resistance, molybdenum provides short-term creep strength, and vanadium, niobium, tungsten, and nitrogen refine the microstructure for long-term creep life. The table below shows the chemical composition requirements for the most frequently ordered grades. Values are taken from A335/A335M-24b.

P11, P12, P22 — The Workhorse Low-Alloy Grades

Element	P11 (1.25Cr-0.5Mo)	P12 (1Cr-0.5Mo)	P22 (2.25Cr-1Mo)
Carbon (C)	0.05 – 0.15	0.05 – 0.15	0.05 – 0.15
Manganese (Mn)	0.30 – 0.60	0.30 – 0.61	0.30 – 0.60
Phosphorus (P), max	0.025	0.025	0.025
Sulfur (S), max	0.025	0.025	0.025
Silicon (Si)	0.50 – 1.00	0.50 max	0.50 max
Chromium (Cr)	1.00 – 1.50	0.80 – 1.25	1.90 – 2.60
Molybdenum (Mo)	0.44 – 0.65	0.45 – 0.65	0.87 – 1.13

These three grades cover the bulk of fossil-fuel power plant and refinery piping. P22 is the single most specified grade in the world for headers, steam lines, and boiler tubes operating at metal temperatures up to about 580 °C. For a closer look at how P22 compares to the carbon-steel A106 used in lower-temperature service, see the comparison article on carbon and alloy steel pipe standards.

P5 and P9 — 5% and 9% Chromium Steels

Element	P5 (5Cr-0.5Mo)	P9 (9Cr-1Mo)
Carbon (C)	0.15 max	0.15 max
Manganese (Mn)	0.30 – 0.60	0.30 – 0.60
Phosphorus (P), max	0.025	0.025
Sulfur (S), max	0.025	0.025
Silicon (Si)	0.50 max	0.25 – 1.00
Chromium (Cr)	4.00 – 6.00	8.00 – 10.00
Molybdenum (Mo)	0.45 – 0.65	0.90 – 1.10

The 5% and 9% chromium grades are used in refinery process units, catalytic cracking, and hydrocracker piping where resistance to sulfur-containing high-temperature streams is needed. Their higher chromium content shifts the operating envelope upward, but the simple Cr-Mo chemistry means lower creep strength than the modern P91 family.

P91 — The Modern High-Temperature Workhorse

Element	P91 Composition (%)
Carbon (C)	0.08 – 0.12
Manganese (Mn)	0.30 – 0.60
Phosphorus (P), max	0.020
Sulfur (S), max	0.010
Silicon (Si)	0.20 – 0.50
Chromium (Cr)	8.00 – 9.50
Molybdenum (Mo)	0.85 – 1.05
Vanadium (V)	0.18 – 0.25
Niobium (Nb) / Cb	0.06 – 0.10
Nitrogen (N)	0.030 – 0.070
Aluminum (Al), max	0.02
Titanium (Ti), max	0.01
Zirconium (Zr), max	0.01

P91 is the steel that changed high-temperature piping. Compared to P22, the addition of about 0.2% vanadium, 0.08% niobium, and tightly controlled nitrogen delivers roughly a tenfold improvement in creep strength at 600 °C. That allows thinner walls, lower pipe weight, and higher steam temperatures in ultra-supercritical power plants. The trade-off is tighter chemistry control, more demanding heat treatment, and the need for a strict forming and welding procedure.

P92 and P122 — Advanced 9% and 11% Chromium Steels

Element	P92 (9Cr-0.5Mo-1.8W-V-Nb)	P122 (11Cr-0.4Mo-2W-Cu-V-Nb)
Carbon (C)	0.07 – 0.13	0.08 – 0.14
Manganese (Mn)	0.30 – 0.60	0.50 max
Chromium (Cr)	8.50 – 9.50	10.0 – 11.5
Molybdenum (Mo)	0.30 – 0.60	0.25 – 0.60
Tungsten (W)	1.50 – 2.00	1.50 – 2.50
Vanadium (V)	0.15 – 0.25	0.15 – 0.30
Niobium (Nb)	0.04 – 0.09	0.04 – 0.10
Copper (Cu)	—	0.30 – 1.70
Boron (B)	0.001 – 0.006	≤ 0.005

P92 and P122 are designed for the most demanding ultra-supercritical and advanced ultra-supercritical boilers, where steam temperatures reach 620 – 650 °C. Tungsten and boron additions push creep strength beyond P91, while the higher chromium content in P122 provides better oxidation resistance in the hottest sections of the boiler.

Mechanical Properties and Hardness

Composition is only half the story. Heat treatment converts that chemistry into useful mechanical properties. A335/A335M specifies both the heat-treatment route and the minimum tensile, yield, elongation, and hardness values for each grade.

Tensile and Hardness Requirements

Grade	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness
P11	≥ 415	≥ 220	≥ 30	≤ 89 HRB
P12	≥ 415	≥ 220	≥ 30	≤ 89 HRB
P22	≥ 415	≥ 205	≥ 30	≤ 89 HRB
P5	≥ 415	≥ 205	≥ 30	≤ 89 HRB
P9	≥ 415	≥ 205	≥ 30	≤ 89 HRB
P91	≥ 585	≥ 415	≥ 20	190 – 250 HBW (≈ 91 HRB – 25 HRC)
P92	≥ 620	≥ 440	≥ 20	≤ 250 HBW
P122	≥ 620	≥ 400	≥ 20	≤ 250 HBW

A few practical notes on these values:

Low-alloy grades (P11, P12, P22, P5, P9) cluster around the same minimum tensile and yield numbers because their strength comes mostly from the tempered bainite / ferrite microstructure, not from alloy content.
P91 and the higher-chromium grades deliberately have higher minimum yield strength. The goal is to push allowable stress at elevated temperature, which directly reduces wall thickness and pipe weight.
Hardness is the easiest field check. If P91 measures below 190 HBW, the heat treatment was likely not correct, and the pipe should not be put into service without further investigation.

Heat Treatment Requirements

The standard gives each grade a specific heat-treatment route. The route has to be followed carefully because creep strength in ferritic alloy steels is strongly tied to grain size and precipitate structure, both of which are fixed during tempering.

Grade	Heat Treatment	Subcritical / Tempering Temperature
P11	Full or isothermal anneal, or normalize and temper	≥ 1200 °F (650 °C)
P12	Full or isothermal anneal, normalize and temper, or subcritical anneal	1200 – 1300 °F (650 – 705 °C)
P22	Full or isothermal anneal, or normalize and temper	≥ 1250 °F (675 °C)
P91	Normalize and temper, or quench and temper	Austenitizing 1900 – 1975 °F (1040 – 1080 °C); tempering 1350 – 1470 °F (730 – 800 °C)
P92	Normalize and temper	Austenitizing 1900 – 1975 °F; tempering 1380 – 1455 °F (750 – 790 °C)
P122	Normalize and temper	Austenitizing ≥ 1900 °F; tempering 1350 – 1470 °F

For the higher-alloy grades, the temperature window is small and the time at temperature has to be controlled. That is why A335 P91, P92, and P122 orders are usually released with additional specification requirements, such as mandatory hardness surveys, microstructure checks, and creep testing for critical headers. The same project discipline is applied to industrial valves and complementary piping components sourced for the same system.

How to Choose the Right A335 Grade for Your Application

Selecting an A335 grade is not just about chemistry and strength. It is about the combination of operating temperature, allowable stress, fabrication method, and total installed cost. Three questions usually drive the choice.

1. What Is the Design Metal Temperature?

This is the most important parameter. Each grade has an upper temperature limit beyond which allowable stress drops too low to give a reasonable wall thickness.

Up to about 540 °C: P11, P12, P22 are usually adequate.
540 – 600 °C: P22 is still widely used, but P91 begins to win on weight.
600 – 620 °C: P91 is the default. P22 walls become too thick.
620 – 650 °C: P92 and P122 are required.

2. Is Corrosion or Oxidation a Concern?

Higher chromium gives better resistance to oxidation and to high-temperature sulfidic attack. For high-sulfur refinery streams, 5% Cr (P5) or 9% Cr (P9, P91) pipe is often selected. In the hottest sections of an advanced ultra-supercritical boiler, 11% Cr (P122) helps control steam-side oxidation.

3. What Is the Fabrication Plan?

P11, P12, and P22 are forgiving. They can be bent, formed, and welded with standard low-alloy procedures. P91 needs preheat (typically 200 – 250 °C), controlled interpass temperature, post-weld tempering above 730 °C, and strict procedure qualification. P92 and P122 are even more demanding. If the fabrication shop is not used to creep-resistant steels, the project risk shifts to the welding and heat-treatment stages rather than to the pipe itself.

Testing, Inspection, and Documentation

A335/A335M requires several routine tests on every heat or lot, plus additional tests when the purchase order specifies them. The most common requirements are:

Product analysis: One chemical check per heat, taken from a finished pipe sample.
Tensile test: One per lot, in accordance with A370.
Hardness test: Typically two Brinell or Rockwell readings per pipe, more for P91 and above.
Flattening test: One per end of one pipe per lot.
Flaring test: For pipe ordered for flaring applications.
Hydrostatic or nondestructive electric test: Every length of pipe must pass either a hydrostatic test or an ultrasonic / eddy current test at the manufacturer's option, unless the purchase order specifies otherwise.

In addition, the standard calls out optional supplementary requirements (S1 – S14) that cover things like product verification, transverse tension tests, charpy V-notch impact, ｄｒｏｐ-weight testing, and explicit minimum creep-rupture properties. For critical service, these supplementary requirements are usually written into the purchase order so there is no ambiguity later. The mill test certificate (MTC) should show the heat number, full chemistry, mechanical test results, NDT results, and confirmation that the heat treatment was performed within the specified temperature window.

Common Project Applications

A335/A335M pipe shows up in the most demanding parts of any high-temperature plant. A few representative applications:

Power plant headers and steam lines. P22 for subcritical and older supercritical units; P91 for supercritical and ultra-supercritical; P92 / P122 for advanced ultra-supercritical and next-generation units.
Boiler tubes and superheater loops. P11, P12, and P22 for conventional boilers; P91 and P92 for high-temperature loops.
Refinery process piping. P5 and P9 in catalytic crackers, hydrocracker reactors, and heater tubes; P22 in steam and feed-water lines.
Petrochemical furnace piping. P5, P9, and P91 in ethylene cracking furnaces and reformer outlets.
Heat exchanger and high-temperature process lines. P22 and P91 in steam generator systems and process gas coolers.

If you are designing a full high-temperature pipe package, these A335 lines are usually combined with butt-weld fittings and the matching alloy-steel flanges so that the entire system has the same material pedigree and the same heat-treatment baseline.

Comparing A335 With Related Specifications

A335 does not exist in isolation. It is typically used in combination with three other specifications:

ASTM A213. Covers seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes. Same chemistry as A335, but smaller diameter and thinner walls.
ASTM A234. Covers wrought carbon and alloy-steel pipe fittings (elbows, tees, reducers) of the same WP grade family. For A335 P22 pipe, the matching fitting is WP22.
ASTM A182. Covers forged or rolled alloy-steel pipe flanges, fittings, and valves for high-temperature service. A335 P22 piping is typically paired with A182 F22 flanges.

Holding the same grade across pipe, fittings, and flanges is essential. If a P91 pipe is welded to a P22 fitting, the joint becomes the weak link in the system because the lower-chromium material will creep faster than the pipe.

Sourcing A335 Pipe From EZ Steel Industrial

EZ Steel Industrial Co., Ltd. is a manufacturer and integrated supplier of industrial metal piping systems. Established in 1994 and operating from a three-location production network in Cangzhou, Yangzhou, and Lishui, the company supplies seamless alloy steel pipe under the A335/A335M specification, alongside the related standards (A213, A234, A182) that complete a high-temperature piping package.

The A335 program covers sizes from 1/2" to 26" in the standard schedules, with custom outer diameter and wall thickness available on request. Pipe can be supplied in the normalized, normalized-and-tempered, or quenched-and-tempered condition as required by the grade. Each delivery is accompanied by an EN 10204 3.1 / 3.2 mill test certificate, with third-party inspection from agencies such as SGS, BV, TUV, or Lloyd's available when the project demands it.

For EPC contractors, refinery operators, and boiler fabricators looking to consolidate alloy pipe, fittings, flanges, gaskets, stud bolts, and industrial valves on a single purchase order, EZ Steel Industrial delivers the full package from one source — including petrochemical-grade alloy tubes and the high-efficiency heat exchanger tubes that often run alongside A335 in the same plant.

Key Takeaways

A335/A335M is the go-to specification for seamless ferritic alloy-steel pipe in high-temperature service, covering 17 grades from P1 up to P122.
Each grade has a defined chemistry (Cr, Mo, V, Nb, W, N), a defined heat-treatment route, and defined mechanical properties. Composition drives the heat treatment, and the heat treatment drives the in-service performance.
P11, P12, and P22 cover most conventional power and refinery needs. P91 is the modern workhorse for ultra-supercritical plants. P92 and P122 serve the most advanced boiler designs above 620 °C.
Grade selection should follow operating temperature, corrosion environment, fabrication capability, and total system cost. Pipe, fittings, and flanges should always be matched in grade.
Quality control (chemistry, tensile, hardness, NDT, hydrostatic) and full heat-treatment records are not optional. They are how an A335 order proves it is fit for high-temperature service.