Alloy Steel Pipe (ASTM A335): Chrome-Moly Grades P11, P22, P91 Explained

Quick Answer

ASTM A335 (ASME SA335) is the standard specification for seamless ferritic chrome-molybdenum ("chrome-moly") alloy steel pipe for high-temperature service, used extensively in power generation, refineries, and petrochemical plants where carbon steel would fail. The pipe is designated by a "P" grade based on chromium and molybdenum content: P11 (1.25% Cr, 0.5% Mo) for service up to ~510°C, P22 (2.25% Cr, 1% Mo) for higher temperatures and pressures, P5 and P9 (5–9% Cr) primarily for refinery service, and P91/P92 (9% Cr with vanadium, niobium, and — in P92 — tungsten) for advanced high-pressure power plant steam lines up to ~600°C+. Chromium provides oxidation and corrosion resistance at high temperature; molybdenum provides creep strength (resistance to slow deformation under sustained heat and stress). A335 pipe is seamless, must be heat treated per grade, and requires careful welding with preheat and post-weld heat treatment (PWHT) — especially the higher grades like P91, where weld defects such as delta ferrite can cause in-service cracking. Grades are matched with A234 WP fittings and A182 F flanges of the same chemistry. Available NPS ½" to 24" (larger sizes per A691) in schedules from 20 through XXS.

When steam or process fluid must be carried at temperatures where ordinary carbon steel loses its strength — 500°C, 600°C, and beyond — piping systems turn to chrome-moly alloy steel. ASTM A335, universally known as "chrome-moly pipe" or "P-grade pipe," is the specification that governs it. It is the dominant piping material in coal, gas, and nuclear power plants for main steam lines, reheat lines, and headers, and it is widely used in refineries and petrochemical plants for hot process service.

The reason is metallurgy. Carbon steel begins to lose mechanical strength and suffers from oxidation and creep (slow, permanent deformation under sustained load at high temperature) as temperatures climb. Adding chromium and molybdenum transforms the behavior: chromium resists oxidation and scaling at elevated temperatures and adds strength, while molybdenum — the single most effective additive for high-temperature creep strength — keeps the steel strong and dimensionally stable under sustained heat and pressure. The result is a pipe that carries high-pressure steam and hot hydrocarbons reliably for decades where carbon steel could not survive.

But chrome-moly pipe is not a single product. It spans a family of grades — P5, P9, P11, P22, P91, P92 — each with different chromium and molybdenum content, different temperature capability, different cost, and different welding requirements. Specifying the wrong grade, or welding the higher grades incorrectly, causes premature failure in service where the consequences are severe.

For power plant engineers, refinery and petrochemical specifiers, EPC contractors, and procurement managers — this guide explains ASTM A335 chrome-moly alloy steel pipe: the grades and how they differ, chemical composition and temperature limits, welding and heat treatment requirements, matching fittings and flanges, and how to specify and order.

For related piping materials, see Carbon Steel Pipe: Grades, Sizes & Standards, Stainless Steel Pipe: 304, 316 & ASTM A312, and API 5L Line Pipe.

What Is ASTM A335 Chrome-Moly Pipe?

ASTM A335 / ASME SA335 covers nominal wall and minimum wall seamless ferritic alloy-steel pipe intended for high-temperature service. Key characteristics:

• Seamless construction — no weld seam, giving uniform strength around the full circumference (important for high-pressure cyclic service).

• Ferritic chrome-moly alloy — low- and intermediate-alloy steels containing up to and including 10% chromium.

• Suitable for forming and welding — the pipe is suitable for bending, flanging (vanstoning), and similar forming operations, and for fusion welding (with proper procedures).

• Heat treated per grade — reheated to fully annealed, isothermal annealed, or normalized and tempered condition depending on grade.

The name "chrome-moly" comes from the two defining alloying elements: chromium (Cr) and molybdenum (Mo).

Why Chromium and Molybdenum

Chromium (Cr):

• Resists oxidation and scaling at high temperature (irreplaceable for elevated-temperature oxidation resistance)

• Increases tensile strength, yield strength, and hardness

• Improves corrosion resistance

• (Note: at 12% or more Cr, steel becomes stainless — chrome-moly grades stay below this)

Molybdenum (Mo):

• The single most effective additive for increasing high-temperature creep strength

• Increases strength, elastic limit, wear resistance, and hardenability

• Resists softening at high temperature, restrains grain growth, and reduces embrittlement

Together, these elements make chrome-moly pipe ideal for power plants, refineries, petrochemical plants, and oilfield service where fluids and gases are transported at extremely high temperatures and pressures.

ASTM A335 Grades Explained

The grades carry a "P" prefix (P for pipe), from P1 through P92. The chromium content generally increases with the grade number. Here are the grades that matter most:

P11 (1.25Cr-0.5Mo)

• Composition: ~1.00–1.50% Cr, 0.44–0.65% Mo

• Temperature: service up to approximately 510°C (950°F)

• Use: conventional power plant steam lines, moderate high-temperature service

• Minimum tensile: 415 MPa; minimum yield: 205 MPa

• The most common lower chrome-moly grade

P22 (2.25Cr-1Mo)

• Composition: ~1.90–2.60% Cr, 0.87–1.13% Mo

• Temperature: higher temperature and pressure capability than P11

• Use: power generation and petrochemical plants — higher-temperature steam service, hydrogen service (resistant to hydrogen attack)

• One of the most widely used chrome-moly grades globally

P5 (5Cr-0.5Mo) and P9 (9Cr-1Mo)

• Composition: P5 ~5% Cr; P9 ~9% Cr, with molybdenum

• Use: primarily refinery and petrochemical service — high-sulfur and hydrogen environments where the higher chromium provides better corrosion resistance

• P5 and P9 are the refinery-focused grades

P91 (9Cr-1Mo-V) — The Advanced Grade

• Composition: ~9% Cr, 1% Mo, with controlled additions of vanadium (0.18–0.25%), niobium/columbium (0.06–0.10%), and nitrogen (0.03–0.07%)

• Temperature: advanced high-pressure boilers and steam lines, up to ~600°C+

• Use: modern supercritical and ultra-supercritical power plants — main steam and reheat lines

• Why it matters: the micro-alloying elements (V, Nb, N) form stable MX-type carbonitride precipitates that provide dramatically enhanced creep strength, allowing thinner walls and higher operating temperatures

• Hardness limit: shall not exceed 250 HB / 265 HV (25 HRC)

P92 (9Cr-2W) — Next Generation

• Composition: similar to P91 but with added tungsten (1.50–2.00%) and boron (0.001–0.006%)

• Use: the most advanced ultra-supercritical power plants

• Even higher creep strength than P91

Grade Comparison Table

Applications

Power Generation (the Dominant User)

Power generation is the dominant user of A335 pipe. It is used extensively in coal, gas, and nuclear power plants for:

• Main steam lines

• Reheat lines

• Headers

• Superheater and reheater connections

Anywhere steam is at high pressure and temperature, chrome-moly pipe carries it. Modern supercritical and ultra-supercritical plants rely on P91 and P92 for the highest-temperature service.

Refineries and Petrochemical

• P5 and P9 for high-sulfur and hydrogen environments

• P11 and P22 for hot process piping

• Resistance to hydrogen attack and high-temperature sulfide corrosion

• Some grades usable for NACE MR0175 sour service applications under high temperature and pressure

For the material selection principles that also apply to heat exchanger tubes in these services, see Heat Exchanger Tube Materials Selection.

Fired Heaters and Boilers

Chrome-moly pipe carries hot fluids in and around fired heaters and boilers. For the burners that fire these systems, see Industrial Process Burners.

Welding and Heat Treatment (Critical)

Chrome-moly pipe requires careful welding — this is where most A335 failures originate.

Preheat and PWHT

• Chrome-moly pipes are hygroscopic — they absorb moisture that can cause hydrogen cracking during welding.

• Preheating before welding and post-weld heat treatment (PWHT) after welding are required, especially for higher-alloy grades.

• PWHT relieves residual stress and restores proper microstructure and mechanical properties in the weld and heat-affected zone.

The P91 Delta Ferrite Problem

P91 welding requires special attention. P91 weld metal must be free of delta ferrite, which forms if the composition balance pushes the weld into the two-phase (austenite + delta ferrite) field during solidification. Delta ferrite in P91 welds is a severe defect: it transforms to brittle sigma phase during high-temperature service, creating networks that initiate cracking. Controlling delta ferrite requires filler metals with carefully balanced chromium, nickel, and manganese, and low dilution during welding.

Welding Processes

A335 can be welded using conventional processes — SMAW (stick), GTAW (TIG), and GMAW (MIG) — with proper preheat, interpass temperature control, filler selection, and PWHT.

Don't Mix Grades

A common and dangerous mistake is mixing P11 and P22 components in the same piping class because "they're both chrome-moly." The Cr and Mo content differs significantly. Welding dissimilar grades requires a special welding procedure (WPS) with buttering layers, and the joint may need more frequent inspection in service due to differential thermal expansion. Match grades throughout a piping class.

Matching Fittings and Flanges

ASTM A335 pipe must be paired with fittings and flanges of the same chemical and mechanical grade:

Matching the chemistry across pipe, fittings, and flanges ensures consistent behavior at temperature. For flange selection, see Pipe Flanges: Types, Faces & Pressure Classes.

Sizes, Dimensions & Testing

Sizes

• NPS: ½" to 24" (larger sizes covered by ASTM A691)

• Schedules: 20, 30, STD, 40, 60, 80, 100, 120, 160, XXS

• Dimensions: per ASME B36.10M

• Length: typically 6m, 12m, or single/double random lengths

• Manufacturing: hot finished or cold drawn, seamless

Testing and Inspection

• Chemical composition — verified, often with 100% PMI (Positive Material Identification) and spectrometer analysis

• Mechanical tests — tension test, flattening test (or bend test for larger diameter), hardness test

• Hydrostatic test — applied to every length of pipe

• Non-destructive examination — per specification (ultrasonic, eddy current)

• Heat treatment — verified per grade

Certification

Chrome-moly pipe is typically certified per EN 10204 3.1/3.2, and NACE MR0175 / MR0103 for sour service applications, with full heat number traceability.

How to Order ASTM A335 Pipe

When ordering, specify:

1. Material name — seamless alloy steel pipe (ASTM A335 / ASME SA335)

2. Grade — P5, P9, P11, P22, P91, P92

3. Manufacturing type — hot finished seamless or cold drawn seamless

4. Size — NPS and schedule (per ASME B36.10), or OD/ID and wall thickness

5. Length — per piece and total quantity (feet, meters, or pieces)

6. Ends — plain end (PE) or beveled end (BE)

7. Testing/certification — hydrostatic test, NDE, PMI, EN 10204 3.1/3.2, NACE if required

8. Special requirements — IBR (Indian Boiler Regulations) if applicable, specific PWHT documentation

Common Specification Mistakes

After 15+ years supplying piping materials to power, refinery, and petrochemical projects:

Mistake 1: Wrong Grade for Temperature

Buyer specifies P11 for service above its ~510°C limit. Creep and premature failure result.

Prevention: Match the grade to the actual maximum operating temperature with margin. P11 up to ~510°C; P22 for higher; P91/P92 for advanced high-temperature service to ~600°C+.

Mistake 2: Inadequate Welding Procedure for P91

Buyer welds P91 without controlling delta ferrite. Sigma phase forms in service, causing cracking.

Prevention: Use qualified P91 welding procedures with balanced filler metal, controlled dilution, proper preheat, and PWHT. P91 welding is specialized — do not treat it like ordinary carbon steel.

Mistake 3: Skipping PWHT

Buyer omits post-weld heat treatment to save time. Residual stress and improper microstructure cause weld failures.

Prevention: PWHT is required for chrome-moly grades. Never skip it. Document PWHT charts.

Mistake 4: Mixing Grades

Buyer mixes P11 and P22 in the same class. Differential thermal expansion and dissimilar-metal welding issues develop.

Prevention: Match grades throughout a piping class. Where dissimilar joints are unavoidable, use qualified procedures with buttering layers.

Mistake 5: Mismatched Fittings and Flanges

Buyer pairs P22 pipe with F11 flanges. Chemistry mismatch causes differential behavior at temperature.

Prevention: Match fittings (A234 WPx) and flanges (A182 Fx) to the pipe grade throughout.

Mistake 6: Wrong Certification

Buyer orders standard pipe for boiler or sour service without IBR or NACE certification. Fails project requirements.

Prevention: Specify required certification up front — EN 10204 3.1/3.2, NACE MR0175, IBR — as applicable.

Supply from Kasko Makine

Kasko Makine supplies ASTM A335 / ASME SA335 chrome-moly alloy steel pipe for power generation, refinery, and petrochemical projects:

Grades: P5, P9, P11, P12, P22, P91, P92

Sizes: NPS ½" to 24" (larger per A691), schedules 20 through XXS, seamless (hot finished or cold drawn)

Matching components: A234 WP fittings and A182 F flanges in matching grades

Certification:

• EN 10204 Type 3.1 / 3.2 material certificates

• 100% PMI and spectrometer chemical analysis

• Hydrostatic test and NDE per specification

• NACE MR0175 / MR0103 for sour service

• IBR certification where required

• Full heat number traceability

Engineering support:

• Grade selection for temperature and service

• Welding and PWHT guidance

• Matching fitting and flange specification

• Documentation and certification coordination
  

Need ASTM A335 chrome-moly pipe? Send us your grade (P11, P22, P91, etc.), size and schedule, quantity, required certification (EN 10204, NACE, IBR), and delivery location to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll provide pricing, matching fittings and flanges, certification, and lead times within 48 hours.

Continue Reading: Pipe & Materials Guides

• Carbon Steel Pipe: Grades, Sizes & Standards — Carbon steel piping (for lower-temperature service)

• Stainless Steel Pipe: 304, 316 & ASTM A312 — Stainless piping

• API 5L Line Pipe: PSL1 vs PSL2 & Grades — Line pipe for oil and gas

• Pipe Flanges: Types, Faces & Pressure Classes — Matching flanges

• Heat Exchanger Tube Materials Selection — Materials for high-temperature service

Frequently Asked Questions

Q: What is ASTM A335 pipe?
A: ASTM A335 (ASME SA335) is the standard specification for seamless ferritic chrome-molybdenum ("chrome-moly") alloy steel pipe intended for high-temperature service. It is used extensively in power generation (main steam lines, reheat lines, headers), refineries, and petrochemical plants where carbon steel would lose strength or fail from creep and oxidation. The pipe is designated by "P" grades (P5, P9, P11, P22, P91, P92) based on chromium and molybdenum content. Chromium provides oxidation resistance and strength at high temperature; molybdenum provides creep strength. The pipe is seamless, suitable for bending, flanging, and welding, and must be heat treated per grade.

Q: What is the difference between A335 P11, P22, and P91?
A: The grades differ in chromium and molybdenum content and temperature capability. P11 (1.25% Cr, 0.5% Mo) is used for service up to approximately 510°C in conventional power plant steam lines. P22 (2.25% Cr, 1% Mo) handles higher temperatures and pressures, used in power and petrochemical plants including hydrogen service. P91 (9% Cr, 1% Mo with vanadium, niobium, and nitrogen) is an advanced grade for high-pressure boilers and steam lines up to ~600°C+, used in modern supercritical power plants. The micro-alloying elements in P91 form stable carbonitride precipitates that dramatically enhance creep strength. P91 requires specialized welding to avoid delta ferrite defects.

Q: Why is A335 called chrome-moly pipe?
A: ASTM A335 is called "chrome-moly" pipe because of its two defining alloying elements: chromium (Cr) and molybdenum (Mo). Chromium resists oxidation and scaling at high temperature (it is irreplaceable for elevated-temperature oxidation resistance) and increases tensile strength, yield strength, and hardness. Molybdenum is the single most effective additive for increasing high-temperature creep strength — it keeps steel strong and dimensionally stable under sustained heat and pressure, resists softening, restrains grain growth, and reduces embrittlement. Together they make the pipe ideal for high-temperature, high-pressure service in power plants, refineries, and petrochemical plants. The grades stay below 12% chromium (above which steel becomes stainless).

Q: Does A335 chrome-moly pipe require special welding?
A: Yes. Chrome-moly pipe requires careful welding with preheat before welding and post-weld heat treatment (PWHT) after — especially for higher-alloy grades. The pipe is hygroscopic (absorbs moisture that can cause hydrogen cracking during welding). P91 in particular requires specialized procedures: the weld metal must be free of delta ferrite, which transforms to brittle sigma phase during high-temperature service and causes cracking. Controlling delta ferrite requires balanced filler metal chemistry and low dilution. A335 can be welded with SMAW, GTAW, or GMAW using qualified procedures. Mixing dissimilar grades (like P11 and P22) requires special procedures with buttering layers. PWHT should never be skipped.

Q: What fittings and flanges match A335 pipe?
A: ASTM A335 pipe must be paired with fittings and flanges of the same chemical and mechanical grade. Butt-weld fittings follow ASTM A234 with matching "WP" grades: P11 pipe uses WP11 fittings, P22 uses WP22, P91 uses WP91, and so on. Forged fittings and flanges follow ASTM A182 with matching "F" grades: P11 pipe uses F11 flanges, P22 uses F22, P91 uses F91, and so on. Matching the chemistry across pipe, fittings, and flanges ensures consistent mechanical behavior and thermal expansion at high temperature, which is critical for reliable high-temperature service.

Q: What is A335 P91 pipe used for?
A: ASTM A335 P91 is an advanced chrome-moly grade (9% Cr, 1% Mo, with vanadium, niobium, and nitrogen) used for high-pressure boilers and steam lines in modern supercritical and ultra-supercritical power plants, operating up to approximately 600°C and above. It is used for main steam lines and reheat lines where the highest temperature and pressure capability is needed. The vanadium, niobium, and nitrogen additions form stable MX-type carbonitride precipitates that provide dramatically enhanced creep strength, allowing thinner walls and higher operating temperatures than earlier grades. P91 requires specialized welding to control delta ferrite, careful preheat and PWHT, and a hardness not exceeding 250 HB / 265 HV (25 HRC).