ASTM A335 Alloy Steel Pipes: Grades P5, P9, P11, P22, P91, P92

Quick Reference

ASTM A335 covers seamless ferritic chrome-moly alloy steel pipes for high-temperature service. Key grades: P11/P22 (power generation), P5/P9 (refinery), P91/P92 (advanced power plants). Matching materials: A234 WPx fittings and A182 Fx flanges. Temperature range: up to 600°C+ depending on grade.

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Need pipe dimension charts? Download the ASME B36.10 Pipe Schedule Chart for alloy steel pipe dimensions and weights.

What Is Alloy Steel?

Alloy steel is steel that has been alloyed with elements such as chromium, molybdenum, vanadium, or nickel to improve mechanical properties like strength, hardness, and resistance to corrosion and high temperatures. Low alloy steel contains less than 8% total alloying elements, while high alloy steel (like stainless steel) contains more.

ASTM A335 Specification: Low Alloy Steel Pipes

General Info A335 Pipes

ASTM A335 is the primary specification for low alloy steel seamless pipes for high-temperature service. These pipes are commonly used in high-temperature and high-pressure applications such as power generation, oil refineries, and chemical plants.

Alloy Steel Pipes ASTM A335Here are some key features of ASTM A335 alloy steel pipes:

Feature	Description
Types	ASTM A335 specification covers seamless ferritic alloy-steel pipe for high-temperature service. The pipes are categorized into several grades based on their chemical composition and mechanical properties. Common grades include P1, P5, P9, P11, P22, P91, and P92, among others. Each grade has specific requirements regarding chemical composition, tensile strength, yield strength, and hardness.
Chemical Composition	The chemical composition of ASTM A335 alloy steel pipes varies depending on the grade. However, common alloying elements include chromium, molybdenum, vanadium, and sometimes nickel. The presence of these alloying elements enhances the high-temperature properties of the steel, such as creep resistance, oxidation resistance, and strength at elevated temperatures.
Mechanical Properties	ASTM A335 pipes undergo various mechanical tests to verify compliance with specified requirements. These tests may include tensile testing, hardness testing, impact testing, and flattening/flaring tests. Mechanical properties such as tensile strength, yield strength, elongation, and hardness vary depending on the grade and heat treatment condition.
Dimensions and Sizes	ASTM A335 pipes are available in various sizes and dimensions, ranging from 1/8 inch to 26 inches in nominal diameter. The wall thickness can vary depending on the nominal pipe size and grade. Standard pipe schedules such as SCH 40, SCH 80, SCH 160, and XXS are commonly available for ASTM A335 pipes.
Applications	ASTM A335 alloy steel pipes are widely used in high-temperature and high-pressure applications, such as: power generation (boiler tubes, superheater tubes, and reheater tubes in steam boilers and power plants), oil and gas industry (downhole tubular, production tubing, and piping systems in refineries and petrochemical plants), chemical industry (piping systems for corrosive environments and high-temperature chemical processes), and the energy industry for high-temperature and very low-temperature service (cryogenic), or for applications with very high pressures.
Welding	ASTM A335 pipes can be welded using conventional welding methods such as SMAW (shielded metal arc welding), GTAW (gas tungsten arc welding), and GMAW (gas metal arc welding). Preheating and post-weld heat treatment may be necessary to avoid cracking and maintain proper mechanical properties, especially for higher alloyed grades.

Pro Tip

For P91 piping, PWHT is not optional - it’s mandatory. The as-welded hardness of P91 can exceed 400 HV, far above the maximum allowable. I’ve seen P91 welds crack during hydrotesting because the contractor skipped PWHT. Always verify PWHT temperature (typically 760°C) and time with hardness testing after treatment.

When selecting ASTM A335 pipes for a specific application, consider factors such as grade, size, wall thickness, and end connections to match the operating conditions and design code requirements.

Why Chrome-Moly? The Case for Cr-Mo Alloy Steels

Standard carbon steel pipes (such as ASTM A106 Gr. B) begin to lose strength rapidly above approximately 425°C (800°F). In high-temperature piping systems, three degradation mechanisms drive the need for chrome-moly alloys:

Creep Resistance

Creep is the slow, time-dependent plastic deformation of metal under sustained stress at elevated temperatures. Carbon steel has poor creep resistance: its allowable stress at 500°C drops to a fraction of its room-temperature value. Chrome-moly steels resist creep through solid-solution strengthening (Mo) and carbide precipitation (Cr, V, Nb) that pin grain boundaries and impede dislocation movement.

Hydrogen Attack Resistance

In refinery environments, carbon steel exposed to high-temperature, high-pressure hydrogen undergoes hydrogen attack: atomic hydrogen diffuses into the steel and reacts with carbon to form methane (CH4) at grain boundaries. This internal decarburization causes irreversible loss of strength and ductility. The Nelson curve (API RP 941) defines safe operating limits for each alloy; chrome-moly grades progressively push the safe envelope to higher temperatures and hydrogen partial pressures.

Nelson Curves

Always consult the latest edition of API RP 941 (Nelson curves) when specifying materials for hydrogen service. The curves are periodically updated based on field experience. A material that was considered safe at a given temperature/pressure combination in an earlier edition may have been downgraded in newer revisions. C-0.5Mo steel (P1), for example, has been effectively removed from hydrogen service recommendations.

Oxidation and Sulfidation Resistance

The chromium in Cr-Mo steels forms a protective chromium oxide (Cr2O3) layer on the surface, which resists both oxidation (reaction with oxygen at high temperatures) and sulfidation (reaction with sulfur compounds). Higher-chromium grades like P5, P9, and P91 are essential in refinery units processing sulfur-containing crude oil, such as hydrodesulfurization (HDS) units and fluid catalytic cracking units (FCCU).

Key Takeaway: Chrome-moly alloy steels solve the three critical failure modes of carbon steel at high temperatures: creep, hydrogen attack, and sulfidation. The choice of specific grade depends on the operating temperature, hydrogen partial pressure, and sulfur content of the process fluid.

Alloying Elements (Mo, Cr)

ASTM A335 pipes have superior features than standard carbon steel pipes due to the addition of high-value elements such as Molybdenum, Chromium, and other alloying materials.

The addition of Molybdenum (“Moly”) increases the strength of the steel and its elastic limit, enhancing the steel’s resistance to wear, impact qualities, and hardenability. It also improves the resistance to softening, makes chromium steel less prone to embrittlement, and prevents pitting.

Chromium, a key element also for stainless steel alloys, prevents steel oxidation at elevated temperatures and increases the resistance of steel to corrosion. It enhances the tensile, yield, and hardness properties of low-alloy pipes at room temperature.

Other alloying elements, present in various degrees in pipes of all grades are

Alloying Element	Properties Conferred to Steel
Aluminum (Al)	Increases grain refinement and control, enhancing strength and toughness. Also improves corrosion resistance and acts as a deoxidizer.
Boron (B)	Enhances hardenability, allowing steel to achieve greater depth of hardness with less quench severity.
Cobalt (Co)	Improves strength at high temperatures, making steel more resistant to thermal fatigue and wear.
Manganese (Mn)	Increases hardenability, tensile strength, and wear resistance. Acts as a deoxidizer and improves toughness.
Nickel (Ni)	Enhances overall strength and toughness, especially at low temperatures. Improves corrosion resistance in certain environments.
Silicon (Si)	Strengthens steel and acts as a deoxidizer. In certain amounts, it can also increase magnetic properties and electrical resistivity.
Titanium (Ti)	Combines with carbon to form titanium carbides, preventing grain growth and enhancing strength at high temperatures.
Tungsten (W)	Significantly increases hardness and high-temperature strength, making steel more resistant to wear and deformation.
Vanadium (V)	Improves hardenability while retaining fine grain size, leading to better wear resistance and toughness, especially at high temperatures.

Alloy Vs. Nickel-alloy Pipes

As previously explained, ASTM A335 alloy pipes distinguish themselves from standard carbon steel pipes by containing elevated levels of alloying elements such as Molybdenum (Mo), Chromium (Cr), and occasionally Nickel, among others.

These pipes are categorized as “low-alloy” due to the total concentration of alloying elements typically remaining below 5%. In contrast, a higher concentration of alloying elements would classify the steel as “higher-alloy,” resembling stainless steel, duplex, or even super-alloys like Inconel, Hastelloy, Monel, and others.

All ASTM A335 Grades and Temperature Limits

Each ASTM A335 grade is designed for a specific temperature range and process environment. The following table summarizes the key grades, their Cr-Mo designation, and recommended maximum continuous service temperature per ASME code allowable stress data:

Grade	Cr-Mo Designation	Max Service Temp	Typical Applications
P1	0.5Mo	~475°C (885°F)	Moderate-temperature steam lines; no longer recommended for hydrogen service (API RP 941)
P2	0.5Cr-0.5Mo	~480°C (900°F)	Moderate-temperature steam and hydrocarbon service
P5	5Cr-0.5Mo	~595°C (1100°F)	Refinery heater tubes, FCCU transfer lines, sulfur-bearing hydrocarbon service
P5b	5Cr-0.5Mo-Si	~595°C (1100°F)	Same as P5 with improved oxidation resistance from higher silicon
P9	9Cr-1Mo	~595°C (1100°F)	Refinery heater tubes, reformer effluent piping, sulfidation-resistant service
P11	1.25Cr-0.5Mo	~510°C (950°F)	Power plant steam piping, superheater tubes, headers
P12	1Cr-0.5Mo	~510°C (950°F)	Similar to P11 at slightly lower Cr, boiler components
P22	2.25Cr-1Mo	~565°C (1050°F)	Main steam lines, hydrogen reformer piping, heavy-wall headers
P91	9Cr-1Mo-VNb	~593°C (1100°F)	Ultra-supercritical power plants, main steam lines, hot reheat piping
P92	9Cr-0.5Mo-2W	~620°C (1150°F)	Next-generation ultra-supercritical plants, highest temperature service

Grade Selection Logic

When selecting a grade, start with the design temperature and process environment: (1) below 425°C in non-hydrogen service, carbon steel suffices; (2) 425-510°C, use P11; (3) 510-565°C or hydrogen-rich environments, use P22; (4) above 565°C or where thinner walls are needed, use P91/P92. In sulfur-bearing refinery service, the chromium content drives the selection; P5 or P9 are preferred for their 5-9% Cr sulfidation resistance.

Applications by Grade

P11 and P12 (1-1.25Cr-0.5Mo) are the workhorses of conventional power generation. They are used for superheater and reheater tubes, high-pressure steam headers, and interconnecting piping in coal, gas, and oil-fired power plants operating at steam temperatures up to 510°C.

P22 (2.25Cr-1Mo) is the standard grade for hydrogen reformer piping in refineries and ammonia plants, where its higher Mo content provides superior resistance to hydrogen attack. It is also widely used for main steam and hot reheat lines in power plants operating above 510°C.

P5 and P9 (5-9Cr) are primarily used in the petroleum refining industry where sulfidation resistance is critical. Typical applications include FCCU riser and transfer lines, coker heater tubes, vacuum distillation column transfer lines, and hydrodesulfurization (HDS) reactor piping.

P91 and P92 (9Cr-VNb/W) are advanced creep-strength-enhanced ferritic (CSEF) steels developed for ultra-supercritical (USC) power plants. P91 allows wall thicknesses approximately 50% thinner than P22 at equivalent temperatures, significantly reducing dead weight, thermal stresses during transients, and material cost. P92 extends this advantage further with tungsten additions for the most demanding USC applications.

A335 Properties

Chemical Composition

The types of alloy steel covered by the ASTM A335 / ASME SA335 specification are designed with a “P” prefix, from P5 to P92. Grades P11/P22 and P91/92 are typically found in power stations, whereas grades P5 and P9 are more common for application in the petrochemical industry. Grades P9, and P91 are, in the list, the more expensive (a P91 seamless pipe may cost approx 5EUR per kg.).

ASTM A335 Low-Alloy Steel (Grades)	UNS equivalent	C	Mn	P max	S max	Si	Cr	Mo
P1	K11522	0.10-0.20	0.30-0.80	0.025	0.025	0.10-0.50	—	0.44-0.65
P2	K11547	0.10-0.20	0.30-0.61	0.025	0.025	0.10-0.30	0.50-0.81	0.44-0.65
P5	K41545	0.15 max	0.30-0.60	0.025	0.025	0.50 max	4.00-6.00	0.44-0.65
P5b	K51545	0.15 max	0.30-0.60	0.025	0.025	1.00-2.00	4.00-6.00	0.44-0.65
P5c	K41245	0.12 max	0.30-0.60	0.025	0.025	0.50 max	4.00-6.00	0.44-0.65
P9	S50400	0.15 max	0.30-0.60	0.025	0.025	0.50-1.00	8.00-10.00	0.44-0.65
P11	K11597	0.05-0.15	0.30-0.61	0.025	0.025	0.50-1.00	1.00-1.50	0.44-0.65
P12	K11562	0.05-0.15	0.30-0.60	0.025	0.025	0.50 max	0.80-1.25	0.44-0.65
P15	K11578	0.05-0.15	0.30-0.60	0.025	0.025	1.15-1.65	—	0.44-0.65
P21	K31545	0.05-0.15	0.30-0.60	0.025	0.025	0.50 max	2.65-3.35	0.80-1.60
P22	K21590	0.05-0.15	0.30-0.60	0.025	0.025	0.50 max	1.90-2.60	0.87-1.13
P91	K91560	0.08-0.12	0.30-0.60	0.020	0.010	0.20-0.50	8.00-9.50	0.85-1.05
P92	K92460	0.07-0.13	0.30-0.60	0.020	0.010	0.50 max	8.50-9.50	0.30-0.60

P91 and P92 Additional Elements

Grades P91 and P92 contain controlled additions of Vanadium (0.18-0.25%), Niobium/Columbium (0.06-0.10%), and Nitrogen (0.03-0.07%). P92 additionally contains Tungsten (1.50-2.00%) and Boron (0.001-0.006%). These micro-alloying elements form stable MX-type carbonitride precipitates (VN, NbC) that are critical for the enhanced creep strength of these grades. Any deviation in these elements during steelmaking can dramatically reduce long-term creep performance.

Field Note

Chrome-moly pipes are hygroscopic - they absorb moisture that can cause hydrogen cracking during welding. Store P91 and P92 pipes in covered, dry areas and preheat to at least 150°C before welding to drive off moisture. Never weld chrome-moly in rainy or high-humidity conditions without proper shelter.

Mechanical Properties

Grade	UNS Number	Yield Strength (ksi)	Tensile Strength (ksi)	Elongation (%)	Hardness Rockwell	Hardness Brinell
P1	K11522	30	55	30	—	—
P2	K11547	30	55	30	—	—
P5	K41545	40	70	30	—	207 max
P9	S50400	30	60	30	—	—
P11	K11597	30	60	20	—	—
P12	K11562	32	60	30	—	174 max
P22	K21590	30	60	30	—	—
P91	K91560	60	85	20	—	250 max
P92	K92460	64	90	20	—	250 max

P91 Strength Advantage

Notice that P91 has minimum yield strength of 60 ksi, double that of P11 and P22 (30 ksi). Combined with far superior creep strength, this allows P91 to use significantly thinner walls for the same design conditions. In a typical main steam header (565°C, 170 bar), P91 wall thickness is roughly half that of P22, resulting in approximately 30-40% weight savings and reduced thermal fatigue during startups and shutdowns.

P91 Special Requirements

Grade P91 is a creep-strength-enhanced ferritic (CSEF) steel that demands far more careful handling than conventional Cr-Mo grades. Failures of P91 piping components, particularly at welds, have been widely documented in the power generation industry. The following requirements are critical.

Heat Treatment

P91 must be normalized at 1040-1080°C followed by tempering at 730-780°C. This produces a tempered martensitic microstructure with a fine dispersion of M23C6 carbides and MX carbonitrides. Deviations from these parameters (e.g., over-tempering above 790°C or under-normalizing below 1020°C) destroy the precipitate structure and permanently degrade creep strength, even if room-temperature hardness and tensile values appear acceptable.

PWHT Requirements

Post-weld heat treatment for P91 is mandatory and must be precisely controlled:

• Temperature: 730-770°C (the ASME minimum is 730°C; many specifications target 760°C)
• Hold time: Minimum 2 hours (some codes require 1 hour per 25 mm of thickness)
• Heating/cooling rate: Maximum 80°C/hour above 400°C to prevent thermal shock
• Temperature uniformity: Within 30°C across the weld zone

Critical P91 Warning

PWHT temperature for P91 must never exceed the original tempering temperature of the base metal (typically 760-780°C). Over-tempering dissolves the strengthening precipitates and cannot be reversed in the field. If PWHT is accidentally performed above Ac1 (approximately 800-830°C for P91), the material must be re-normalized and re-tempered, which is virtually impossible on installed piping. Always verify the actual tempering temperature from the mill certificate before establishing the PWHT range.

Hardness Limits

• Base metal: 250 HB / 265 HV / 25 HRC maximum
• Weld metal and HAZ after PWHT: 250 HB maximum (some owner specifications require 248 HB maximum)
• As-welded: Typically 350-420 HV (this is why PWHT is mandatory)

Delta Ferrite Concerns

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 sigma phase during high-temperature service, creating brittle networks that initiate cracking. Controlling delta ferrite requires using filler metals with carefully balanced Cr, Ni, and Mn contents and keeping dilution low during welding.

Matching Fittings and Flanges

ASTM A335 alloy steel pipes must be paired with compatible fittings and flanges of the same chemical and mechanical grade. The following table maps pipe grades to their matching component standards:

Pipe Grade (A335)	BW Fittings (A234)	Forged Fittings & Flanges (A182)	Cast Valve Body (A217)
P1	WP1	F1	WC1
P5	WP5	F5 / F5a	C5
P9	WP9	F9	C12
P11	WP11	F11	WC6
P12	WP12	F12	WC9
P22	WP22	F22	WC9
P91	WP91	F91	C12A
P92	WP92	F92	—

Common Mistake

Mixing P11 and P22 components in the same piping class because “they’re both chrome-moly.” The Cr and Mo content differs significantly, and welding dissimilar grades requires special WPS with buttering layers, and the joint may need more frequent inspection during operation due to differential thermal expansion.

For a complete cross-reference of all piping material grades, see the ASTM Piping Materials Compatibility Table.

Welding Considerations for A335 Chrome-Moly Pipes

Welding chrome-moly alloy steels is more demanding than welding carbon steel. The higher alloy content increases hardenability, meaning the heat-affected zone (HAZ) transforms to martensite during cooling. Without proper preheat and PWHT, the weld is susceptible to hydrogen cracking and remains excessively hard.

Preheat Requirements

Grade	Minimum Preheat Temperature	Notes
P1 / P2	100°C (200°F)	Low hardenability; moderate preheat sufficient
P5 / P9	200°C (400°F)	Higher Cr increases martensite risk
P11 / P12	150-200°C (300-400°F)	Standard for most power plant work
P22	200-250°C (400-480°F)	Higher Mo content demands more preheat
P91	200-300°C (400-570°F)	Must maintain interpass 200-300°C; never allow to cool below 200°C before PWHT
P92	200-300°C (400-570°F)	Same as P91; tungsten does not change preheat requirements

Filler Metal Selection

Pipe Grade	SMAW Electrode (AWS)	GTAW/GMAW Wire (AWS)	Notes
P11	E8018-B2	ER80S-B2	1.25Cr-0.5Mo filler matches base metal
P22	E9018-B3	ER90S-B3	2.25Cr-1Mo filler
P5 / P9	E502 / E505	ER502 / ER505	5Cr-0.5Mo / 9Cr-1Mo
P91	E9015-B9 / E9018-B9	ER90S-B9	Modified 9Cr-1Mo-VNb; must match micro-alloying
P92	E9015-B92	ER90S-B92	9Cr-0.5Mo-2W filler

Welding Tip

For P91 root passes, always use GTAW (TIG) with ER90S-B9 filler wire. The controlled heat input of GTAW minimizes dilution and provides the cleanest root. Fill passes can use SMAW with E9015-B9 (basic low-hydrogen electrode). Never use cellulosic electrodes (EXX10 types) on any chrome-moly grade.

PWHT Summary by Grade

Grade	PWHT Temperature Range	Minimum Hold Time	Cooling Rate
P1 / P2	595-720°C	1 hr per 25 mm	Furnace or controlled
P5 / P9	730-760°C	1 hr per 25 mm	Furnace or controlled
P11 / P12	680-730°C	1 hr per 25 mm	Furnace or controlled
P22	680-730°C	1 hr per 25 mm	Furnace or controlled
P91	730-770°C	2 hr minimum	Max 80°C/hr above 400°C
P92	730-770°C	2 hr minimum	Max 80°C/hr above 400°C

Comparison: P11 vs P22 vs P91

This is the most frequent selection question in high-temperature piping design. The three grades cover overlapping but distinct operating windows:

Parameter	P11 (1.25Cr-0.5Mo)	P22 (2.25Cr-1Mo)	P91 (9Cr-1Mo-VNb)
Chromium	1.00-1.50%	1.90-2.60%	8.00-9.50%
Molybdenum	0.44-0.65%	0.87-1.13%	0.85-1.05%
Max service temp	~510°C (950°F)	~565°C (1050°F)	~593°C (1100°F)
Yield strength	30 ksi (205 MPa)	30 ksi (205 MPa)	60 ksi (415 MPa)
Tensile strength	60 ksi (415 MPa)	60 ksi (415 MPa)	85 ksi (585 MPa)
Creep strength at 550°C	Low	Moderate	High (2-3x P22)
Wall thickness (relative)	Thick	Thick	Thin (~50% of P22)
Sulfidation resistance	Low	Moderate	High
Hydrogen attack resistance	Good	Very good	Excellent
Welding complexity	Moderate	Moderate	High
PWHT temperature	680-730°C	680-730°C	730-770°C
Material cost (relative)	1x	1.2-1.5x	2-3x
Typical application	Conventional power plant steam lines	Hydrogen reformers, main steam	USC power plants, thin-wall headers

When to use which: Choose P11 for conventional power plant piping up to 510°C where cost matters and wall thickness is acceptable. Choose P22 when operating above 510°C, in hydrogen service, or when moderate sulfidation resistance is needed. Choose P91 for the highest temperatures (above 565°C), when wall thickness reduction is essential (large headers, thick sections prone to thermal fatigue), or in ultra-supercritical power plants. Be prepared, however, for significantly higher fabrication requirements.

For a detailed head-to-head analysis, see A335 P11 vs P22 and A335 P11 vs P22 vs P91.

Testing Requirements A335 Pipes

• Transverse/longitudinal: tension and flattening, hardness, bend tests. For material that has been heat-treated in batch furnaces, these tests shall be made on the 5% of the pipes from each heat lot number. For smaller lots, one pipe at a minimum has to be tested
• ASTM A335 Gr. P91 shall have a hardness of 250 HB / 265 HV (25 HRC)
• Hydro testing: shall be applied to every length of pipe
• The non-destructive electric test is optional

Tolerances

Diameter

NPS [DN]	Over (in.)	Over (mm)	Under (in.)	Under (mm)
1/8 to 1 1/2 [DN 6 to 40]	1/64 (0.015)	0.4	1/64 (0.015)	0.4
Over 1 1/2 to 4 [DN 40 to 100]	1/32 (0.031)	0.79	1/32 (0.031)	0.79
Over 4 to 8 [DN 100 to 200]	1/16 (0.062)	1.59	1/32 (0.031)	0.79
Over 8 to 12 [DN 200 to 300]	3/32 (0.093)	2.38	1/32 (0.031)	0.79
Over 12 [> DN 300]	+/- 1% OD	+/- 1% OD	+/- 1% OD	+/- 1% OD

Wall Thickness

The tolerances in WT, in %, from specified are:

• 1/8 to 2 1/2 [6 to 65] incl., all t/D ratios: over 20%, under 12.5%
• Above 2 1/2 [65], t/D < or = 5%: over 22.5%, under 12.5%
• Above 2 1/2 [65], t/D > 5%: over 15%, under 12.5%

t = Specified Wall Thickness; D = Specified Outside Diameter

Marking Requirements A335

The marking requirements for ASTM A335 pipes are specified for proper identification and traceability. According to the ASTM A335 standard, the marking requirements typically include the following information:

Marking	Description
Manufacturer’s Name or Logo	The manufacturer’s name or logo is usually stamped or printed on the pipe. This identifies the source of the pipe and provides accountability for its quality.
Pipe Grade and Size	The grade of the pipe (e.g., P5, P9, P11, etc.) and its nominal size are commonly marked on the surface of the pipe. This information indicates the material composition and dimensions of the pipe.
Heat Number	Each heat of steel used to produce the pipes is assigned a unique heat number during the manufacturing process. The heat number is typically stamped or marked on the pipe for traceability purposes.
ASTM A335 Specification	The ASTM A335 specification number is often marked on the pipe to indicate compliance with the standard requirements for seamless ferritic alloy-steel pipe for high-temperature service.
Lot Number or Batch Number	In addition to the heat number, pipes may also be marked with a lot number or batch number for traceability and quality control.
Additional Markings	Depending on the specific requirements of the purchaser or applicable regulations, additional markings may be required. These may include symbols indicating compliance with certain standards or specifications, country of origin, and special handling instructions.

Specific marking requirements may vary depending on the contractual agreement between buyer and manufacturer, as well as any applicable regulations or industry standards.

Alloy Pipe Cross-Reference Table: ASTM vs. EN Grades

Chrome Moly Pipes: Werkstoff vs EN vs ASTM

Werkstoff/DIN	EN	ASTM
1.5415	16Mo3	A335 Grade P1
1.7335	13CrMo4-5	A335 Grade P11, P12
1.7380	10CrMo9-10	A335 Grade P22
1.7362	X11CrMo5	A335 Grade P5
—	—	A335 Grade P9
1.4903	X10CrMoVNb9-1	A335 Grade P91
1.4901	X10CrMoWVNb9-2	A335 Grade P92

Standards Reference Table

The following standards govern the specification, design, fabrication, and inspection of ASTM A335 chrome-moly piping systems:

Standard	Title / Scope
ASTM A335	Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service
ASME SA-335	ASME Boiler and Pressure Vessel Code adoption of ASTM A335
ASTM A234	Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High Temperature Service
ASTM A182	Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves
ASTM A217	Steel Castings, Martensitic Stainless and Alloy, for Pressure-Containing Parts
ASME B31.1	Power Piping (governs most power plant chrome-moly piping)
ASME B31.3	Process Piping (governs refinery and petrochemical chrome-moly piping)
ASME B36.10	Welded and Seamless Wrought Steel Pipe (pipe dimensions)
API RP 941	Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries (Nelson Curves)
AWS A5.5	Low-Alloy Steel Electrodes for SMAW
AWS A5.23	Low-Alloy Steel Electrodes and Fluxes for SAW
AWS A5.28	Low-Alloy Steel Electrodes and Rods for GMAW/GTAW

Top Manufacturers Alloy Steel Pipes

Pipe Mill	Country
Jiangu Chengde Steel Tube Co Ltd. Ltd	China
Chomutov	Czech
Eschweiller(Bentler)	Germany
Vallourec Tubes	Germany, France, & Brazil
Tenaris	Italy, Mexico, & Argentina
Sumitomo Metal	Japan
JFE	Japan
Productos Tubulares	Spain
Tubos Reunidos	Spain
Wyman Gordon	USA
Michigan Seamless	USA
Rockwell Collins	USA

We recommend purchasing the ASTM A335 specification from the ASTM website to get a complete understanding of this topic.

Frequently Asked Questions

What is the difference between ASTM A335 P11, P22, and P91?

P11 (1.25Cr-0.5Mo) is used for service up to approximately 510°C (950°F) in conventional power plant steam lines. P22 (2.25Cr-1Mo) extends the temperature range to approximately 565°C (1050°F) and provides better hydrogen attack resistance for refinery applications. P91 (9Cr-1Mo-VNb) provides dramatically higher creep strength up to 593°C (1100°F), allowing approximately 50% thinner pipe walls compared to P22 for the same design conditions. Each successive grade adds more chromium and micro-alloying elements for improved high-temperature performance.

Is PWHT mandatory for ASTM A335 chrome-moly pipe welding?

Yes, post-weld heat treatment (PWHT) is mandatory for most ASTM A335 grades per ASME B31.1 and B31.3. For P91, the PWHT temperature must be precisely controlled at 730-770°C with a minimum hold time of 2 hours. Skipping PWHT leaves the weld with hardness values above 400 HV, far exceeding the allowable maximum of 250 HB, which risks brittle fracture and hydrogen cracking. Even lower grades like P11 and P22 require PWHT at 680-730°C to temper the martensitic microstructure in the heat-affected zone.

What fittings and flanges match ASTM A335 alloy pipes?

ASTM A335 pipes are matched with ASTM A234 WPx series buttweld fittings (e.g., WP11, WP22, WP91) and ASTM A182 Fx series forged flanges and fittings (e.g., F11, F22, F91). For cast valve bodies, ASTM A217 grades are used (e.g., WC6 for P11 service, WC9 for P22 service, C12A for P91 service). All matching components must share the same chemical composition and mechanical property ranges for proper weldability and uniform in-service performance.

Why is chrome-moly steel used instead of carbon steel at high temperatures?

Chrome-moly (Cr-Mo) steels provide three critical advantages over carbon steel at elevated temperatures: (1) superior creep resistance, maintaining strength under sustained load where carbon steel would deform permanently; (2) resistance to hydrogen attack, which causes irreversible internal decarburization in carbon steel above 230°C in high-pressure hydrogen environments; and (3) improved oxidation and sulfidation resistance from the chromium content, which forms a protective oxide layer. Carbon steel loses allowable stress rapidly above 425°C and is unsuitable for most refinery and power plant high-temperature piping.

What is the maximum service temperature for ASTM A335 P91 pipes?

ASTM A335 P91 pipes can operate at continuous temperatures up to approximately 593°C (1100°F), with ASME code allowable stresses extending slightly higher in some design scenarios. P91 achieves this through micro-alloying with vanadium, niobium, and nitrogen, which form stable MX-type precipitates (VN, NbC) that pin grain boundaries and resist creep deformation far more effectively than conventional Cr-Mo grades like P22. For service above 593°C, P92 (with tungsten additions) should be considered.