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What Are P-Numbers?

P-numbers are the base metal classification system used in ASME Section IX to group materials by composition and weldability characteristics. The system was developed to reduce the number of procedure qualifications required while ensuring that materials within the same group behave similarly during welding. P-numbers appear in QW/QB-422 of Section IX, which lists every ASME-approved base metal specification with its assigned P-number and group number.

The P-number system covers a much broader range of materials than D1.1 Table 5.6, which only groups structural steels into Groups I through V. ASME IX P-numbers range from P-Number 1 (carbon steels) through P-Number 62 (zirconium alloys), covering ferrous metals, nickel alloys, aluminum alloys, copper alloys, titanium, and reactive metals. Each P-number represents a family of materials with similar welding behavior, meaning a procedure qualified on one material in the group generally produces acceptable results on other materials in the same group.

Major P-Number Assignments

P-Number 1 — Carbon Steels

The most commonly encountered group in pressure vessel and piping fabrication. Includes SA-36 (structural plate), SA-516 Gr.60 and Gr.70 (pressure vessel plate), SA-106 Gr.B (seamless pipe), SA-105 (forgings), SA-53 (welded and seamless pipe), and SA-179 (heat exchanger tubing). P-Number 1 is subdivided into Groups 1 through 4 based on carbon content and strength level. Group 1 covers lower-carbon, lower-strength materials. Group 2 covers medium-carbon steels with higher tensile strength.

P-Number 3 — Alloy Steels (up to 0.75% Cr)

Covers low-alloy steels with chromium content up to 0.75% and total alloy content that exceeds P-Number 1 limits. Includes SA-335 P1 and P2 (alloy pipe), SA-387 Gr.2 (alloy plate). These materials typically require preheat and often require post-weld heat treatment (PWHT). A procedure qualified on P-Number 3 does not qualify for P-Number 1, and vice versa.

P-Number 4 — Alloy Steels (0.75% to 2% Cr)

Covers chromium-molybdenum steels in the 1Cr-0.5Mo and 1.25Cr-0.5Mo range. Includes SA-335 P11 and P12 (alloy pipe) and SA-387 Gr.11 (alloy plate). These materials require mandatory PWHT per the construction code. The higher chromium content makes them more sensitive to hydrogen cracking, requiring strict preheat and hydrogen control.

P-Number 5A — Alloy Steels (2.25% to 3% Cr)

Covers 2.25Cr-1Mo steels, the workhorse alloy for high-temperature refinery and petrochemical service. SA-335 P22 (pipe) and SA-387 Gr.22 (plate) are the most common specifications. PWHT is mandatory. These materials are also subject to temper embrittlement concerns in long-term elevated-temperature service.

P-Number 5B — Alloy Steels (5% to 9% Cr)

Covers 5Cr-0.5Mo through 9Cr-1Mo steels, including SA-335 P5 and P9. These materials typically require mandatory PWHT per the construction code due to their air-hardening tendency.

P-Number 8 — Austenitic Stainless Steels

Covers the 300-series austenitic stainless steels including SA-240 Type 304 and 316 (plate), SA-312 TP304 and TP316 (pipe), SA-182 F304 and F316 (forgings). Austenitic stainless steels generally require no preheat and no PWHT. The welding concerns are sensitization (carbide precipitation at grain boundaries), hot cracking, and distortion due to the higher coefficient of thermal expansion compared to carbon steels.

P-Number 9A/9B — Nickel Alloy Steels

Covers nickel steels used in cryogenic service. P-9A includes 2.5% Ni steels (SA-203 Grade A), P-9B includes 3.5% Ni steels (SA-203 Grades D and E). These materials are designed for low-temperature applications such as LNG storage and ethylene plants where carbon steels lose ductility.

P-Number 10 — Various Chromium Stainless Steels

P-Number 10 covers a broad range of chromium-containing stainless steels across multiple subgroups. P-10A and P-10B cover ferritic stainless steels (409, 410S types). P-10H covers duplex and super duplex stainless steels (2205, 2507) that combine austenitic and ferritic microstructures. Duplex grades require careful heat input control to maintain the austenite-ferrite phase balance. P-10I and P-10J cover additional specialty grades. The subgroup distinction matters when the construction code requires impact testing.

P-Number 11 — High-Nickel Low-Temperature Steels

Covers high-nickel steels for low-temperature and cryogenic service. P-11A materials include 9Ni steels (SA-353, SA-333 Gr.8, SA-420 WPL8) and 3.5Ni castings used in LNG storage, cryogenic piping, and low-temperature pressure vessels. PWHT must be carefully controlled — excessive temperatures or holding times can degrade the toughness properties these steels are selected for. Toughness requirements are mandatory for all P-11 materials.

P-Number Group Directory

Table QW/QB-422 contains over 2,000 material specifications across approximately 70 distinct P-Number groups. The table below lists every P-Number family with its material category, typical specifications, and number of listed materials.

Nonferrous P-Numbers (P21+) do not use Group Numbers per QW-421.2. “—” indicates no group subdivision.

Group Numbers Explained

Within each P-number, materials are further subdivided into group numbers based on composition ranges and strength levels. The group number becomes an essential variable only when the referencing construction code (Section I, Section VIII, B31.3, etc.) requires impact testing. When impact testing is required, changing group number within the same P-number requires a new PQR. When impact testing is not required, all groups within a P-number are treated as equivalent for qualification purposes.

This dual treatment is specific to ASME IX and has no equivalent in D1.1 or API 1104. It reflects the ASME philosophy that materials within the same P-number have similar weldability, but their toughness behavior after welding may vary enough to warrant separate qualification when impact testing is specified.

“The P-number system is the backbone of ASME IX qualification. Get the P-number wrong and the entire WPS is invalid — it doesn’t matter how good the welds look.”

— Standard CWI instruction, reflecting ASME IX QW-403 essential variable requirements

How P-Numbers Compare to D1.1 Base Metal Groups

D1.1 Table 5.6 organizes structural steels into five groups (I through V) based on strength and weldability. This system is much simpler than ASME IX P-numbers because D1.1 covers only structural carbon and low-alloy steels, while ASME IX covers the full range of metallic materials used in pressure equipment. The rough equivalencies are P-Number 1 corresponding to D1.1 Groups I and II, and P-Number 3 corresponding to D1.1 Groups III and IV. D1.1 Group V (A913 Gr.80 HSLA steel) has no direct P-number equivalent because it is not commonly used in pressure equipment.

The two systems are not interchangeable. A qualification under D1.1 Group I does not satisfy an ASME IX P-Number 1 qualification requirement, and vice versa. Fabricators working under both codes must maintain separate qualification records. For questions about D1.1 base metal groups and preheat requirements, use the preheat calculator and check your steel’s group using your mill test report.

Qualification Ranges

A procedure qualified on a base metal within a P-number establishes a qualification range. The key rule is that changing P-number is an essential variable — it always requires a new PQR. Within the same P-number, the qualification range depends on whether impact testing is required. Without impact testing, all groups are covered. With impact testing, only the specific group (and groups of lesser impact test severity) are covered.

For dissimilar metal joints (welding two different P-numbers together), ASME IX QW-424 provides specific rules. A separate qualification is required for each P-number combination. A P-1 to P-1 qualification does not cover a P-1 to P-8 joint. The filler metal selection for dissimilar joints depends on both base metals and must be compatible with both P-numbers. The F-number of the filler metal is a separate essential variable.

More standards guides: API 1104 · CSA W59 · CSA W47.1 · Pipeline Welding · D1.2 Aluminum · D1.3 Sheet · D1.4 Rebar · D1.6 Stainless · D1.8 Seismic · D1.9 Titanium · D1.5 Bridges · AS/NZS 1554 · Preheat Calculator · D1.5 Preheat · D1.4 Preheat

Reference data from ASME BPVC IX:2025 — QW/QB-422, QW-424. Not affiliated with ASME.