ASME P-Number Table: Complete Reference from P-No.1 to P-No.15F

ASME P-Numbers are the cornerstone of the welding qualification system defined in ASME BPVC Section IX, classifying base metals into groups so that a single Welding Procedure Specification (WPS) can cover an entire family of materials. Whether you are writing a WPS for a pressure vessel in carbon steel, qualifying a procedure for a P91 creep-strength-enhanced ferritic (CSEF) superheater, or checking whether an existing PQR covers your new stainless-steel piping, understanding P-Numbers is non-negotiable. This reference article provides the most complete, practical explanation of ASME P-Numbers (P-1 through P-15F), Group Numbers, F-Numbers, and A-Numbers available in one place — with tables, worked examples, and engineering context drawn directly from Section IX.

The P-Number system was designed with a single economic goal: to minimise the sheer quantity of qualification records that industry would otherwise need. Without grouping, every change in base metal grade would demand a new PQR — an expensive, time-consuming mechanical test campaign. By assigning comparable materials to the same P-Number, ASME allows one PQR to serve an entire product family. Alongside P-Numbers, three companion systems complete the qualification framework: Group Numbers (toughness sub-groupings), F-Numbers (filler metal usability groupings), and A-Numbers (weld metal chemical composition). Together, these four systems define the essential and supplementary essential variables that govern whether your WPS is properly qualified for a given job.

This guide covers every P-Number from P-1 (carbon and low-alloy steel) through P-15F (high-chromium martensitic/ferritic alloys), explains how Group Numbers interact with impact testing requirements under ASME Section VIII Division 1, explains F-Number and A-Number logic, and provides practical engineering guidance on applying qualification scope correctly. All references are to ASME BPVC Section IX, Table QW-422 / QB-422, QW-432, and QW-442.

Overview: The Four ASME Section IX Classification Numbers

ASME BPVC Section IX governs welding and brazing procedure and performance qualifications for pressure boundary applications worldwide. Its classification number system is built on four inter-related designations, each targeting a different variable in the welding equation:

Each of these four systems reduces the qualification burden in a different way. P-Numbers reduce the number of base metal combinations requiring separate PQRs. F-Numbers reduce the number of filler metal combinations requiring separate welder performance qualifications. A-Numbers ensure that the deposited weld metal composition is consistently recorded and tracked as an essential variable. Group Numbers add precision when toughness properties are at stake.

Figure 2 — ASME Section IX classification number system overview as published in the BPVC.

P-Number — Base Metal Classification

P-Numbers are alphanumeric designations assigned by ASME to base metals in Table QW-422 (and QB-422 for brazing). Each designation represents a group of metals sharing similar composition, weldability, and mechanical properties such that a single qualified procedure can be applied across the entire group. The system is hierarchical: P-Numbers identify major material families, and Group Numbers further subdivide P-Numbers where impact toughness requirements apply.

A practical implication worth emphasising: when ASME says “P-Number qualification,” it means qualification of the welding procedure, not the welder. Welder performance qualification (WPQ) also uses P-Numbers but under a different — and generally more permissive — set of cross-qualification rules in QW-423. A welder qualified on P-1 through P-15F, P-34, or P-41 through P-49 is broadly qualified across these groups, which is significantly more permissive than procedure qualification rules.

The P-Number system runs from P-1 through P-15F for ferrous materials, and extends through P-21 to P-26 (aluminium and alloys), P-31 to P-35 (copper and alloys), P-41 to P-49 (nickel and alloys), P-51 to P-53 (titanium and alloys), and P-61 to P-62 (zirconium) for non-ferrous materials. This article focuses on the ferrous range P-1 to P-15F, which covers the vast majority of industrial pressure vessel and piping applications.

Figure 3 — Table reference locations in ASME Section IX. P-Numbers and Group Numbers are found in QW-422; F-Numbers in QW-432; A-Numbers in QW-442.

Full P-Number Reference Table: P-No.1 to P-No.15F

The following table provides a comprehensive reference for all ferrous P-Numbers from P-1 through P-15F as defined in ASME BPVC Section IX, Table QW-422. The “Typical Grades” column lists representative ASME/ASTM material designations. Consult the current edition of Section IX for the complete and authoritative material listing, including UNS numbers and product forms.

P-No.1 Carbon Steel in Depth

P-Number 1 is by far the most widely used classification in pressure equipment fabrication, covering the full range of carbon and low-alloy steels used in general pressure vessel and piping construction. The defining characteristic of P-1 materials is a carbon and manganese-dominated composition with no intentional alloying additions beyond minor quantities of silicon and deoxidising elements. Tensile strength typically ranges from 380 MPa (55 ksi) to 485 MPa (70 ksi) depending on grade.

Group Number Sub-classification within P-No.1

Within P-No.1, ASME Section IX defines three Group Numbers based primarily on specified minimum tensile strength, which in turn determines notch toughness behaviour. The Group Number becomes a supplementary essential variable under UG-84 of ASME Section VIII Division 1 when Charpy impact qualification is required:

Chrome-Moly Steels: P-3, P-4, P-5A, P-5B, and P-5C

The chrome-molybdenum (Cr-Mo) family of steels is critical to high-temperature pressure equipment in power generation, petroleum refining, and petrochemical processing. Chromium improves oxidation resistance and elevated-temperature strength; molybdenum resists creep and temper embrittlement. These steels form the backbone of boiler pressure parts, catalytic reformer tubing, hot-wall hydroprocessing reactors, and high-temperature piping systems operating between approximately 400°C and 650°C.

The Cr-Mo P-Numbers are separated based on chromium content and alloying additions. All Cr-Mo steels require Post Weld Heat Treatment (PWHT) — a mandatory Code requirement in virtually all applicable construction codes. See our detailed article on heat treatment requirements for fabricators for tempering temperature ranges and holding times for each grade.

P-No.6 Martensitic and P-No.7 Ferritic Stainless Steels

P-No.6 covers martensitic stainless steels in the 11–13% chromium range. These materials harden on cooling due to their martensite transformation, requiring preheat and PWHT to prevent hydrogen cracking and brittle fracture in the weld heat-affected zone. Common applications include valve trim components, pump shafts, and equipment handling moderately corrosive media where moderate strength is required. SA-182 F6a (AISI 410) is the most widely used P-6 material in pressure equipment.

P-No.7 covers ferritic stainless steels, characterised by a body-centred cubic (BCC) crystal structure that does not transform to austenite on heating and therefore cannot be hardened by quenching. The chromium content of P-7 materials typically ranges from 11% to 30%. These steels are prone to grain growth in the heat-affected zone and sensitisation at elevated temperatures, but they do not require PWHT in most applications. SA-268 TP430 (17% Cr) is a typical P-7 grade. Welding of P-7 steels should be done with low heat input to minimise HAZ grain growth.

P-No.8 Austenitic Stainless Steel

P-Number 8 is one of the most important classifications in the chemical process, pharmaceutical, food, and cryogenic industries. It covers the full range of austenitic stainless steels — materials with a face-centred cubic (FCC) crystal structure that remains stable at all temperatures from cryogenic to approximately 870°C. P-8 steels do not undergo phase transformation and cannot be hardened by heat treatment, which means PWHT is generally not required (and in many cases is actively harmful due to sensitisation risk).

The P-8 family includes the workhorse 300-series grades: TP304/304L, TP316/316L, TP321, TP347, and their variants. A PQR qualified on TP304 (P-8) to TP304 (P-8) also qualifies procedures for TP316, TP316L, TP321, and TP347 — all of which carry P-8 — subject to filler metal compatibility and A-Number consistency. For dissimilar welding of P-8 to P-1 (e.g., stainless steel cladding on carbon steel vessels), see our guide on stainless steel weld decay and sensitisation.

P-No.9A and P-No.9B — Nickel Alloy Steels

P-No.9A covers 2½% nickel steels and P-No.9B covers 3½% nickel steels. These materials are specifically designed for low-temperature service, where the added nickel content improves fracture toughness at sub-zero temperatures. They are widely used in LPG storage spheres, LNG processing facilities, and cryogenic piping systems. SA-333 Gr.3 (P-9A) is a common pipe grade rated to approximately –73°C, while SA-333 Gr.8 (P-9B) is rated to –101°C. Impact testing requirements under ASME Section VIII UG-84 are critical for these materials, and Group Numbers play a significant role in qualification scope when toughness testing governs.

P-No.10 High-Alloy Steels: Duplex, Super Duplex, and Lean Duplex

The P-No.10 sub-group designations (P-10A, P-10C, P-10H, P-10I, P-10J, P-10K) cover a diverse range of high-alloy steels that do not fit neatly into the simpler carbon, Cr-Mo, or straightforward austenitic classifications. The most commercially important of these sub-groups are the duplex and super-duplex stainless steels.

Duplex stainless steels (typically P-10C, such as SA-240 Gr.2205, UNS S31803) have a two-phase microstructure of approximately equal proportions of austenite and ferrite, which provides superior strength and resistance to stress corrosion cracking compared to standard austenitic grades. Super-duplex grades (P-10H, such as S32750/S32760) offer even higher PREN (Pitting Resistance Equivalent Number) values, suitable for aggressive chloride environments. For a complete technical guide, see our article on welding duplex stainless steels.

P-No.11 Through P-No.15F: Specialty Alloys

Several P-Numbers between P-11 and P-15F cover specialty alloy groups that are less frequently encountered in general fabrication but are critically important in their specific industries. Key groups in this range include:

Group Numbers and Impact Toughness Testing

Group Numbers are sub-classifications within P-Numbers, found in the same Table QW-422 of ASME Section IX. They exist for one specific reason: to provide a finer level of control over WPS qualification when the governing Code Section (such as ASME Section VIII Division 1, Section I, or B31.1) requires the welding procedure to be qualified by toughness (Charpy V-notch impact) testing.

Under normal P-Number qualification rules, a PQR qualified on one Group Number covers all Group Numbers within the same P-Number. However, when toughness testing is invoked as a Supplementary Essential Variable (which happens automatically when UG-84 of Section VIII or its equivalent applies), the Group Number becomes independently essential. In that context, a change in Group Number — even within the same P-Number — requires WPS requalification with toughness testing at the appropriate test temperature.

Figure 5 — Group Number sub-grouping within P-No.1 as illustrated in ASME Section IX Table QW-422. Group Numbers partition materials by strength level, which correlates with impact toughness behaviour.

Figure 6 — Simplified P-Number classification table as structured in ASME Section IX QW-422, showing the relationship between P-Number and material category.

Understanding when Group Numbers become relevant requires knowing whether your construction Code Section invokes Supplementary Essential Variables. For ASME Section VIII Division 1 pressure vessels, UG-84 Charpy impact testing requirements determine this. For power piping, ASME B31.1 Para. 127.4 applies. Always consult the applicable Code Section — not just Section IX itself — to determine whether Group Numbers are essential variables for your specific application.

F-Number — Filler Metal Usability Grouping

F-Numbers, defined in ASME Section IX Table QW-432, classify electrodes and welding rods by their usability characteristics — the properties that determine a welder’s ability to consistently produce sound welds with a given filler metal. The grouping logic is straightforward: if a welder can make good welds with one electrode in a group, they can make good welds with all electrodes in that group, because the handling and deposition behaviour is comparable.

This grouping applies primarily to Welder Performance Qualification (WPQ) and is also used as an essential variable in WPS qualification. For further detail on electrode classification systems, see our guide on welding consumable nomenclature and the comparison of cellulosic vs rutile electrodes.

Figure 7 — F-Number electrode grouping per ASME Section IX Table QW-432. Note that F-6 covers all bare wire processes (GTAW, GMAW, SAW, PAW) and has the broadest qualification scope.

A-Number — Weld Metal Chemical Composition

The A-Number is the fourth and final classification in the ASME Section IX system. It identifies the chemical composition of the deposited weld metal, as opposed to the base metal (P-Number) or the filler metal product (F-Number). A-Numbers are recorded on both the PQR (during procedure qualification) and the WPS, and a change in A-Number is an essential variable requiring WPS requalification.

A-Numbers are particularly important when the same filler metal designation may be used across a range of base metal combinations or dilution conditions, resulting in different weld metal compositions. The A-Number provides a single, chemistry-based reference that captures the actual as-deposited composition rather than the nominal filler metal specification.

Essential Variables Quick Reference

To use the P-Number, Group Number, F-Number, and A-Number system correctly, you need to understand exactly when each becomes an essential variable requiring WPS requalification, and when it functions as a supplementary essential variable requiring additional toughness qualification. The table below provides a practical reference:

Recommended Reference Books

The following books are essential references for engineers and inspectors working with ASME Section IX qualification, P-Numbers, and related welding codes.

Welder Performance Qualification Scope Across P-Numbers

Welder performance qualification rules in QW-423 are considerably more permissive than WPS procedure qualification rules. A welder who demonstrates competence on one P-Number group is considered qualified for a broader range of materials than the equivalent WPS qualification would allow. This is because welder qualification focuses on the welder’s skill rather than the procedure’s metallurgical fitness for service. The diagram below illustrates how welder qualification scope extends across P-Number groups:

Frequently Asked Questions

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