9.1 Base Metal P-Numbers, Group Numbers, and S-Numbers

9.1 Base Metal P-Numbers, Group Numbers, and S-Numbers

Core Principle: The primary objective of ASME Section IX base metal grouping is economic and engineering efficiency: to avoid requiring fabricators to qualify separate welding procedures for every commercial alloy specification. By grouping metals with similar weldability, chemical composition, and mechanical properties into P-Numbers, and subdividing them by notch toughness into Group Numbers, Section IX establishes a standardized framework that balances metallurgical integrity with practical manufacturing flexibility.

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1. Engineering Purpose and Architecture of Table QW/QB-422

Under paragraph QW-420, base metals are assigned alphanumeric designations known as P-Numbers (and for brazing, QB-Numbers). There are tens of thousands of individual material specifications worldwide—ranging from ASTM, ASME Section II, AWS, and API to European (EN), Japanese (JIS), and proprietary mill grades. Requiring an individual Procedure Qualification Record (PQR) for every distinct steel grade would paralyze industrial fabrication.

To resolve this challenge, ASME Section IX establishes Table QW/QB-422, one of the largest and most critical reference tables in the entire Boiler and Pressure Vessel Code. Table QW/QB-422 categorizes thousands of ferrous and nonferrous materials by assigning them specific classification attributes:

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|                     ANATOMY OF TABLE QW/QB-422 SPECIFICATION DATA                       |
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| Specification & Grade | ASTM / ASME SA or SB designation (e.g., SA-516 Grade 70)        |
| UNS Number            | Unified Numbering System alloy identifier (e.g., K02700)         |
| Minimum Tensile       | Specified minimum ultimate tensile strength in ksi and MPa       |
| Nominal Composition   | Elemental alloy breakdown (e.g., Carbon-Manganese, 1-1/4Cr-1/2Mo)|
| Product Form          | Plate, seamless pipe, welded tube, forging, casting, or fitting |
| Assigned P-Number     | Metallurgical weldability family (e.g., P-No. 1)                 |
| Assigned Group Number | Notch-toughness sub-tier within the P-Number (e.g., Group 2)     |
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The Fundamental Rule of P-Numbers

When a welding procedure specification (WPS) is qualified using a base metal assigned a specific P-Number, that qualification extends to all other base metals assigned to that same P-Number, subject only to thickness limits, process variables, and supplementary essential variables (such as notch toughness). For example, qualifying a procedure on SA-106 Grade B seamless pipe (P-No. 1) qualifies the fabricator to weld SA-516 Grade 70 plate (P-No. 1), SA-36 structural plate (P-No. 1), and SA-105 forged flanges (P-No. 1) without retesting.

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2. Master Taxonomy of ASME P-Numbers

Section IX organizes base metals into numbered categories based on atomic structure, alloy content, transformation behavior, and cracking susceptibility. Inspectors and welding engineers preparing for the AWS Endorsement examination must recognize the primary ferrous and nonferrous P-Number groupings.

Nonferrous Base Metal P-Number Families

ASME Section IX also categorizes nonferrous materials into dedicated P-Number blocks:

• P-No. 21 through P-No. 26: Aluminum and aluminum-base alloys (e.g., P-21 for 1000 series pure aluminum; P-22 for 3000 series Al-Mn; P-23 for 6000 series Al-Mg-Si; P-25 for 5000 series Al-Mg).
• P-No. 31 through P-No. 35: Copper and copper-base alloys (e.g., P-31 pure copper, P-34 copper-nickel 70/30 and 90/10).
• P-No. 41 through P-No. 49: Nickel and nickel-base alloys (e.g., P-41 pure nickel 200; P-42 Inconel 600; P-43 Inconel 625, Hastelloy C-276; P-45 Incoloy 800).
• P-No. 51 through P-No. 53: Titanium and titanium-base alloys (reactive metals requiring trailing argon shields).
• P-No. 61 through P-No. 62: Zirconium and zirconium-base alloys.

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3. Group Numbers and Notch-Toughness Mechanics (QW-420 & QW-403.5)

Within ferrous P-Numbers (P-No. 1, 3, 4, 5, 9, 10, 11, and 15), materials are further subdivided into Group Numbers (e.g., P-No. 1 Group 1, P-No. 1 Group 2). Understanding when Group Numbers matter—and when they are completely irrelevant—is one of the most frequently tested distinctions on the CWI Endorsement examination.

The Dual Rules of Group Numbers

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|                        THE GROUP NUMBER OPERATIONAL DICHOTOMY                           |
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| CASE A: IMPACT TESTING IS NOT REQUIRED BY THE CONSTRUCTION CODE                         |
|  - Group Numbers have NO regulatory significance!                                       |
|  - A PQR welded on ANY Group Number within a P-Number qualifies ALL Group Numbers       |
|    within that same P-Number.                                                           |
|  - Example: A PQR on SA-516 Gr. 70 (P-1 Gr. 2) qualifies SA-36 (P-1 Gr. 1),            |
|    SA-516 Gr. 60 (P-1 Gr. 1), and SA-537 Cl. 1 (P-1 Gr. 3).                             |
|-----------------------------------------------------------------------------------------|
| CASE B: IMPACT TESTING IS MANDATED (BY B31.3, B31.1, OR SECTION VIII)                   |
|  - Group Numbers become a SUPPLEMENTARY ESSENTIAL VARIABLE under QW-403.5!              |
|  - Qualification of a procedure on one Group Number qualifies ONLY that specific       |
|    Group Number (or combinations tested).                                               |
|  - Example: A PQR on SA-516 Gr. 70 (P-1 Gr. 2) with impact tests qualifies ONLY         |
|    P-1 Gr. 2 base metals for low-temperature service. It CANNOT weld P-1 Gr. 1!        |
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Why Do Group Numbers Exist Metallurgically?

Group Numbers are assigned to group materials that demonstrate comparable notch toughness (Charpy V-notch impact transition behavior). For instance, in P-No. 1:

• Group 1 comprises lower-strength structural carbon steels with specified minimum tensile strength $\le 65\text{ ksi}$ (e.g., SA-36, SA-106 Gr. B, SA-516 Gr. 60). These steels typically have higher ductile-to-brittle transition temperatures.
• Group 2 comprises fine-grain, higher-strength carbon steels with specified minimum tensile strength between $70\text{ ksi}$ and $80\text{ ksi}$ (e.g., SA-516 Gr. 70, SA-106 Gr. C). These steels are frequently killed and normalized for low-temperature toughness.
• Group 3 and 4 encompass quenched and tempered or high-strength microalloyed carbon steels (e.g., SA-537 Class 1 and Class 2).

When a pressurized piping system (such as ASME B31.3 low-temperature service below $-20\text{°F}$) mandates Charpy impact testing, welding parameters that produce acceptable toughness in a Group 2 steel may fail catastrophically in a Group 1 steel due to grain coarsening in the heat-affected zone (HAZ). Thus, paragraph QW-403.5 locks the procedure strictly to the Group Number(s) qualified.

Critical Code Rule: Welder Performance Immunity to Group Numbers

While Group Numbers can severely restrict Procedure Qualification (WPS/PQR), they have zero effect on Welder Performance Qualification (WPQ) under Article III! A welder who tests on any P-No. 1 steel (whether Group 1, Group 2, or Group 3) is fully qualified to weld all P-No. 1 through P-No. 15F base metals in production under Table QW-423.1. Welders are tested for manual dexterity and sound deposit ability, not metallurgical notch toughness.

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4. The S-Number Legacy and 2009 Code Unification (QW-420.2)

Candidates studying older piping procedures or legacy PQRs will encounter references to S-Numbers. Understanding their history and current legal status prevents critical auditing errors.

Why Did S-Numbers Exist?

Historically, the ASME Boiler and Pressure Vessel Code (Sections I, III, VIII) only permitted materials that had been formally reviewed, accepted, and published in ASME Section II Part A or Part B (designated with an "SA" or "SB" prefix, such as SA-106). Conversely, the ASME B31 Pressure Piping Codes (such as B31.1 Power Piping and B31.3 Process Piping) routinely permitted the use of standard ASTM specifications that had not been adopted into Section II (designated with an "A" or "B" prefix, such as ASTM A106 or ASTM A333).

To accommodate piping fabricators qualifying procedures under Section IX, ASME created S-Numbers in Table QW/QB-422. S-Numbers mirrored P-Numbers exactly (e.g., an ASTM material with properties identical to P-No. 1 was assigned S-No. 1; an alloy matching P-No. 4 was assigned S-No. 4).

The 2009 Code Elimination

In the 2009 Edition of ASME Section IX, the committee completely eliminated S-Numbers. All ASTM materials previously categorized under S-Numbers were either incorporated directly into ASME Section II or assigned official P-Numbers in Table QW/QB-422.

Current Grandfathering Rule (QW-420.2)

What happens to a legacy PQR qualified in 1998 that lists base metal "S-No. 1 Group 2"? Under paragraph QW-420.2, all previous qualifications using S-Numbers remain fully valid. An S-Number assignment is considered legally equivalent to its corresponding P-Number. When revising or auditing a WPS supported by an older S-Number PQR, the inspector simply treats the S-Number as the corresponding P-Number.

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5. Procedure Qualification of Unassigned Materials (Table QW-424.1)

Not all engineering materials are listed in Table QW/QB-422. What happens when a fabricator must weld a foreign steel (e.g., DIN 17175, EN 10028, JIS G3106), an unlisted ASTM material, or a proprietary mill alloy? Such materials are classified by Section IX as unassigned base metals.

Paragraph QW-424 and Table QW-424.1 govern the base metals qualified in production when test coupons are welded using assigned and unassigned materials.

Crucial Nuances of Table QW-424.1

1. The Dissimilar Assigned-to-Unassigned Bonus: Look closely at row 3 of Table QW-424.1: If a fabricator qualifies a PQR by welding SA-516 Gr. 70 (P-No. 1) to an unassigned alloy (e.g., proprietary alloy Incoloy 925), what does the PQR qualify?

   • It qualifies Incoloy 925 to any P-No. 1 carbon steel.
   • It also qualifies Incoloy 925 to itself!
   • However, it does NOT qualify P-No. 1 to P-No. 1! (A separate PQR welded P-1 to P-1 is required to qualify carbon steel to carbon steel).

2. Unassigned Metals Never Qualify Other Unassigned Metals: Qualifying a procedure on unassigned alloy "X" qualifies only alloy "X". It provides zero qualification for unassigned alloy "Y", even if both alloys have almost identical nominal chemistries and mechanical properties.

3. Essential Variable Status (QW-403.11 & QW-403.13): Under paragraph QW-403.11, a change from a base metal listed under one P-Number to a metal listed under another P-Number, or to an unassigned base metal, is an essential variable. Under QW-403.13, changing from one unassigned base metal to another unassigned base metal is also an essential variable requiring a completely new PQR.

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6. Practical Exam Traps & Code Navigation Strategies

• Trap 1: The "Unassigned Qualifies Assigned" Fallacy: An exam question states: "A manufacturer qualifies a PQR on P-No. 1 welded to unassigned alloy Z. Can this PQR support a production WPS to weld P-No. 1 to P-No. 1?" The answer is NO. Under Table QW-424.1, welding P-No. 1 to an unassigned metal qualifies only that unassigned metal to P-No. 1, and that unassigned metal to itself. It never qualifies P-No. 1 to P-No. 1.
• Trap 2: The Group Number Over-Application Trap: When a question presents two P-No. 1 steels with different Group Numbers (e.g., P-1 Gr. 1 welded to P-1 Gr. 2), immediately check whether the question mentions Charpy impact testing or notch-toughness requirements. If impact testing is NOT specified, Group Numbers are completely irrelevant—the procedure is fully qualified for all P-No. 1 metals!
• Trap 3: The Welder Base Metal Trap: Watch out for questions asking what base metals a welder is qualified to weld after passing a test on P-No. 1. Candidates often waste time checking Group Numbers in Table QW/QB-422. Group Numbers have zero application to welders; under Table QW-423.1, testing on P-No. 1 qualifies the welder for P-No. 1 through P-No. 15F, all Group numbers!