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P-Number Group in Welding: Explanation, Importance, and Usage

The P-Number is a classification system used in welding to group materials based on their similar welding characteristics. This system is maintained by the American Society of Mechanical Engineers (ASME) in their Boiler and Pressure Vessel Code (BPVC), specifically in Section IX.

The primary purpose of the P-Number system is to reduce the number of welding procedure qualifications by grouping materials that have similar properties, such as chemical composition, mechanical properties, and weldability. This allows a single welding procedure to be applicable to multiple materials within the same P-Number group.


  1. Standardization: P-Numbers standardize the classification of materials, ensuring consistency and reliability in welding procedures across different projects and industries.

  2. Efficiency: By grouping similar materials, the P-Number system reduces the number of welding procedure specifications (WPS) needed, saving time and resources in qualifying procedures.

  3. Safety: Ensuring that materials with similar welding characteristics are grouped together helps in maintaining the integrity of welded joints, thus enhancing safety in critical applications such as pressure vessels, boilers, and structural components.

  4. Cost-Effectiveness: Reducing the need for multiple welding procedure qualifications lowers costs associated with testing, documentation, and qualification processes.

  5. Compliance: Adherence to the P-Number system ensures compliance with industry standards and codes, which is essential for regulatory approvals and certifications.

P-Number Group in Welding: Explanation, Importance, and Usage


To use the P-Number system effectively, follow these steps:

  1. Identify the Material: Determine the material specification and grade that needs to be welded.

  2. Find the P-Number: Refer to ASME BPVC Section IX, Table QW/QB-422, which lists materials and their corresponding P-Numbers. Alternatively, consult material manufacturer specifications or databases that provide this information.

  3. Qualify the Procedure: Develop and qualify a Welding Procedure Specification (WPS) for the P-Number group. Ensure that the WPS addresses the essential variables for the specific welding process, as defined in ASME Section IX.

  4. Perform Welding: Execute the welding process according to the qualified WPS, ensuring adherence to the specified parameters and techniques.

  5. Inspection and Testing: Conduct necessary inspections and tests (e.g., visual inspection, radiographic testing, ultrasonic testing) to verify the quality and integrity of the welded joint.

  6. Documentation: Maintain detailed records of the WPS, Procedure Qualification Record (PQR), and Welder Performance Qualification (WPQ) for future reference and compliance purposes.


  • Example 1: Carbon steels such as A36, A516 Gr 70, and A105 fall under P-Number 1, allowing a single WPS to be used for these materials.

  • Example 2: Stainless steels like 304 and 316 are classified under P-Number 8, enabling streamlined qualification for welding procedures involving these materials.

By understanding and utilizing the P-Number system, welding professionals can ensure efficient, cost-effective, and compliant welding operations across a wide range of materials and applications.


General Description

Typical Material Specification within This P-Number Group


Carbon Steel (C, Mangane Steels)

SA-36, SA-106 Gr. B, SA-350 Gr. LF1, SA-516 Gr. 70


1/2 Cr & 1/2 Mo Steels

SA-209 T1, SA-213 T2, SA-234 WP1, SA-335 P1 & P2


1-1/4 Cr-Mo Steels

SA-182 F11 CL. 1, SA-213 T11, SA-335 P11


2-1/4 Cr-Mo Steels up to 75 ksi

SA-182 F22 CL. 1, SA-213 T22, SA-335 P22


5 to 9 Cr-Mo Steels up to 85 ksi

SA-182 F5 & F9, SA-213 T5 & T9, SA-234 WP5 & WP9


2-1 /4 to 9 Cr-Mo Steels over 85 ksi

SA-182 F3V, SA-336 F3V, SA-508 3V, SA-541 3V


Stainless Steel - Martensitic

SA-216 TP410 & TP429, SA-182 F6B, F6NM & F429, A473 410


Stainless Steel - Ferritic

SA-240 TYPE 405, 409, & 41 OS, SA-240 TYPE 430


Stainless Steel - Austenitic

SA-182 F304L, SA-182 F316L, SA-240 TYPE 304L & 316L


2% Ni Steel

SA-182 FR, SA-234 WPR, SA-333 GR. 9, SA-350 LF9


3% Ni Steel

SA-333 3, SA-350 LF3, CL 2


4.5% Ni Steel

SA-352 LC4


Low Alloy Steel

SA-225 C & D, SA-487 GR. 1, CL A & B


Low Alloy Steel

SA-213 T17


Low Alloy Steel

SA-612 UNS K02900


Stainless Steel - Duplex

SA-240 S31803, S32205 & S32750, SA-789 S32205


Stainless Steel - High Cr

SA-182 FXM-27CB, SA-240 S44635, SA-336 FXM-27CB


Stainless Steel - High Cr & Mo

SA-240 S44700, SA-268 S44735, SA-479 S44700


Stainless Steel - High Cr, Mo & Ni

SA-240 S44700, SA-268 S44735, SA-479 S44700


HSLA (High Strength Low Alloy)

SA-333 8, SA-420 WPL8, SA-533 TYPE A, CL 3


HSLA (High Strength Low Alloy)

SA-517 A, E, F, & B, SA-592 F


Age Hardening Alloy Steel Forgings



9% Cr, Creep strength enhanced

SA-182 F92, SA-213T91 &T92, SA-335 P91 & P92


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