What is a P-Number in welding?

To reduce the number of welding and brazing procedure qualifications required, the ASME (American Society of Mechanical Engineers) committee classified base metals with P-Numbers in table QW/QB-422 of ASME IX. Ferrous base metals with specific impact test requirements were further subdivided into Group Numbers within each P-Number.

These classifications are based on similarity between base metals in terms of:

• Chemical composition
• Mechanical properties
• Weldability
• Brazeability

In a WPS (Welding Procedure Specification), the P-Number is an essential variable: any change requires requalification. So a WPS qualified with P-Number 1, which is the P-Number for carbon steels, can be used on every carbon steel listed under P-Number 1.

The P-Numbers can be looked up here.

The table below is a guide to which materials fall under each P-Number:

P-Number	Base metal (typical example)
1	Carbon-manganese steels
2	Not used
3	1/2Mo or 1/2Cr-1/2Mo
4	1 1/4Cr-1/2Mo
5A	2 1/4Cr-1Mo
5B	5Cr-1/2Mo or 9Cr-1Mo
5C	Cr-Mo-V
6	Martensitic stainless steels (410, 415, 429)
7	Ferritic stainless steels (409, 430)
8	Austenitic stainless steels:
Group 1 - 304, 316, 317, 347
Group 2 - 309, 310
Group 3 - high Mn
Group 4 - high Mo
9A, B, C	Steels with 2 to 4% Ni
10A, B, C, F	Various low-alloy steels
10H	Duplex and super duplex (S31803, S32750)
10I	High-Cr stainless
10J	High Cr-Mo stainless
10K	High Cr-Mo-Ni stainless
11A	HSLA steels
11B	HSLA steels
12 to 20	Not used
21	Aluminium 1000 and 3000 series
22	Aluminium 5000 series (5052, 5454)
23	Aluminium 6000 series (6061, 6063)
24	Not used
25	Aluminium 5000 series (5083, 5086, 5456)
26 to 30	Not used
31	High copper
32	Brass
33	Cu-Si
34	Cu-Ni
35	Cu-Al
36 to 40	Not used
41	High nickel
42	Ni-Cu (Monel 500)
43	Ni-Cr-Fe (Inconel)
44	Ni-Mo (Hastelloy B2, C22, C276, X)
45	Ni-Cr
46	Ni-Cr-Si
47	Ni-Cr-W
48 to 50	Not used
51-53	Titanium alloys
61-62	Zirconium alloys

Putting the P-Number to work in your WPS

Choosing the right P-Number is the first step towards a WPS that passes qualification. Download our free WPS template in Excel to ASME IX 2025 ed. - bilingual, with every QW-402 to QW-410 block and a worked example.

Want to go beyond the template? In the course Writing a WPS - ASME IX (2025 ed.) I teach the complete process: P-Number, Group Number, consumables, parameters, PQR and databook.