A Welding Procedure Specification (WPS) is not just documentation. It is the technical basis for every production weld made on ASME-coded pressure equipment. When a WPS is correctly developed and qualified, the welder has a clear, tested set of parameters to work within. When it is poorly developed or incorrectly qualified, it creates a false sense of compliance while production welds are made under conditions that have never been demonstrated to produce acceptable results.
This guide walks through WPS development under ASME Section IX, step by step, from understanding why the WPS exists to the most common errors encountered in practice.
Why ASME Section IX Governs Welding Procedures
ASME Section IX is the part of the ASME Boiler and Pressure Vessel Code that establishes qualification requirements for welding and brazing procedures and for welders and brazing operators. It applies to all pressure-retaining welds made under ASME Section I (power boilers), Section III (nuclear components), Section IV (heating boilers), and Section VIII (pressure vessels), as well as ASME B31.1 and B31.3 piping systems.
The reason Section IX requires procedure qualification testing is that welding is not a deterministic process in the way that machining is. The same nominal parameters can produce acceptable or unacceptable welds depending on variables that are difficult to control absolutely: operator technique, base material heat, heat variations, consumable moisture content, preheat uniformity. The only way to validate that a set of parameters actually produces a sound weld in the applicable material combination is to make a test weld and mechanically test it.
A WPS supported by a valid PQR provides documented evidence that the procedure was tested and the test weld passed mechanical examination. Without this, the procedure is theoretical.
Essential Variables, Nonessential Variables, and Supplementary Essential Variables
ASME Section IX divides WPS variables into three categories that govern when requalification is required:
- -Essential variables: Parameters whose change affects the mechanical properties of the weld. A change in an essential variable requires a new PQR test before the WPS can be revised to reflect that change. Examples: change in P-number of base material, change in F-number of filler metal, change in PWHT from without PWHT to with PWHT, change in electrode process (SMAW to GTAW).
- -Supplementary essential variables: Parameters that affect notch toughness. These become essential variables when the applicable construction code (Section VIII, B31.3, etc.) requires Charpy impact testing of the weld. If impact testing is not required for the application, supplementary essential variables are treated as nonessential. Examples: base metal thickness range, filler metal diffusible hydrogen designation, preheat and interpass temperature limits.
- -Nonessential variables: Parameters that do not significantly affect mechanical properties. Changes to nonessential variables can be made with a WPS revision and authorized representative sign-off, without a new PQR test. Examples: changes in joint design within qualified range, changes in groove angle within limits, changes in cleaning method.
Step-by-Step WPS Development
Developing a WPS under ASME Section IX follows a defined sequence:
- -Step 1: Define the application: Identify the base materials by ASME P-number, the filler metal by F-number and A-number (for ferrous materials), the process, the position(s) required in production, the joint geometry, the preheat and interpass temperature requirements, and whether PWHT is required.
- -Step 2: Check for existing PQR coverage: Before conducting a new test, review existing PQR records. A PQR tested for P1 to P1 material may already cover the production application. Check that all essential variables in the proposed WPS fall within the qualified ranges of the supporting PQR.
- -Step 3: Conduct the test weld: If no existing PQR covers the application, make a test weld using the proposed essential variables. Record all actual parameters (heat input, interpass temperature, preheat, wire feed speed, voltage, travel speed) as-recorded, not as-specified. These recorded values form the PQR.
- -Step 4: Mechanical testing: Prepare and test specimens per QW-463: tensile tests, guided bend tests, and CVN testing if required. Laboratory reports with actual test values become part of the PQR. All specimens must meet the minimum acceptance criteria.
- -Step 5: Complete the PQR: Compile all recorded welding data and mechanical test results into the PQR. The PQR must be signed by the manufacturer's or contractor's authorized representative.
- -Step 6: Write the WPS: Using the PQR as the technical basis, write the WPS. The WPS specifies ranges for all variables. Essential variable ranges must stay within the limits established by the PQR and the applicable Section IX tables. The WPS must reference the supporting PQR(s).
- -Step 7: Sign and control the WPS: The WPS is signed by the authorized representative and assigned a unique document number. Copies are distributed to supervisors and welders. Revision control tracks any subsequent changes.
P-Numbers, F-Numbers, and A-Numbers
ASME Section IX groups base materials into P-numbers based on comparable chemistry and weldability. A PQR tested on a P-1 (carbon steel) to P-1 material qualifies WPS documents for any P-1 to P-1 combination within the qualified thickness range, without testing every individual material specification.
Filler metals are grouped into F-numbers by metallurgical similarity. F-number groupings control the range of filler metals that one PQR qualification supports. The A-number system groups deposited weld metal by chemical composition and is relevant to the WPS.
These grouping systems allow one carefully designed test program to support a broad range of production WPS documents. Understanding the P-number and F-number structure is essential for designing an efficient PQR testing program.
Not all base materials fall within standard P-numbers. Duplex stainless steels, high-alloy materials, and certain specialty grades require individual qualification because they are not grouped with similar materials. This is frequently overlooked when WPS packages are prepared for projects involving high-alloy materials.
When PWHT Affects Qualification
Post-weld heat treatment (PWHT) is an essential variable in ASME Section IX. A PQR qualified with PWHT does not support a WPS that specifies no PWHT, and vice versa. This means that if PWHT is required for some welds on a project but not for others, separate PQR qualifications are required for the PWHT and the as-welded conditions.
On complex projects involving both pressure piping (where PWHT may be required by the applicable piping code and material) and structural attachments (where PWHT may not be required), separate WPS documents with separate PQR support are needed for the two applications.
PWHT temperature, hold time, and cooling rate requirements must be specified in the WPS and, where applicable, confirmed as supplementary essential variables consistent with the PQR.
Remote WPS Development and Review
WPS and PQR development is well-suited to remote delivery. The application requirements, material specifications, joint geometries, and applicable codes are shared by document. The WPS package is developed, reviewed against ASME Section IX requirements, and returned as a complete, signed package.
For fabricators without in-house Senior CWI or CWB Level 2 expertise, WPS packages are sometimes submitted to clients or regulators with errors that create compliance risk. An independent review before submission identifies deficiencies early, when correction is straightforward.
Norman QC provides WPS/PQR development, review, and sign-off services per ASME Section IX, CSA W47.1, AWS D1.1, and API 1104. For scope and fee information, contact via the services page.
FAQs
How many PQRs are needed to support a typical fabrication shop's WPS library?
This depends on the range of materials, processes, and service conditions the shop handles. A carbon steel pressure vessel shop doing standard P1 to P1 SMAW and GTAW work may get by with 4 to 8 PQRs covering a range of thicknesses, positions, and PWHT conditions. A shop handling alloy steel, stainless, and dissimilar metal welds may need significantly more. A proper WPS qualification program assessment is the starting point for determining what is needed.
Can the PQR be performed by the fabricator themselves?
Yes. Under ASME Section IX, the manufacturer or contractor is responsible for performing the test weld and mechanical testing. The fabricator makes the test weld and arranges for laboratory testing. The authorized representative (not a third-party inspector) certifies the PQR. However, independent third-party witnesses are often specified by clients for major PQR qualifications to provide additional confidence in the recorded data.
Does a WPS need to be re-approved periodically?
ASME Section IX does not set a time-based expiry for WPS qualification. A WPS supported by a valid PQR remains qualified indefinitely as long as the variables remain within the qualified ranges. However, if the applicable code changes in a way that affects qualification requirements, the WPS should be reviewed against the new requirements. Additionally, if the supporting PQR test records cannot be located, requalification may be required.