WPS and PQR Demystified: How W...

Ask a room of engineers to explain the difference between a WPS and a PQR and the answers will vary more than they should. These documents are the legal and technical foundation of every code weld, yet they are frequently misunderstood, misapplied or confused. A clear understanding of how welding procedures are qualified prevents both quality failures and costly requalification.

Three Documents, Three Purposes

Welding Procedure Specification (WPS)

The WPS is the instruction to the welder. It specifies the ranges of variables to be used in production: process, base metal groupings, filler metal, thickness range, joint design, position, preheat and interpass temperature, postweld heat treatment (PWHT), shielding gas, and electrical characteristics. It is written in terms of permitted ranges, not single values.

Procedure Qualification Record (PQR)

The PQR is the evidence. It records the actual variables used when welding a test coupon and the results of the mechanical tests performed on it. A PQR is a historical record of what was done and what was achieved; it should not be revised to change the variables used, although editorial corrections and additional supporting information are permitted under ASME Section IX.

Welder Performance Qualification (WPQ)

The WPQ demonstrates that an individual welder or welding operator can deposit sound weld metal using a qualified procedure. Procedure qualification proves the procedure can produce welds with the required properties; performance qualification proves the person can produce sound welds.

ASME Section IX: The Variable System

ASME Boiler and Pressure Vessel Code, Section IX, Welding, Brazing, and Fusing Qualifications, is referenced by ASME Section I, Section VIII, ASME B31.1, ASME B31.3 and many other codes. Its logic rests on classifying each welding variable by its effect on the weldment.

• Essential variables are those that, when changed beyond the stated limits, are considered to affect the mechanical properties of the weldment. Changing an essential variable requires a new or additional PQR.
• Supplementary essential variables apply when notch-toughness (impact) testing is required by the referencing code. Changing them requires requalification when toughness is a requirement.
• Nonessential variables may be changed by revising the WPS without requalification, provided the change is documented.

To keep the number of qualifications manageable, Section IX groups materials. Base metals are assigned P-Numbers (and Group Numbers within them) based on composition, weldability and mechanical properties. Filler metals are grouped by F-Numbers according to usability characteristics, and weld metal chemical composition is classified by A-Numbers. Qualifying on one P-Number can qualify others under the rules in Section IX.

Testing the procedure coupon

For groove welds, Section IX typically requires tension tests to verify strength and guided bend tests to verify ductility and soundness. Notch-toughness tests are added when the referencing code requires them. The coupon thickness determines the range of base metal and deposited weld metal thickness qualified, according to the tables in QW-451; for many common coupon thicknesses, the maximum qualified thickness is twice the coupon thickness.

Welder qualification and continuity

Welders may be qualified by mechanical bend tests or, for certain processes and materials, by volumetric examination. Section IX also contains continuity provisions: a welder's qualification for a process lapses if that process has not been used within a specified period, which is six months under the general rules, unless continuity is maintained as permitted by the code.

A WPS tells the welder what to do. The PQR proves that doing it works. Neither substitutes for the other, and neither substitutes for a qualified welder.

AWS D1.1: Prequalified and Qualified by Testing

AWS D1.1, Structural Welding Code — Steel, governs welded steel structures such as buildings and many industrial structures. It offers two routes to an approved WPS:

• Prequalified WPSs are exempt from procedure qualification testing because the code recognizes combinations of processes, base metals, filler metals, joint details, preheat and other parameters with a long history of satisfactory performance. Every condition listed in the code for prequalification must be met; any deviation removes prequalified status.
• WPSs qualified by testing are required when conditions fall outside prequalified limits — for example, unlisted base metals, nonstandard joint geometries or processes not eligible for prequalification. Testing follows the D1.1 qualification requirements with its own variable rules.

Even with a prequalified WPS, AWS D1.1 still requires welders to be qualified. A prequalified procedure removes the need for a PQR, not the need for competent welders.

Other Codes Worth Knowing

Pipeline welding in the United States commonly follows API Standard 1104, which has its own procedure and welder qualification requirements. Internationally, procedure qualification is often performed to ISO 15614-1. Standard Welding Procedure Specifications published by AWS under the AWS B2.1 framework can be adopted within their stated limits in certain applications. The engineer must always confirm which qualification code the contract or jurisdiction requires.

Common Errors to Avoid

• Writing a WPS with ranges not supported by any PQR.
• Overlooking supplementary essential variables when impact testing is required.
• Applying a WPS qualified without PWHT to a joint that will be heat treated, or vice versa.
• Failing to maintain welder continuity records.
• Assuming a prequalified WPS remains prequalified after small parameter changes.

Key Takeaways

• The WPS instructs, the PQR proves, and the WPQ qualifies the individual welder.
• ASME Section IX classifies variables as essential, supplementary essential and nonessential.
• P-Numbers, F-Numbers and A-Numbers reduce the number of qualifications needed.
• AWS D1.1 allows prequalified WPSs only when every prequalification condition is met.
• Welder qualification and continuity are required regardless of how the procedure is qualified.

To master procedure writing and review, consider IIME's Welding Procedure and Welder Qualification training course.