Welding Procedure Qualification: Specification, Testing, and Range of Approval
What a procedure qualification actually proves
A Welding Procedure Specification (WPS) is a production document; the Procedure Qualification Record (PQR) is the evidence behind it. Qualification demonstrates that a procedure, made with a specific combination of process, base metal, filler, position, and essential variables, can produce a weld meeting the specified mechanical and soundness requirements. The candidate must know the difference between essential variables (change them and requalification is required), nonessential variables (may be changed and recorded in the WPS without requalification), and supplementary essential variables, which apply only when notch toughness is specified.
The testing sequence
Typical qualification follows: prepare a test coupon per the qualified joint detail, weld it under recorded conditions, allow specified cooling, then cut specimens. Common tests are transverse tension, guided bend (face and root), and, where required, impact, hardness, or macro-examination. Acceptance criteria usually require the tension specimen to fail in the base metal or exceed a specified minimum tensile strength, and bend specimens to show no open defect exceeding the code limit in the weld or heat-affected zone after bending.
Traps to expect
Questions often swap the roles of WPS and PQR, or imply that a PQR expires. A PQR generally remains valid indefinitely unless a code or contract says otherwise; it is the welder's qualification that may require periodic renewal. Another frequent trap is position: qualifying in the flat position gives a narrow range of approval, while vertical-up or overhead qualification broadens it. Thickness ranges are also asymmetric: a thick coupon qualifies a wide band, a thin coupon qualifies only thin material, and impact-tested coupons add further restrictions.
Rule of thumb
Ask three questions for any proposed change: is it essential, does toughness apply, and does the change fall inside the recorded range of approval? If any answer is uncertain, requalify.
Worked example: a procedure qualified on 10 mm plate in the vertical-up position with toughness testing does not automatically cover 25 mm plate; thickness qualification limits and impact-testing rules must both be checked before the WPS is issued.