Welding Procedure Qualification: PQR, WPS, and Essential Variables
From PQR to WPS
The procedure qualification record (PQR) documents an actual test weld and its destructive or nondestructive results. The welding procedure specification (WPS) is the production instruction derived from a qualified PQR. The trap: candidates treat the WPS as the test record. It is not. The PQR is the evidence; the WPS is the instruction. Every WPS must be supported by at least one PQR within its qualified ranges.
Essential, supplementary essential, and nonessential variables
Essential variables affect mechanical properties and must be requalified if changed. Supplementary essential variables matter only when notch toughness is required. Nonessential variables can be changed by revising the WPS without requalification. Typical essential variables include process, base metal specification and grade, filler metal classification, preheat and interpass temperature, post-weld heat treatment, and joint design type. Position and thickness ranges are also governed, though the exact rules differ by code.
Test assembly and mechanical testing
The qualification test typically includes a groove weld for procedure coverage and may include fillet tests. Mechanical tests usually include tensile, guided bend, and, where required, impact tests. Trap: confusing the number of test specimens with the number of welders or the number of procedures. Also watch for the requirement that test coupons be welded under the same essential variables recorded on the PQR.
Worked example
A PQR is qualified with 1/2 in (12 mm) plate in the vertical position with preheat of 200 F (93 C). A production WPS proposes 3/8 in (9.5 mm) plate in the same position with preheat of 250 F (121 C). Because the thickness is within the qualified range and preheat is higher (typically more conservative), the change is often acceptable without requalification, but the WPS must be revised to reflect the actual preheat. Always confirm the code's specific direction on preheat increases.
Common traps
Using a WPS with no supporting PQR; changing filler metal classification and assuming no requalification; ignoring supplementary essential variables when toughness is specified; failing to record actual test values versus proposed ranges.