WPS — Welding Procedure Specification
The document that tells a welder how to make a weld: process, materials, joint preparation, position, parameters, preheat and any post-weld heat treatment.
A WPS is an instruction. It is only valid within the range established by the qualification record that supports it, which is why a WPS on its own — without the PQR behind it — proves nothing.
The recurring mistake
Using a WPS outside its qualified range. Rarely deliberate: the thickness on this job is simply different from the thickness that was tested, and nobody had the range in front of them at the moment of assignment.