WPS: the welding procedure specification explained
The sheet that tells you exactly how a particular joint is to be welded — and the document an inspector reaches for first when something has gone wrong.
What a WPS is
A welding procedure specification is an instruction for one joint configuration: this material, this thickness, this process, these consumables, these parameters, this position, this sequence of runs. It is not general guidance. Two joints that look similar can need two WPS documents if the material group or the thickness range differs.
It exists because welding cannot be verified by looking at it. A joint that was welded too cold, or without preheat, or with the wrong filler, can look perfect and fail in service. Qualifying the procedure in advance, on a test piece that is destroyed and examined, is the only practical answer.
The chain of documents
pWPS — preliminary. The technologist writes down what should work.
Test piece — welded to the pWPS, under witness. It then goes for destructive and non-destructive examination to EN ISO 15614-1: visual, radiography or ultrasonics, transverse tensile tests, bend tests, impact tests, macro-section and hardness survey.
WPQR — the report of all that, with the ranges the test qualifies. A test on 12 mm plate does not qualify 40 mm.
WPS — issued to the shop floor, backed by the WPQR number.
Reading one
The fields a welder actually uses are near the bottom: the run sequence table. For each run it gives the process, the consumable and diameter, the current range, the voltage range, the travel speed or heat input limit, and the polarity. Above it sit the joint sketch, the material group to ISO/TR 15608, the preheat and interpass temperatures, the gas and flow rate, and any post-weld heat treatment.
Two numbers are worth checking before you strike an arc. Preheat: if it says 100 °C, that is 100 °C measured 25 mm from the joint, before every run, not just the first — the reasoning is in the preheat calculator. Heat input: if a range is given, it is an essential variable, and staying inside it means knowing your travel speed rather than guessing it. The heat input calculator is the arithmetic.
The WPS and your certificate are different things
The WPS qualifies the procedure. Your EN ISO 9606 certificate qualifies you. Both must be valid and both must cover the joint in front of you, and they cover it in different ways — the WPS by material and thickness range, the certificate by process, position, thickness and diameter. What your own certificate covers is worked out in the qualification range calculator, and the test behind it is described in the EN ISO 9606 examination.
On structural work to EN 1090 and on pressure equipment, welding without a qualified procedure is not a shortcut — it is a non-conformity that surfaces at handover, when the cost of cutting out and re-welding is at its highest. Ask for the document before the job starts, not after.
Frequently asked questions
What is the difference between a pWPS, a WPS and a WPQR?
A pWPS is the proposal — the technologist's intended settings, not yet proven. The test piece is welded to it and examined; the report of that examination is the WPQR. Once the WPQR is accepted, the pWPS becomes a WPS. In short: pWPS is the plan, WPQR is the proof, WPS is the approved instruction.
Does a WPS travel between companies?
The qualification belongs to the manufacturer, not to the paper. Another company generally cannot simply adopt your WPQR; it qualifies its own, or has the transfer accepted by the certifying body under EN ISO 15614. The welder's certificate, by contrast, belongs to the welder and does travel.
What if I cannot hold the parameters in the WPS?
Stop and say so. Ranges in a WPS are essential variables: welding outside them means the joint was not made by a qualified procedure, whatever it looks like. A revised or additional WPS is a normal request, and far cheaper than cutting the joint out.
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