5 min read
WPS, PQR and WPQ: what procurement should ask for
A plain-language guide to welding procedures, procedure qualification records and welder qualifications under ASME Section IX, and what to check on a spec.
Welding documentation arrives with most pressure equipment submittals as a stack of three-letter acronyms. They are not interchangeable, and each answers a different question. Knowing which is which makes submittals quicker to review and gaps easier to spot before they become schedule problems.
Three documents, three questions
| Document | Question it answers | What it contains |
|---|---|---|
| WPS, Welding Procedure Specification | How will this joint be welded? | The instructions a welder follows: process, base metals, filler metal, thickness range, positions, preheat and interpass temperature, heat treatment, shielding gas and electrical settings, each as an allowable range |
| PQR, Procedure Qualification Record | Has that procedure been proven? | The actual values used to weld a test coupon, and the results of mechanical tests on it: tension, guided bend and, where required, impact tests |
| WPQ, Welder Performance Qualification | Can this person make the weld? | A record that an individual welder or welding operator produced a sound test weld, and the positions, thicknesses, diameters and filler metal groups that qualifies them for |
When the work is built to an ASME code, including Section VIII vessels and B31 piping, all three are governed by ASME Section IX.
The WPS and its supporting PQR
The WPS is the working document. It gives ranges rather than single values, so the welder has room to work while staying inside what has been proven. The PQR is the proof: a record of one test coupon, welded and tested, and certified by the manufacturer. Every WPS must be supported by at least one PQR, and a single PQR can support several WPSs.
A PQR records what happened on the day and is never edited to cover something it did not test. A new material, thickness range or process means a new coupon.
Essential variables, in plain terms
Section IX sorts welding variables into three categories:
- Essential variables affect the mechanical properties of the weld. Change one beyond its qualified range and the procedure must be requalified with a new PQR. Examples include the welding process, the base metal P-Number, the filler metal F-Number and the qualified thickness range.
- Supplementary essential variables apply only when the construction code requires impact (notch toughness) testing. They include heat input and some limits on interpass temperature and postweld heat treatment. A procedure qualified without impact tests cannot simply be used on a job that needs them.
- Nonessential variables, such as groove design details and cleaning methods, can be changed by revising the WPS without new testing.
P-Numbers group base metals with similar welding characteristics, so one qualification covers a family of materials. Carbon steels are generally P-No. 1; austenitic stainless steels such as 304 and 316 are P-No. 8. F-Numbers group filler metals by usability in the same way.
Thickness is the variable most often overlooked. A test coupon qualifies a range of base metal and deposited weld metal thickness tied to the coupon thickness, commonly up to twice the coupon thickness for mid-range plate, with specific limits at both ends. A procedure qualified on 10 mm plate will not cover a 50 mm nozzle forging.
Dissimilar metals
Joining two different P-Numbers, such as a carbon steel nozzle to a stainless shell, needs a procedure qualified for that combination. A PQR for carbon steel to carbon steel and another for stainless to stainless do not add up to a qualification for carbon steel to stainless.
Filler metal choice matters as much as the paperwork. Over-alloyed austenitic fillers such as ER309L are common for carbon-to-stainless joints at moderate temperatures, because they tolerate dilution from the carbon steel side without forming a brittle weld. For higher temperatures or heavy thermal cycling, nickel-based fillers are often chosen to manage the difference in thermal expansion. Postweld heat treatment of dissimilar joints needs particular care, and the procedure has to be qualified with the heat treatment the joint will actually receive.
The welder's qualification
A qualified procedure in the hands of an unqualified welder is still a nonconformance. The WPQ ties a named welder to a process and a set of ranges: positions, thickness, pipe diameter and filler metal group. A welder's qualification for a process lapses if they have not welded with that process for six months, so shops keep a continuity log to show each qualification is current.
What to ask for on your spec
Most of these checks take minutes if the documents are requested up front:
- WPSs with their supporting PQRs, submitted for review before production welding. Make this a hold point in the inspection and test plan.
- Coverage. Base metal P-Numbers, including any dissimilar combinations, thickness ranges, positions and pipe diameters should cover every joint on the drawings.
- Toughness. If the design requires impact testing, the PQRs should include impact tests at or below the minimum design metal temperature.
- Heat treatment. If the equipment will be postweld heat treated, the test coupons should have been too.
- WPQs and continuity records for the welders on the job.
- New procedures, flagged early. If a procedure has to be developed, welding and mechanically testing a coupon adds real time to the schedule.
For pressure equipment in Canada, welding procedures are also subject to the provincial jurisdiction's requirements under CSA B51, which can include registration with the regulator. Rigid Welding develops and qualifies Section IX procedures, including for dissimilar metal joints, and these are the questions we would expect a careful buyer to ask of any shop.
- Welding procedure development
- Quality documentation