Welding procedures: WPS, PQR and prequalification

Devon has followed welding procedures since his first week, reading the sheet taped to the booth wall and setting the machine inside its ranges. In this lesson he learns how those sheets are written, what stands behind them, and who has to sign them before anyone may weld to them. Elena gives him a project for the week: take one real joint on the Coldwater skid, a flange-to-pipe butt weld in the injection line, and trace its procedure back to the test that proved it. By Friday he has the procedure in one hand, the test record in the other, and a list of every number on the first that the second has to support.

Why a written procedure

A welding procedure is a set of instructions that, followed by a qualified welder, produces a weld with the properties the design needs. Writing it down does three things. It makes the weld repeatable, because every welder on every shift uses the same variables. It makes the weld checkable, because an inspector can compare what is happening at the booth with what is written. And it makes the weld defensible, because the procedure can be traced to a test or to the code's prequalified limits, so that the shop can show why the weld can be trusted. A weld made without a procedure may be sound, but nobody can prove it.

The documents and their names

American practice uses three documents. The welding procedure specification, or WPS, is the instruction the welder follows: process, joint design, base and filler metals, position, preheat and interpass temperature, electrical characteristics, technique, and the ranges allowed for each. The procedure qualification record, or PQR, records the actual values used to weld a test coupon and the results of the tests made on it; it is a record of what happened, not an instruction, and it is never revised to change what was done. The welder performance qualification record shows that a welder has passed a test for a set of variables. Under D1.1 a WPS is either prequalified, with no PQR behind it, or qualified by test, with one or more PQRs behind it.

The difference between the instruction and the record is the most important idea in this lesson. The procedure sheet tells the welder what to do and gives ranges. The test record tells the inspector what was proved and gives the single values that were actually used. Every range on the procedure sheet must fall within what the code allows the test to support. A procedure sheet can be revised; a test record cannot, because it records a test that has already happened.

Prequalified procedures

For the commonest structural joints, D1.1's prequalification clause lists prequalified joint details: square, bevel, V, J and U grooves, flare grooves and fillets, with their root openings, root faces, bevel angles and tolerances, for the processes it accepts, together with tables of matching filler metals and minimum preheat. A WPS that keeps every variable within those limits needs no test; Elena writes it, signs it, and keeps it available to the inspector and the engineer. Most of Hartwell's structural WPSs are of this kind.

Prequalification is a privilege the code grants for well-proven combinations, and its limits are firm. Every variable must be within them: the process, the transfer mode, the filler classification and size, the steel group, the joint detail and its tolerances, the position, the preheat, the bead width and thickness of each layer, and the heat input where it is limited. A procedure that moves outside any one of them, such as a wider root opening than the figure allows, a filler that is not in the matching table, or a process the code does not prequalify, must be qualified by test.

Qualifying a procedure by test

A procedure qualification test is planned like an experiment. Elena writes a preliminary procedure with the values she intends to use. A welder welds a test coupon of the right material, thickness and joint, in the right position, while someone records the actual values: current, voltage, travel speed, wire feed, gas, preheat, interpass temperature and the sequence of passes. The coupon is then examined and tested as the code requires: visually, and often by radiography or ultrasonics, and then cut into specimens for tensile tests, bend tests, macro-etch sections, and impact or hardness tests if the job needs them. If every test passes, the recorded values become the procedure test record, and the production procedure is written with ranges that the code allows the test to support.

Under D1.1 the contractor qualifies its own procedures; the test may be witnessed by an inspector, and the results are recorded on the PQR and certified by the contractor. Elena did this for the Riverside flange splices, where D1.5 required qualification by test with Charpy specimens. The first plate failed for a slag line; the second passed every test, and the WPS was written with production ranges drawn from it.

Section IX works the same way for pressure work, with its own test requirements. A groove weld procedure test on plate or pipe needs two tensile specimens and four bend specimens, face and root bends for thinner coupons and side bends for thicker ones, plus any toughness testing the construction code demands. The PQR lists every essential variable as it was in the test, and the WPS that refers to it may use the ranges Section IX allows for each variable.

Essential variables and ranges

Not every number on a procedure needs a test behind it. Each code classifies variables by how much they can change the weld's properties. A change in the base metal group, the filler metal classification, the process, a big change in preheat, or, when impact tests are required, a big increase in heat input, can change strength or toughness, so it needs a new test. Changes in things like the cleaning method or the electrode angle generally do not. Structural codes list the variables whose change requires requalification and the amount of change allowed; Section IX lists essential, supplementary essential and nonessential variables for each process.

Ranges come from those rules. If the test was welded at a certain current, the code may allow production within a band around it. If the test coupon was a certain thickness, the procedure covers the thickness range in the table. If the test was welded with preheat, production preheat may not be lower than the tested value by more than the allowed margin. Elena's job, when she writes the production procedure, is to put on it ranges that are both useful to the welder and inside what the test supports.

# Check the ranges on a production procedure against what its test record supports
# (illustrative rules of the kind the codes use; Elena takes the real ones from the edition in force)
pqr = {'process': 'GTAW+SMAW', 'p_no': 8, 'f_no_root': 6, 'f_no_fill': 5,
       'thickness': 0.5, 'preheat_min': 50, 'interpass_max': 350, 'heat_input': 40.6}
wps = {'process': 'GTAW+SMAW', 'p_no': 8, 'f_no_root': 6, 'f_no_fill': 5,
       'thick_range': (0.1875, 1.0), 'preheat_min': 50, 'interpass_max': 350, 'heat_input_max': 45.7}

checks = []
checks.append(('process', wps['process'] == pqr['process']))
checks.append(('base metal group', wps['p_no'] == pqr['p_no']))
checks.append(('filler groups', (wps['f_no_root'], wps['f_no_fill']) == (pqr['f_no_root'], pqr['f_no_fill'])))
lo, hi = wps['thick_range']
checks.append(('thickness range', lo >= 0.1875 and hi <= 2 * pqr['thickness']))
checks.append(('preheat not below test minus 100', wps['preheat_min'] >= pqr['preheat_min'] - 100))
checks.append(('interpass not above test plus 100', wps['interpass_max'] <= pqr['interpass_max'] + 100))
checks.append(('heat input not above test (impact tested)', wps['heat_input_max'] <= pqr['heat_input']))
for name, ok in checks:
    print(f"{name:<44} {'supported' if ok else 'NOT SUPPORTED'}")
print('procedure OK' if all(ok for _, ok in checks) else 'revise the procedure or run a new test')

process                                      supported
base metal group                             supported
filler groups                                supported
thickness range                              supported
preheat not below test minus 100             supported
interpass not above test plus 100            supported
heat input not above test (impact tested)    NOT SUPPORTED
revise the procedure or run a new test

Devon's check on the Coldwater flange weld finds one problem. The production procedure allows a maximum heat input of 45.7 kJ/in, but the test record shows 40.6 kJ/in, and because the line is impact tested, production heat input may not exceed the tested value. Elena revises the procedure to the tested value and issues the new revision; she does not touch the test record. Lucia removes the old revision from the booth the same afternoon.

Reading a procedure sheet

A procedure sheet has a standard layout, and Devon learns to read it from top to bottom. The heading gives the procedure number, revision, date and the documents that support it. Then come the joint: type, sketch, groove angle, root opening, root face, backing, and the passes. The base metals and thickness range. The filler: classification, size and any supplementary requirements. Position and progression. Preheat and interpass limits. The electrical characteristics: current type and polarity, and the ranges for amperage, voltage and travel speed for each pass and size. Technique: stringer or weave, cleaning between passes, back-gouging, peening. Post-weld heat treatment if any. Signatures and the approval or acceptance stamp.

Two habits help at the booth. Read the whole sheet before striking the first arc, not just the amperage. And check the revision against the controlled copy list in the office; a procedure that has been superseded may still be taped to a wall somewhere. Lucia's inspection rounds include checking that the revision at each booth is current, and she pulls any copy that is not.

Controlling procedures

Hartwell's quality manual describes how procedures are controlled. Elena drafts and signs each WPS and its PQRs; the engineer of record may review them, and on the bridge job the owner's inspector did. Controlled copies go to the shop with a revision number and a list of where they are posted. When a WPS is revised, the new revision is approved before use and the old copies are withdrawn and marked superseded. The AISC auditor checks this system on each visit, comparing the procedures in use with the approved list.

Procedures also link to everything else in the quality system. Each welder's qualification must cover the procedure's process, position and ranges. Each filler on the procedure must be in stock with its certificates. Each weld on a weld map or inspection record names the procedure used. If a procedure is revised, the inspector checks whether any work in progress was made under the old revision and whether that matters.

Standard procedures and procedures from others

Some procedures come from outside the shop. Standard welding procedure specifications, published by AWS and accepted by ASME for Section IX work within their limits, let a shop use a procedure backed by tests done by others, provided it follows the standard procedure exactly and meets the conditions of use. Pipeline companies issue their own procedures to contractors. A customer may supply procedures for its own equipment. In each case the shop must check that the procedure is valid for the work, that it is the current revision, and that its welders are qualified for it, and the quality manual says how such procedures are accepted into the shop's system.

Procedures on the shop floor

A procedure is only useful if it reaches the person holding the torch. Hartwell posts a laminated controlled copy at each booth for the joints welded there, and the travelers that go with each assembly name the procedure for every weld. Before a welder starts a new joint, the foreman or the inspector checks that the welder has the right sheet and has read it. During welding, Lucia's rounds include a quick comparison: the current on the clamp meter, the voltage on the machine, the filler on the rod oven label and the preheat crayon mark, each against the sheet. If a value is outside the range, the welder stops, the cause is found, and any welds made outside the range go on the nonconformance log for review.

Devon notices that the best welders treat the procedure as a tool, not a nuisance. Marta reads every new sheet twice and asks Elena about anything that looks odd, and twice in Devon's time she has found a typing error, once a current range that did not match the electrode size, once a preheat written in the wrong units. Both were fixed by a revision the same day. A procedure that nobody reads cannot be corrected, and a welder who reads it carefully is part of the shop's quality system.