Knowing how to prepare a WPS is one of the most useful skills a welding QA/QC engineer can have. A good WPS is short, clear and fully supported by a PQR. A weak one either confuses the welder or claims ranges the PQR never qualified – and that is exactly what a client auditor looks for.
This guide follows ASME Section IX, which is the most common code for pressure vessels and process piping. If you are new to the terms, read WPS vs PQR vs WPQ first.
- Start from a qualified PQR – the WPS can never go beyond what the PQR supports.
- Fill every variable listed for your process in the ASME IX variable tables.
- Calculate the ranges (thickness, P-number, preheat, heat input) from the PQR, not from habit.
- Review, sign and control revisions like any other quality document.
What you need before you start
- The supporting PQR(s) – with test results and the actual values used to weld the coupon.
- The construction code – for example ASME VIII or ASME B31.3 – for preheat, PWHT and impact test requirements.
- The client and project specifications – they often add limits stricter than the code.
- Material and filler details – base metal specifications, P-numbers, and filler SFA specification and AWS classification.
- A WPS format – ASME IX gives a suggested form, QW-482, but any layout is acceptable if all the required variables are covered.
Step 1: Fill the header
Give the WPS a unique number and revision, the date, the organization name, and the supporting PQR number(s). State the welding process(es) – for example GTAW root and SMAW fill – and whether each is manual, semi-automatic, machine or automatic.
Step 2: Joint design (QW-402)
Describe the joint type, groove design, root opening, root face and backing. A simple sketch helps the welder far more than a paragraph. For most processes, joint design is a non-essential variable, so the WPS can show the range of joints you actually use.
Step 3: Base metals and thickness range (QW-403)
List the base metal P-number and group number, not only the material specification. A PQR on one P-number can qualify other materials according to the rules in QW-424.
Then calculate the qualified thickness range from the PQR coupon using QW-451. Example: a groove coupon 12 mm thick qualifies base metal from 1.5 mm to 24 mm (2T). Do the same for deposited weld metal thickness for each process – a common error is to forget that each process in a combination WPS has its own limit.
Step 4: Filler metals (QW-404)
Record the SFA specification, AWS classification, F-number and A-number for each process, plus electrode or wire size. A change of F-number or A-number is an essential variable, so these must match the PQR or another qualified PQR. Electrode diameter is usually non-essential and can be given as a range.
Step 5: Positions (QW-405)
For procedure qualification, position is generally a non-essential variable, so the WPS can list all positions you need. Show the progression for vertical welding (uphill or downhill) – it becomes a supplementary essential variable when impact testing is required.
Step 6: Preheat and interpass temperature (QW-406)
State the minimum preheat and the maximum interpass temperature. A decrease in preheat of more than 55 °C (100 °F) below the PQR value is an essential variable. When impact tests apply, an increase in maximum interpass temperature of more than 55 °C above the PQR is a supplementary essential variable. Always check the construction code and client specification, which often require a higher minimum preheat than the PQR.
Step 7: Post-weld heat treatment (QW-407)
Specify whether PWHT is required, with temperature range and holding time. PWHT condition is an essential variable – a WPS for PWHT service must be supported by a PQR that was itself heat treated, and vice versa. Take the actual temperature range and holding time from the construction code and client specification, and check them against the PQR.
Step 8: Gas (QW-408)
For GTAW, GMAW and FCAW, give shielding gas composition and flow rate, and backing (purge) gas when required – for example for stainless steel or alloy roots.
Step 9: Electrical characteristics (QW-409)
For each process and each pass type (root, hot pass, fill, cap), record current type and polarity, amperage range, voltage range and travel speed range. Ranges should reflect what welders really use, but they must stay realistic around the PQR values. When impact tests apply, heat input becomes a supplementary essential variable: the WPS heat input must not exceed the PQR value, so calculate it and show it.
Step 10: Technique (QW-410)
Describe stringer or weave bead, method of initial and interpass cleaning, back gouging, single or multiple passes per side, and any peening limits. These are mostly non-essential but they matter a lot on the shop floor.
Step 11: Review, approve and control
- Cross-check every essential variable against the PQR – a simple checklist with "PQR value / WPS range / OK" columns is the fastest way.
- Have the WPS reviewed and signed by the responsible welding engineer or QA/QC lead.
- Submit it for client or third-party approval where the project requires it.
- Add it to the WPS / PQR register and issue controlled copies to the shop floor.
Example: what a WPS range looks like next to its PQR
| Variable | PQR (actual) | WPS (range) |
|---|---|---|
| Process | GTAW root + SMAW fill/cap | GTAW root + SMAW fill/cap |
| Base metal | P-No. 1 Gr. 1, 12 mm | P-No. 1, 1.5 – 24 mm |
| Filler | ER70S-2 (F-6), E7018 (F-4) | ER70S-2 (F-6), E7018 (F-4) |
| Preheat | 15 °C | Min. 10 °C (or per code / spec, if higher) |
| PWHT | None | None |
| Position | 6G uphill | All positions, uphill |
Illustrative values only. Your ranges must be calculated from your own PQR, the current code edition and your client specification.
Common mistakes that cause audit findings
- Thickness or P-number ranges wider than the PQR supports.
- A PWHT WPS supported by a non-PWHT PQR, or the other way round.
- Preheat copied from an old WPS instead of the code and client specification.
- Missing heat input when impact testing is required.
- A WPS revised on the shop floor without review, register update or approval.
Frequently asked questions
Who is responsible for preparing a WPS?
Under ASME Section IX, the organization that does the welding is responsible for its WPSs and the supporting PQRs. A consultant can prepare the document, but the organization must own, sign and control it.
Is form QW-482 mandatory?
No. QW-482 is a suggested format in a non-mandatory appendix. Any format is acceptable as long as it includes all the essential, non-essential and, when required, supplementary essential variables.
Can I use another company's WPS?
Generally no. Under ASME Section IX, a WPS must be supported by a PQR qualified by your own organization. Standard Welding Procedure Specifications listed in the code are a separate, limited exception.
How many PQRs can support one WPS?
One or more. For example, a combination WPS for GTAW root and SMAW fill can be supported by one PQR covering both processes or by separate PQRs.
What is the difference between essential and non-essential variables?
A change in an essential variable requires a new PQR. A change in a non-essential variable only requires a revised WPS. Supplementary essential variables become essential when notch toughness is required.
This guide is a practical summary. Always work to the current edition of ASME Section IX, the construction code, the client specification and your approved quality plan.
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