A WPS is the instruction sheet that tells a welder exactly how to make a weld. It specifies the process, the filler metal, the base metals, the electrical settings, the preheat, the position — every variable that affects the finished joint. If a building inspector or quality auditor asks "how was this weld made?" — the WPS is the answer.
Why Does a WPS Matter?
Every structural, pressure vessel, or code-governed weld needs a qualified procedure behind it. This is not optional — AWS D1.1, ASME Section IX, and API 1104 all require it.
Without a valid WPS:
- Your welds aren't code-compliant, period
- You'll fail an audit before the inspector looks at a single weld
- You're personally liable if something fails and someone gets hurt
A railing, a stairway, a pressure line — if the procedure behind the weld is wrong, everything downstream is wrong. This is why welding audits start with paperwork, not weld inspection.
What's in a WPS?
A WPS covers every essential variable defined by the governing code. Most come straight from the code tables — AWS D1.1 Table 4.5 for structural work, or ASME IX QW-250 for pressure work.
Joint Design
The joint type, groove geometry, and backing configuration. This includes butt, T, corner, and lap joints; groove types (V, bevel, J, U); groove angle; root opening; and root face dimensions. Under ASME IX, joint design is a nonessential variable — you can change it without requalifying. Under AWS D1.1, prequalified joints must match the configurations in Clause 3.
Base Metals
The material you're welding, listed by specification and grouped by P-Number (ASME) or Group Number (AWS). For example, ASTM A36 is P-Number 1, Group 1 under ASME IX. SA-516 Gr.70 is also P-1 but Group 2. If you're welding two different metals, both go on the WPS. Changing the P-Number is an essential variable — it triggers a new PQR.
Filler Metals
The rod, wire, or electrode classified by AWS designation and grouped by F-Number and A-Number. E7018 is F-4, A-1. ER70S-6 is F-6, A-1. The SFA specification number and electrode diameter are also listed. Changing the F-Number is an essential variable under ASME IX.
Welding Process
SMAW (stick), GMAW (MIG), FCAW (flux-core), GTAW (TIG), SAW (sub-arc), or a combination. A combo WPS might use GTAW for the root pass and FCAW for fill and cap. Changing the process always requires a new PQR.
Electrical Parameters
Amps, volts, wire feed speed, travel speed, polarity (DCEP, DCEN, AC), and transfer mode for GMAW (short-circuit, spray, pulsed). These are listed per pass — root, fill, and cap often have different settings. For example, a typical FCAW fill pass might run 240-280 amps, 26-29 volts, DCEP.
Position
The welding position qualified by the procedure. 1G through 6G for groove welds, 1F through 5F for fillets. The position on the WPS must fall within what the supporting PQR qualifies — a test in 3G qualifies 1G and 3G under most codes.
Preheat, Interpass, and PWHT
Minimum preheat temperature and maximum interpass temperature in degrees F or C. For thicker carbon steel (over 1" A572 Gr.50), AWS D1.1 Table 3.3 typically requires 225 F minimum preheat. PWHT, if required, specifies temperature range, hold time, and heating/cooling rates.
Shielding Gas and Technique
Gas type and flow rate — 75/25 Ar/CO2 is standard for GMAW, 100% Argon for GTAW, with back purge if applicable. Technique covers stringer vs. weave, single or multi-pass, cleaning method, and contact-tip-to-work distance.
What Is a Prequalified vs. Qualified WPS?
A prequalified WPS is one where the code has already validated the combination of process, joint, and material — no destructive testing required. A qualified WPS requires a PQR to prove it works.
| Prequalified WPS | Qualified WPS | |
|---|---|---|
| Code basis | AWS D1.1 Clause 3 only | AWS D1.1 Clause 4, ASME IX, API 1104 |
| PQR required? | No | Yes |
| Processes allowed | SMAW, GMAW, FCAW, SAW | Any |
| Typical use | Structural steel (A36, A572) | Pressure work, exotic alloys, non-standard joints |
Every variable must stay within the prequalified limits. One variable outside the range — wrong groove angle, wrong preheat — and it's no longer prequalified. See our deep-dive on prequalified vs. qualified WPS for the full breakdown.
What Is a pWPS (Preliminary WPS)?
A pWPS is a draft procedure written before the qualification test. It specifies target values for the test coupon. After the PQR test passes, the pWPS becomes a full WPS with qualified ranges derived from the actual test data.
What Are the Most Common WPS Mistakes?
- Ranges that exceed the PQR — Your WPS can't claim 3/16" to 3/4" thickness if the PQR test coupon was 3/8", which qualifies roughly 3/16" to 3/4" under ASME IX but only the tested thickness under some AWS provisions
- Missing essential variables — Every essential variable from the code must be on the WPS. Missing one means the WPS is incomplete and an auditor will cite it
- No revision control — Using an outdated WPS is a common audit finding. Every revision needs a date and approval signature
- Not matching the actual work — If the welder is running 250 amps and the WPS says 180-220, you have a compliance problem
Frequently Asked Questions
What is the difference between a WPS and a PQR?
A WPS tells the welder what to do — it lists allowable parameter ranges. A PQR is the destructive test record that proves those parameters produce a sound weld. The WPS derives its ranges from the PQR's actual test values. One PQR can support multiple WPSs, and one WPS can reference multiple PQRs. See our full PQR guide for details.
Can you weld without a WPS?
Not on code-governed work. AWS D1.1, ASME Section IX, and API 1104 all require a qualified WPS for every production weld. Welding without one means your work is non-compliant and you will fail an audit. Even shop tack welds on structural members need to follow a qualified procedure.
How many WPSs does a typical shop need?
It depends on the range of work. A structural steel shop doing mostly A36 and A572 with FCAW might need 3-5 WPSs. A pressure vessel shop working across multiple alloys, thickness ranges, and processes could need 20 or more. Every unique combination of essential variables requires its own procedure.
Does a welder need to be qualified to a specific WPS?
A welder needs a WPQ (Welder Performance Qualification) that covers the essential variables of the WPS they're running — process, position, base metal group, and thickness range. The 6-month continuity rule also applies: if a welder hasn't used a process in 6 months, their qualification lapses.
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