A Welding Procedure Specification – called a WPS – is the written set of instructions that tells you exactly how to weld a specific joint. It covers everything from the base metal and filler metal to the welding position, amperage range, travel speed, and preheat requirements. In structural, pressure, and fabrication work, the WPS is the standard that makes every weld repeatable and verifiable.
This guide teaches how to read a WPS. It does not teach how to write, approve, qualify, or override one. If you are preparing for a certification test or working to a code, this article will help you understand what each section means and what to verify before you weld. If you need to modify a WPS, consult the responsible engineer or CWI.
For safety-critical welds – structural, pressure, lifting, or code-regulated applications – always follow the approved WPS and qualified supervision. Never change a WPS parameter without approved qualification.
What Is a WPS and Why Does It Matter?
A WPS documents the welding parameters that were tested and qualified so a welder can reproduce a sound weld consistently. It:
- Standardizes welding conditions across different welders, shifts, and job sites.
- Provides traceability – if a weld is questioned, the WPS shows what parameters were used.
- Is used in many code-regulated welding applications, including AWS D1.1 (structural steel), ASME Section IX (pressure vessels), and API 1104 (pipelines).
A WPS is backed by a Procedure Qualification Record (PQR) that documents the test results. Together they form the technical foundation for welding that is aligned with the applicable WPS, procedure, and code requirements.
Key Sections of a WPS
Every WPS follows the same basic structure, though the exact format varies by code and company. Here are the major sections and what each tells you.
| Section | What It Tells You | Why It Matters |
|---|---|---|
| Process | Which welding process qualifies (SMAW, GMAW, FCAW, GTAW, etc.) | Each process has different variables – welding to the wrong process voids the WPS |
| Base Metal | Material specification (e.g., ASTM A36) and thickness range qualified | Welding outside the qualified thickness is not covered by the WPS |
| Filler Metal | AWS classification and diameter of the electrode or wire | Changing filler metal can alter mechanical properties and weld soundness |
| Position | Welding positions the WPS covers (1G, 2G, 3G, 4G, etc.) | A WPS qualified in flat position may not cover vertical or overhead |
| Joint Design | Groove type, bevel angle, root opening, backing, tolerances | Incorrect joint prep can prevent full penetration or cause slag problems |
| Preheat / Interpass | Minimum preheat temperature and maximum interpass temperature | Wrong preheat can cause cracking – especially on thicker or high-carbon materials |
| Shielding Gas | Gas composition, flow rate range, and backing gas if used | Wrong gas or flow can cause porosity and poor mechanical properties |
| Electrical Characteristics | Current type, polarity, amperage and voltage ranges | Set your machine within the qualified range – going outside the range voids the WPS |
| Travel Speed | Range in inches per minute (ipm) | Too fast or too slow affects bead profile, penetration, and heat input |
| Technique | Stringer vs. weave, gun or rod angle, oscillation pattern | Technique affects fusion, bead shape, and heat distribution |
| Pass Sequence | Number of passes, bead arrangement, interpass cleaning notes | Multi-pass sequences must be followed for proper fill and strength |
| Qualification Reference | PQR number, applicable code edition, revision date | Links the WPS to the test data that qualified it – essential for traceability |
How to Read a WPS for Your Specific Weld
When you pick up a WPS, verify that it covers your specific weld by checking these items in order:
- Check the process and position – does the WPS cover the process you are using and the position you are welding in?
- Verify the base metal – is your material specification listed? Is your thickness within the qualified range?
- Check the filler metal – does the WPS list the AWS classification you are using?
- Read the electrical parameters – set your machine within the qualified amperage and voltage range.
- Follow the technique and pass sequence – are you using the right travel speed, angle, and bead arrangement?
If any of these do not match, ask your supervisor or CWI before welding. Welding outside the WPS range can void the qualification and may produce an unsafe weld.
Essential vs Non-Essential Variables
Welding codes divide WPS variables into two categories:
- Essential variables are parameters that, if changed outside the qualified range, require re-qualification of the WPS. Common examples include changing the process, base metal group, filler metal classification, or position.
- Non-essential variables can be changed within the WPS range without re-qualification, as long as the change is recorded. Examples may include welding technique (stringer vs. weave) or the brand of welding machine, depending on the code.
The specific classification of essential vs non-essential variables depends on the governing code edition. Even if a variable is non-essential under a particular code, shop procedures, contract requirements, and the approved WPS may still control it. Never assume a variable is non-essential without checking the code requirements. Do not change WPS variables without qualified approval. When in doubt, ask the CWI.
Prequalified vs Qualified WPS
Some codes, such as AWS D1.1, allow certain joint configurations to be used without separate testing when the applicable code conditions are met. These are called prequalified joints. A prequalified WPS still has limits and must be followed exactly. It is not a substitute for following the approved WPS parameters.
A qualified WPS is backed by a PQR that documents actual test welding, mechanical testing, and results. Most codes require that production welding stays within the ranges proven by the PQR.
Both prequalified and qualified WPS require the welder to work within the documented parameters. Neither is a substitute for following the WPS exactly.
Limitations and Safety-Critical Framing
This article teaches how to read a WPS. It does not authorize you to write, modify, or override one. The following points are critical and must be followed:
- For structural, pressure, lifting, code-regulated, or safety-critical work, always follow the approved WPS and consult qualified supervision.
- Never change or override any WPS variable without approval from the responsible engineer or CWI.
- If any part of a WPS is unclear, ask your supervisor or CWI before welding.
- An improper WPS – or working outside its range – can produce unsafe welds.
- Preheat and interpass temperature requirements exist to prevent cracking. Follow them exactly.
- Welding outside qualified parameters can void certification and create liability.
Summary
Reading a WPS is a practical skill that every welder in structural, pressure, or fabrication work should know. Start with the process and position, verify the base metal and filler metal, then set your parameters within the WPS range. If you are not sure about any section, ask before welding.
See the related guides on welding symbols, welding positions, joint types, stick electrode classification, certification test preparation, and MIG vs TIG vs Stick comparison for deeper coverage of each topic.
