How to Prepare a Welding Procedure Specification (WPS)

Introduction

A Welding Procedure Specification (WPS) is one of the most critical documents in any fabrication or construction project. Whether you are working on a pressure vessel, piping system, structural steel, or storage tank, a WPS defines exactly how a weld must be performed — and ensures it meets the required quality and safety standard.

Without a properly prepared and qualified WPS, welding work cannot be accepted on any serious industrial project — including Saudi Aramco, EPC contracts, or third-party inspected fabrication shops.

In this guide, you will learn exactly what a WPS is, what it must contain, and how to prepare one step by step.

What Is a Welding Procedure Specification (WPS)?

A Welding Procedure Specification is a written document that provides direction to the welder (or welding operator) for making a production weld in accordance with Code requirements.

It defines all the variables that control weld quality, including the base metal, filler metal, joint design, preheat, interpass temperature, heat input, and post-weld heat treatment.

A WPS does not stand alone — it must be supported by a Procedure Qualification Record (PQR), which is the test evidence that the procedure produces acceptable mechanical properties.

The governing codes for WPS preparation include:

Step 1 — Identify the Applicable Code and Project Requirements

Before writing a single line on the WPS, you must confirm:

• Which welding code applies (ASME IX, AWS D1.1, ISO 15614, or project specification)
• The applicable material specifications — base metals involved
• Any client or project-specific supplementary requirements such as Aramco SAES, TOTAL GS EP, or similar

This step prevents you from writing a WPS that qualifies under the wrong code or misses a project holdpoint.

Step 2 — Define the Joint Design

The WPS must fully describe the weld joint, including:

Step 3 — Specify the Base Metal (P-Number)

ASME Section IX groups base metals into P-Numbers based on their composition and weldability. You must specify:

• The P-Number and Group Number of the base metal(s)
• The material specification — for example, ASTM A106 Gr. B or ASTM A516 Gr. 70
• The thickness range qualified — not just the coupon thickness, but the qualified range per code

For example, welding a 20mm coupon under ASME IX qualifies from 5mm to 40mm (the 2T rule applies for impact-tested joints).

Step 4 — Select the Welding Process

The WPS must clearly state the welding process or combination of processes:

Many WPS documents cover a combination of processes, such as GTAW root pass followed by SMAW fill and cap — which is very common in piping work.

Step 5 — Specify the Filler Metal (F-Number and A-Number)

ASME Section IX classifies filler metals by F-Number (weldability grouping) and A-Number (deposited weld metal chemistry). You must specify:

• AWS classification of the electrode or wire — for example, E7018 or ER70S-6
• Filler metal size range
• Flux designation when SAW process is used
• Whether the filler is a matching, over-matching, or under-matching consumable relative to the base metal

Step 6 — Define Preheat and Interpass Temperature

Preheat is applied before welding begins, and interpass temperature is the temperature maintained between each weld pass. Both must be stated on the WPS:

Step 7 — Define Welding Parameters (Essential Variables)

This is the core of the WPS — the actual electrical and travel parameters the welder must follow. For each welding pass or pass range, specify:

These parameters are taken directly from the PQR test record and then expressed as a qualified range on the WPS.

Step 8 — Specify Shielding Gas and Flow Rate

For GTAW, GMAW, and FCAW processes, the WPS must document:

• Shielding gas composition — for example, 100% Argon, or 75% Ar / 25% CO₂
• Gas flow rate — typically expressed in litres per minute (LPM)
• Backing gas (purge gas) for root pass on pipe — gas type and flow rate

Step 9 — Define Post-Weld Heat Treatment (PWHT)

Not all welds require PWHT, but when it is required by code or project specification, the WPS must include:

ASME Section IX mandatory essential variables require the WPS to be requalified if PWHT conditions change beyond allowed limits.

Step 10 — Complete the WPS Format and Reference the PQR

Once all variables are defined, the WPS document must be formally prepared and include:

• WPS number — unique document identifier
• Revision number and date
• Supporting PQR number(s)
• Applicable code and edition
• Authorized signature — typically the Welding Engineer or QA Manager
• Position(s) qualified — 1G, 2G, 5G, 6G, etc.

Essential Variables vs. Non-Essential Variables

A common mistake when writing a WPS is not understanding the difference between essential and non-essential variables under the applicable code.

Always check the applicable code clause carefully before revising a WPS. A wrong decision here can invalidate months of production welding.

Free Download Sample wps

Common Mistakes to Avoid

Several errors appear repeatedly during WPS preparation and third-party review:

• Specifying a thickness range wider than what the PQR coupon actually qualifies
• Using filler metal not covered by the qualified F-Number
• Not matching the PWHT condition between WPS and PQR
• Missing position qualifications — for example, WPS states 5G but PQR was done in 1G only
• Incorrectly calculating heat input — leading to compliance issues during inspection
• Signing the WPS without verifying PQR test results are acceptable for tensile, bend, and hardness requirements

Conclusion

Preparing a Welding Procedure Specification correctly requires a solid understanding of the applicable welding code, material behavior, and the qualification test results documented in the PQR. A WPS is not just a form to fill — it is a controlled technical document that governs every production weld on your project.

Whether you are a Welding Engineer, QA/QC Inspector, or Piping Supervisor preparing for certification, mastering WPS preparation is an essential skill in the industrial and EPC world.

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