How to Write a Welding Procedure Specification (WPS): Step-by-Step Tutorial

A Welding Procedure Specification (WPS) is the cornerstone document of any code-compliant welding programme. It translates the metallurgical requirements of the applicable fabrication standard — whether AWS D1.1, ASME Section IX, or ISO 15614-1 — into actionable welding parameters that the welder follows in production. This tutorial walks through every element of a WPS in the correct sequence: from identifying the governing code to completing the mandatory fields, linking to a qualifying PQR, and managing essential variable limits.

1. Understand the Governing Code Before You Write Anything

The first act in writing a WPS is identifying which construction or fabrication code governs the work. The code dictates every subsequent decision: which variables are essential, what tests are required to qualify the procedure, and what format the completed WPS must take.

Once you have identified the governing code, obtain the current edition. Code editions change; an ASME Section IX 2025 edition may have revised essential variable tables compared to 2021. Write and qualify to the edition specified in the purchase order or contract.

2. Identify the Base Metals and Assign P-Numbers / Material Groups

Base metal classification is a foundational step. Each code assigns base metals to material groups that define the scope of qualified procedures:

• ASME Section IX: P-Numbers (and Group Numbers within P-Numbers for toughness-sensitive applications). Welding P-1 to P-1 (carbon steel to carbon steel) qualifies you to weld any combination within P-1 without re-qualifying for each individual alloy within the group.
• AWS D1.1: Base metals are listed in Table 4.9 (approved metals) or require supplemental testing. No P-Number system — qualification is by material specification.
• ISO 15614-1: Material Groups per ISO/TR 15608. Group 1 (carbon steels) is the broadest; Groups 8 (austenitic stainless), 9 (nickel alloys), and 10 (copper alloys) follow their own rules.

For the WPS document header, record the base metal specification (e.g., ASTM A516 Grade 70, BS EN 10028-2 P355GH), the P-Number, and, for ASME applications, the Group Number. Misidentifying the material group is one of the most common qualification errors and requires a complete re-start of the PQR programme.

For further background on how base metal composition governs weldability and HAZ response, see the article on HAZ microstructure in steel welds and the guide to hydrogen-induced cracking.

3. Select the Welding Process and Classify Filler Metals

The welding process (or combination of processes) must be declared in the WPS. Each process listed in a multi-process procedure is subject to its own essential variable requirements. Common production processes and their code designations:

3.1 Filler Metal Classification (F-Numbers and A-Numbers)

Under ASME Section IX, filler metals are classified by F-Number (affecting welder qualification scope) and A-Number (chemical composition of the deposited weld metal, relevant to essential variables for most processes). The F-Number grouping allows one filler metal qualification to cover other fillers in the same group without re-testing. For example, qualifying with an E7018 (F-4) covers all other F-4 electrodes.

AWS and ISO classification strings must be fully recorded in the WPS, e.g., AWS A5.1/A5.1M E7018-1 H4. The complete classification string, not just the trade name, is the code-required entry. Abbreviating to a trade name or brand name alone is non-conforming.

4. Define the Joint Design and Applicable Positions

Joint design parameters in the WPS include: joint type (butt, fillet, corner, T-joint, lap), groove configuration (V, double-V, U, J, bevel), root opening, root face dimension, groove angle, and backing type. These parameters govern access, fusion, and the required number of passes.

4.1 Welding Position

Positions are standardised per code. Under AWS/ASME: 1G/1F (flat), 2G/2F (horizontal), 3G/3F (vertical), 4G/4F (overhead), 5G and 6G for pipe. Qualifying in the most restrictive position (typically 6G for pipe, or 3G + 4G for plate) provides the widest coverage of positions in production. The WPS must list all positions in which production welding will be performed; the PQR must cover those positions.

5. Determine Preheat and Interpass Temperature Requirements

Preheat is applied to slow the cooling rate in the HAZ, reducing the risk of hydrogen-induced cold cracking by allowing hydrogen to diffuse out before the microstructure becomes susceptible martensite. The minimum preheat temperature is calculated from the carbon equivalent (CE) of the base metal.

5.1 Carbon Equivalent Formulae

IIW Carbon Equivalent (CEIIW):
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

(all values in wt%; applicable for CE < 0.65)

Pcm (Ito-Bessyo, for low-carbon HSLA steels):
Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B

Minimum preheat guidance (AWS D1.1, Table 4.5 or equivalent):
CE < 0.40  →  No preheat required (for t ≤ 19 mm)
CE 0.40–0.60 →  50–150 °C (thickness-dependent)
CE > 0.60  →  150 °C minimum; consult specialist calculation

The WPS must specify the minimum preheat temperature and the maximum interpass temperature. For carbon and low-alloy steels, maximum interpass temperature is typically 250–315 °C (to avoid excessive grain growth and property degradation). For austenitic stainless steels, maximum interpass temperature is usually 175 °C (to control sensitisation risk); see the corrosion mechanisms article for sensitisation background.

6. Specify Electrical Parameters and Calculate Heat Input

The electrical parameters — polarity, current type (AC or DC), current range, and voltage range — must be specified for each electrode diameter and/or wire feed speed listed. These directly control the heat input delivered to the joint.

6.1 Heat Input Calculation

Heat Input (H) in kJ/mm:

H = (V × I × 60) / (S × 1000)

Where:
  V = arc voltage (volts)
  I = welding current (amperes)
  S = travel speed (mm/min)

Thermal efficiency factor (when applicable):
H_eff = η × H

  η = 1.00 for SMAW and SAW (conservatively)
  η = 0.80 for GMAW/FCAW
  η = 0.60 for GTAW

Example:
  V = 24 V, I = 180 A, S = 270 mm/min
  H = (24 × 180 × 60) / (270 × 1000) = 259,200 / 270,000 = 0.96 kJ/mm

Heat input determines the width and peak temperature of the heat-affected zone. High heat input coarsens HAZ grains and reduces toughness; in creep-resistant steels such as P91 (9Cr-1Mo-V), excessively high heat input promotes Type IV cracking susceptibility at the fine-grained HAZ. Low heat input increases cooling rate and raises the risk of martensite formation and cold cracking. The WPS must specify minimum and maximum heat input, typically as a range with 25% tolerance on each side of the PQR-qualified value, unless the code specifies otherwise.

7. Specify Post-Weld Heat Treatment (PWHT)

PWHT requirements are typically mandated by the construction code based on material type and section thickness. PWHT changes the mechanical properties and residual stress state of the weldment; it is therefore an essential variable — qualifying with PWHT does not cover without-PWHT conditions, and vice versa.

The WPS must record: heating rate (°C/hr), PWHT temperature range (minimum and maximum), hold time, and cooling rate. For thick sections, temperature uniformity across the weld joint is critical and is typically verified by calibrated thermocouples attached directly to the workpiece.

8. Completing the WPS Document: Mandatory Fields

The following step-by-step sequence covers the mandatory fields for an ASME Section IX – compliant WPS. AWS D1.1 and ISO 15614-1 have analogous requirements with minor format differences.

9. Essential Variables: What Triggers Re-qualification

Understanding essential variables is critical to maintaining a valid WPS. The following table summarises common essential variables for ASME Section IX groove weld qualification (QW-250 to QW-290 series). Always verify against the current edition tables for the specific process.

10. Thickness and Diameter Qualification Ranges

A PQR qualifies the WPS over a range of production base-metal thicknesses and pipe diameters defined by the applicable code. Under ASME Section IX, QW-451.1 (groove welds, tension and bend tests), the following qualification ranges apply:

For pipe diameter qualification under AWS D1.1, qualifying on pipe of nominal diameter ≥ 600 mm (24 in) qualifies all pipe sizes. For ASME Section IX, qualifying on pipe ≥ 73 mm OD covers all pipe diameters in that position.

11. Pre-Qualified WPS Under AWS D1.1

AWS D1.1 Clause 7 provides a pre-qualification route that allows certain joint designs, base metals, filler metals, and process combinations to be used without the expense of PQR testing. This is a significant advantage for structural fabricators working exclusively within D1.1-approved material and joint combinations.

11.1 Conditions for Pre-Qualification

• Base metal must be listed in D1.1 Table 4.9 (pre-approved base metals).
• Filler metal must be an approved consumable per the applicable SFA specification for the process.
• Joint dimensions must comply with the dimensional requirements of Clause 7 (root opening, groove angle, root face tolerances).
• Preheat must be at or above the minimum specified in Table 4.5 of D1.1.
• Heat input limits must remain within the electrode manufacturer's recommendations and the code's pass size limits.

12. Common WPS Non-Conformances and How to Avoid Them

13. Metallurgical Considerations Behind WPS Parameter Selection

Every WPS parameter has a metallurgical rationale. The selection is not arbitrary — it is derived from the thermodynamic and kinetic behaviour of the material system being joined.

13.1 Cooling Rate and HAZ Microstructure

The cooling rate in the HAZ determines the transformation products formed from the austenite created during welding. Rapid cooling (high carbon equivalent, thick section, low preheat, low heat input) produces martensite, which is hard and susceptible to hydrogen cracking. Controlled cooling by preheat and controlled heat input produces tempered martensite, bainite, or ferrite–pearlite mixtures with better toughness. The martensite formation article and the bainite microstructure guide provide detailed background on HAZ transformation products.

13.2 Grain Coarsening in the Coarse-Grained HAZ (CGHAZ)

The region immediately adjacent to the fusion line is heated above approximately 1100 °C during welding and undergoes rapid austenite grain growth. Grain size is strongly influenced by heat input; higher heat input produces a wider and coarser CGHAZ. Grain size governs toughness through the Hall-Petch relationship: coarser grains reduce toughness (lower CVN absorbed energy). For toughness-critical applications, heat input must be bounded above as a supplementary essential variable.

13.3 Solidification and Dilution Effects

The deposited weld metal composition is a mixture of filler metal and melted base metal. The dilution ratio (typically 20–40% for SMAW, up to 70% for SAW) affects the weld metal chemistry and therefore the Fe-C phase diagram position of the weld bead. In dissimilar-metal welds (e.g., carbon steel to stainless steel), dilution can move the weld deposit into an unfavourable composition range (hot-cracking-susceptible, or sensitisation-prone). Filler metal selection compensates for dilution effects by over-alloying relative to the target composition.

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