ASME Section IX Article III: Welding Data
Article III is the part of ASME Section IX that most working welding engineers and inspectors reach for constantly, even if they could not name it by number: it is the collection of variable tables, QW-250 through QW-290, that define exactly which changes to a welding procedure are essential, supplementary essential, or nonessential for each process. This module continues the training series from Part QG and Articles I and II, moving from the general framework and procedure qualification rules into the process-specific detail that actually governs whether a WPS needs requalification after a change.
By the end of this module you should be able to explain what distinguishes an essential variable from a supplementary essential or nonessential variable, locate the correct QW-25X table for a given process, and recognize the variables most commonly missed during WPS review. A short timed quiz at the end lets you check your understanding before moving to Article IV.
Where this fits: Part QG covers general requirements across the whole code. Article I sets welding general requirements. Article II establishes the procedure qualification framework and the QW-200 series rules. Article III, covered here, supplies the actual variable tables that Article II’s rules reference for each specific process.
Scope of Article III
Article III does not state welding rules on its own; instead it supplies the detailed variable tables that Article II’s qualification framework points to. Each welding process covered by Section IX has its own table, identified by a QW-25X number, listing every variable Article II recognizes as relevant to that process and classifying each one as essential, supplementary essential, or nonessential. When you qualify or review a WPS, Article III’s table for that process is the document you actually check line by line.
This structure exists because a variable that matters enormously for one process may be irrelevant for another. Preheat interpass temperature control matters differently for SMAW than it does for GTAW, and shielding gas composition is essential for GMAW and GTAW but does not apply to SMAW at all. Article III’s process-specific tables capture exactly these differences rather than forcing one generic rule set onto every process.
Essential, Supplementary Essential, and Nonessential Variables
Every variable in a QW-25X table falls into one of three categories, and understanding the distinction is the single most important skill this module covers.
Essential Variables
An essential variable is one that, if changed beyond the qualified range, requires the WPS to be requalified with new procedure qualification testing before production welding can rely on it. Essential variables affect the mechanical properties of the weld directly, so a change here is treated as creating an effectively new, unproven procedure.
Supplementary Essential Variables
Supplementary essential variables only apply when notch toughness testing is a requirement of the construction code, typically for applications where impact properties matter, such as low-temperature service. When toughness testing is required, these variables are treated with the same weight as essential variables; when it is not required, they are effectively treated as nonessential.
Nonessential Variables
A nonessential variable can be changed within the WPS without requalification, though the WPS document itself must still be revised to reflect the change. These variables affect workmanship and technique but are not considered to meaningfully change the mechanical properties the procedure qualification already proved.
Structure of a QW-25X Table
Every process table in Article III follows the same basic layout: a row for each variable, a QW paragraph reference number, a short description of the variable, and columns indicating whether it applies to the WPS, the PQR, or both, and which category it falls into. Reading these tables efficiently is a core skill for anyone reviewing a WPS or preparing procedure qualification test plans.
| Column | What It Tells You |
|---|---|
| QW Paragraph | The specific sub-paragraph number defining that variable in detail |
| Brief of Variables | A short description of what is being controlled, such as “T-No” or “Preheat temp. decrease” |
| Applies to WPS | Whether the variable must be recorded and controlled on the WPS document |
| Applies to PQR | Whether the variable must be recorded on the procedure qualification record |
| Essential / Supp. Essential / Nonessential | The classification governing whether a change requires requalification |
Process-Specific Variable Tables
Article III provides a dedicated table for each welding process recognized by Section IX. Knowing which number corresponds to which process saves real time when you are reviewing a WPS package and need to check a specific variable’s classification.
| Table Number | Process |
|---|---|
| QW-252 | Shielded Metal Arc Welding (SMAW) |
| QW-253 | Submerged Arc Welding (SAW) |
| QW-254 | Gas Metal Arc Welding (GMAW) |
| QW-255 | Gas Tungsten Arc Welding (GTAW) |
| QW-256 | Plasma Arc Welding (PAW) |
| QW-257 | Electron Beam Welding (EBW) |
| QW-258 | Laser Beam Welding (LBW) |
| QW-259 | Electroslag Welding (ESW) |
| QW-260 | Flux Cored Arc Welding (FCAW) |
| QW-261 | Stud Welding |
Practical use: When a WPS combines two processes, such as a GTAW root followed by a SMAW fill and cap, both process tables apply to their respective portions of the joint. Review each process’s variables against its own table rather than trying to apply one table across the entire weld.
Variables That Appear Across Most Processes
While each table has process-specific entries, several variable categories recur across nearly every QW-25X table, and reviewers who know these by heart move through WPS reviews far faster.
Base Metal Variables
P-Number and Group Number changes are essential variables across all processes, since they define the base metal chemistry and weldability characteristics the procedure was actually tested on. A change from one P-Number to another, or in some cases between groups within the same P-Number, generally requires requalification.
Filler Metal Variables
F-Number, defining the general filler metal grouping, and in some processes A-Number, defining the deposited weld metal chemical analysis grouping, are essential variables for most arc welding processes. Filler metal diameter is typically nonessential unless it affects a supplementary essential variable like heat input for notch toughness applications.
Position
Welding position is an essential variable for procedure qualification in most processes, since orientation affects heat input, penetration, and cooling rate. A procedure qualified in the flat position does not automatically extend to overhead welding without additional qualification, consistent with the position rules covered in the site’s welding positions guide.
Preheat and PWHT
A decrease in minimum preheat temperature below what was qualified is essential, since it affects cooling rate and the risk of hydrogen cracking. Post-weld heat treatment is essential when it is performed, since PWHT changes the mechanical and metallurgical properties of the completed weld, and this becomes a supplementary essential variable when notch toughness testing applies.
Electrical Characteristics
Current type and polarity (AC, DCEP, DCEN) are essential variables for most arc welding processes, since they directly affect penetration profile and deposition characteristics. A change in amperage or voltage range within the qualified heat input band is typically nonessential unless notch toughness testing makes heat input a supplementary essential variable.
Common review mistake: Treating a variable as nonessential because it “seems minor” without checking the actual table. Heat input, for instance, silently shifts from nonessential to supplementary essential the moment notch toughness testing becomes a code requirement — a distinction reviewers frequently miss.
Practice Quiz: Article III — Welding Data
Test your understanding of Article III with this 10-question timed quiz. You have 20 seconds per question. Your best score is saved automatically so you can track improvement across attempts.
10 questions · 20 seconds per question · Instant feedback after each answer
Next in this series: Article IV covers Performance Qualification, the rules governing welder and welding operator testing, which builds directly on the variable framework established here in Article III.