WPS, PQR & Welder Qualification: ASME Section IX Explained

Quick Answer

Three documents control welding on any code-built equipment, and they are frequently confused. A WPS (Welding Procedure Specification) is the instruction — it tells the welder how to make the weld: process, material, filler, joint design, position, preheat, interpass temperature, electrical parameters, and post-weld heat treatment. A PQR (Procedure Qualification Record) is the evidence — it records the actual parameters used on a test coupon and the mechanical test results (tensile, bend, impact, hardness) that prove the procedure produces a sound joint. One or more PQRs support a WPS; the WPS cannot exist without qualified backing. A WPQ (Welder Performance Qualification) proves that a specific welder can execute that procedure, tested by mechanical or radiographic examination of their coupon. Under ASME Section IX, variables are classified as essential (changing them requires requalification — a new PQR), non-essential (can be changed by revising the WPS without requalification), and supplementary essential (become essential when impact testing is required by the construction code). Materials are grouped by P-number (base metals) and filler metals by F-number and A-number, which is what allows one qualification to cover a range of materials rather than requiring a test for every grade. In practice, the most common audit findings are a WPS with no supporting PQR, welders working outside their qualified range, and expired continuity records.

Every weld on a pressure vessel, a boiler, a storage tank, or a piping system is made by a person following an instruction, and the code's position is that both the instruction and the person must be proven before the weld is made. Not inspected afterwards — proven beforehand. That is the entire logic of welding qualification.

It exists because welding is the most variable operation in fabrication. The same joint, same material, and same filler can produce sound metal or a brittle crack depending on heat input, preheat, interpass temperature, electrode condition, and technique. Radiography and ultrasonic testing catch many defects but not all, and they cannot detect a microstructure that is wrong but geometrically sound — the exact failure mode in Cr-Mo steels and duplex stainless. Qualification is the control that addresses what NDE cannot see.

For buyers, this matters commercially as well as technically. Welding documentation is the most common source of audit findings and delivery delays on fabricated equipment. A vessel arriving with a WPS that has no supporting PQR, or welders whose continuity has lapsed, is a vessel that cannot be accepted until the paperwork is rebuilt — which sometimes means re-welding.

For QA/QC engineers, project engineers, inspectors, and procurement teams buying fabricated equipment — this guide explains WPS, PQR, and welder qualification under ASME Section IX: what each document does, how variables work, the qualification process, and what to check.

For the equipment these documents accompany, see Heat Exchangers and API 650 Storage Tanks.

The Three Documents

WPS — Welding Procedure Specification

The instruction. A written document giving direction to the welder for making production welds in accordance with code requirements.

A WPS states the ranges permitted for:

• Welding process — SMAW, GTAW, GMAW, FCAW, SAW
• Base metal — P-number group and thickness range
• Filler metal — specification, classification, F-number, A-number
• Joint design — groove type, backing, root gap
• Position — and progression for vertical welding
• Preheat — minimum temperature and maintenance
• Interpass temperature — maximum
• Post-weld heat treatment — temperature, holding time, heating and cooling rates
• Gas — shielding, backing, trailing; composition and flow rate
• Electrical characteristics — current, polarity, amperage and voltage ranges, travel speed, heat input where controlled
• Technique — stringer or weave, cleaning, back gouging, multi-pass or single

The WPS gives ranges, because it must cover production variation. Those ranges are limited by what the supporting PQR qualified.

PQR — Procedure Qualification Record

The evidence. A record of the actual values used when welding a test coupon, and the test results obtained from it.

A PQR records:

• The actual welding parameters used (single values, not ranges)
• Tensile test results — the weld must meet or exceed the base metal's specified minimum tensile strength
• Bend test results — face, root, or side bends demonstrating ductility and soundness
• Impact test (Charpy) results where required, with test temperature
• Hardness survey where required — mandatory for sour service and often for Cr-Mo
• Macro examination where required
• Chemical analysis of weld deposit where required

The relationship: one or more PQRs support a WPS. The WPS's permitted ranges are derived from the PQR's actual values by applying the rules in Section IX. A WPS with no supporting PQR has no validity whatsoever.

WPQ — Welder Performance Qualification

The person. Proof that an individual welder can deposit sound weld metal following a qualified WPS.

The welder makes a test coupon, which is examined by bend testing or radiography. The resulting WPQ certificate states the ranges the welder is qualified for:

• Process
• P-number range of base metals
• F-number range of filler metals
• Thickness range
• Position — and this is where welders are most often working outside their range
• With or without backing
• Pipe diameter range

Continuity: a welder's qualification lapses if the process is not used for a period defined by the code (commonly six months). Continuity records must be maintained and current. An expired welder qualification invalidates the welds made after expiry.

Essential, Non-Essential and Supplementary Essential Variables

This classification is the heart of Section IX and the source of most confusion.

Essential Variables

Changes that affect the mechanical properties of the weld. Changing an essential variable requires requalification — a new PQR.

Typical essential variables include:

• Change of welding process
• Change to a different P-number group
• Change of F-number or A-number of filler
• Significant change in thickness beyond the qualified range
• Deletion or significant reduction of preheat
• Addition, deletion, or change of PWHT
• Change from one transfer mode to another in GMAW

Non-Essential Variables

Changes that do not affect mechanical properties. They can be changed by revising the WPS — no requalification, no new PQR.

Typical non-essential variables include:

• Joint groove design details
• Method of back gouging
• Cleaning method
• Stringer vs weave technique (in many cases)
• Single vs multiple electrodes

Supplementary Essential Variables

Variables that become essential only when the construction code requires impact (toughness) testing — typically for low-temperature service.

Typical supplementary essential variables include:

• Position (which is otherwise non-essential for procedure qualification)
• Heat input limits
• Change in PWHT temperature or time beyond ranges
• Vertical progression (uphill vs downhill)

The practical consequence: a procedure perfectly acceptable for ambient-temperature service may require full requalification for low-temperature service simply because impact testing brings supplementary essential variables into play. This catches projects out when a line class is upgraded late in design.

P-Numbers, F-Numbers and A-Numbers

Qualification would be impossibly expensive if every material grade required its own test. Section IX solves this with grouping.

P-numbers group base metals with similar weldability, mechanical properties, and composition. Qualifying on one material in a P-number group generally qualifies others in the same group (with some restrictions and Group-number sub-divisions for impact-tested applications).

Broad examples:

• P-No. 1 — carbon steels (A106, A516, A105 and similar)
• P-No. 4 / 5A / 5B — Cr-Mo alloy steels, including A335 P11, P22, P91 — see Alloy Steel Pipe A335
• P-No. 8 — austenitic stainless steels (304, 316 and similar)
• P-No. 10H — duplex stainless steels — see Duplex & Super Duplex Pipe
• P-No. 4x — nickel alloys

F-numbers group filler metals by usability characteristics — how they run and the technique required. This governs welder qualification range.

A-numbers classify the chemical composition of the deposited weld metal, ensuring the deposit chemistry matches the intended service.

Practical point: a welder qualified on P-No. 1 carbon steel is not automatically qualified on P-No. 8 stainless or P-No. 10H duplex. Fabricators working across material families need separate qualifications.

The Qualification Process

1. Define the requirement. What material, thickness range, positions, and service conditions must be covered? Does the construction code require impact testing? Is sour service or a specific hardness limit applicable?

2. Draft a preliminary WPS (pWPS). The intended parameters, before qualification.

3. Weld the test coupon under the pWPS, recording actual values — not the intended ranges. An independent inspector may witness this, and for Type 3.2 style requirements usually does.

4. Test the coupon per Section IX and any additional code or specification requirements: tensile, bend, impact, hardness, macro.

5. Record results on the PQR. If tests pass, the PQR is certified. If they fail, adjust and repeat.

6. Write the WPS citing the supporting PQR(s), with ranges derived by applying Section IX rules to the qualified values.

7. Qualify welders to the WPS with individual test coupons, issuing WPQ certificates stating each welder's qualified ranges.

8. Maintain continuity records so qualifications remain current.

Additional Requirements Beyond Section IX

Section IX is the baseline. Construction codes and project specifications add to it.

ASME Section VIII / B31.3 / API 650 each impose their own requirements on which procedures are acceptable, PWHT, and NDE extent.

NACE MR0175 sour service requires hardness surveys across the weld and heat-affected zone, with the 22 HRC / 250 HV10 limit applying to weld and HAZ as well as parent metal. A procedure qualified without hardness testing is not qualified for sour service. See NACE MR0175.

Duplex stainless requires ferrite balance control — filler selection (ER2209/ER2594), nitrogen in the shielding gas, and interpass temperature limits, with ferrite measurement on the qualification coupon.

Cr-Mo steels (P91 in particular) require control of delta ferrite in the weld metal, tight preheat and interpass control, and mandatory PWHT. Weld failures in Cr-Mo equipment are common enough that these procedures deserve specific scrutiny.

Low-temperature service brings impact testing and therefore supplementary essential variables.

EN/ISO equivalents: ISO 15614 (procedure qualification) and ISO 9606 (welder qualification) are the European counterparts, used for PED and EN-code work. A fabricator working internationally often holds both. The concepts are parallel but the variables and ranges differ — they are not interchangeable, and a project specifying ASME cannot be satisfied with ISO 15614 documentation without explicit agreement.

What Buyers Should Check

When receiving fabricated equipment, the welding documentation package should allow you to trace every production weld to a qualified procedure and a qualified welder.

Check:

1. Every WPS cites a supporting PQR, and that PQR is actually included in the package
2. The PQR test results pass — read the tensile, bend, and where applicable impact and hardness values, do not assume
3. The production material falls within the WPS's qualified P-number and thickness range
4. The welding position used in production is within the WPS range — and where impact testing applies, position is supplementary essential
5. Every welder has a current WPQ covering the process, P-number, F-number, thickness, and position used
6. Continuity records are current — no lapsed qualifications
7. A weld map / weld register links each production weld to its WPS, welder, and NDE report
8. PWHT charts are present where required, showing temperature and time achieved
9. Hardness surveys where sour service or Cr-Mo applies
10. Ferrite measurements for duplex
11. NDE reports correspond to the welds in the register
12. Revision control — the WPS revision used in production matches what was approved

Common Findings and Mistakes

After 15+ years supplying fabricated equipment and reviewing welding documentation:

Mistake 1: WPS With No Supporting PQR

A WPS is issued and used in production, but no qualification record backs it. The document has no validity.

Prevention: Require the PQR alongside every WPS at document review, before fabrication starts — not at final handover.

Mistake 2: Welder Working Outside Qualified Position

Welder qualified in 1G (flat) welding a 6G (fixed pipe) joint in production. Position is the most common range violation.

Prevention: Check every welder's WPQ position range against the actual production joints, particularly for pipe work.

Mistake 3: Lapsed Continuity

Welder qualifications valid on paper but the welder has not used the process within the code's continuity period.

Prevention: Require current continuity records, dated, as part of the documentation package.

Mistake 4: Procedure Not Qualified for Impact Testing

Existing WPS reused for low-temperature service. Because impact testing is now required, supplementary essential variables apply and the procedure is not qualified.

Prevention: Confirm at design stage whether impact testing applies; requalify with impact testing where it does.

Mistake 5: No Hardness Data for Sour Service

Sour service equipment supplied with procedures qualified without weld and HAZ hardness surveys.

Prevention: Specify NACE MR0175 hardness requirements in the enquiry, and require the qualification to include HAZ hardness.

Mistake 6: Mixing ASME and ISO Documentation

Project specifies ASME Section IX; fabricator supplies ISO 15614 procedure qualifications.

Prevention: State the required qualification standard explicitly and verify at document review. They are not interchangeable without agreement.

Mistake 7: No Weld Map

Documentation package contains WPSs, PQRs, and WPQs but no register linking them to individual production welds.

Prevention: Require a weld map or register identifying, for each weld: joint number, WPS, welder ID, NDE performed, and result.

Mistake 8: Reviewing Documentation at Handover

Welding documentation reviewed only when the equipment is complete. Problems then require re-welding or acceptance concessions.

Prevention: Review WPS/PQR at the start of fabrication as a hold point in the ITP, and audit welder qualifications during production.

Supply from Kasko Makine

Kasko Makine fabricates and supplies code equipment with complete welding documentation for refining, petrochemical, power, chemical, and industrial projects:

Fabricated equipment:

• Pressure vessels and reactors (ASME Section VIII Div 1 with U-stamp, PED)
• Shell and tube heat exchangers (ASME VIII + TEMA)
• Storage tanks (API 650 / API 620)
• Columns, separators, and process vessels
• Piping spools and fabricated assemblies
• Structural steel fabrication

Welding qualification held and supplied:

• WPS and supporting PQRs per ASME Section IX
• ISO 15614 procedure qualification for EN/PED projects
• WPQ welder qualifications with current continuity records
• Procedures qualified for carbon steel, stainless, duplex and super duplex (with ferrite control), Cr-Mo grades including P11, P22 and P91 (with delta ferrite and PWHT control), and nickel alloys
• Procedures qualified with impact testing for low-temperature service
• Procedures qualified with hardness surveys for NACE MR0175 sour service

Documentation package supplied:

• WPS, PQR, and WPQ certificates with continuity
• Weld map / weld register linking every production weld to procedure, welder, and NDE
• PWHT charts with temperature and time records
• NDE reports (RT, UT, PT, MT) with acceptance
• Hardness survey results where applicable
• Ferrite measurements for duplex
• EN 10204 Type 3.1 / 3.2 material certificates — see Material Test Certificates
• ASME U-1 data reports
• Hydrostatic test certificates
• Third-party inspection reports (Bureau Veritas, SGS, TÜV, Lloyd's)

Quality process: documentation submitted for client review at the start of fabrication as an ITP hold point, not at handover — so procedure or qualification issues are resolved before welding rather than after.

Logistics: Fabricated equipment shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Delivery by scope; documentation dossiers supplied with shipment and electronically in advance.

Need fabricated equipment with full welding documentation? Send us your equipment specification, applicable construction code (ASME VIII, API 650, B31.3, PED), materials, service conditions including any low-temperature or sour service requirements, and your documentation and inspection requirements to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll confirm the qualifications held, identify any requalification needed for your materials or service, and provide a quotation within 72 hours.

Continue Reading: Fabrication & QA Guides

• TEMA Data Sheets — Specifying a heat exchanger for fabrication
• API 650 Storage Tanks — Tank welding and testing
• Material Test Certificates — Material certification
• NACE MR0175 Sour Service — Weld and HAZ hardness requirements
• Duplex & Super Duplex Pipe — Ferrite control in welding
• Alloy Steel Pipe A335 — Cr-Mo welding and PWHT

Frequently Asked Questions

Q: What is the difference between a WPS and a PQR?
A: A WPS (Welding Procedure Specification) is the instruction given to the welder for making production welds — it states the permitted ranges for process, base material P-number and thickness, filler metal, joint design, position, preheat, interpass temperature, post-weld heat treatment, shielding gas, and electrical parameters. A PQR (Procedure Qualification Record) is the evidence supporting it — a record of the actual single values used when welding a test coupon, together with the mechanical test results obtained from that coupon, including tensile, bend, and where required impact and hardness testing. One or more PQRs support a WPS, and the WPS's permitted ranges are derived from the PQR's qualified values by applying the rules of ASME Section IX. A WPS with no supporting PQR has no validity.

Q: What are essential variables in ASME Section IX?
A: Essential variables are those changes that affect the mechanical properties of a weld, and changing one requires requalification — a new PQR. Typical essential variables include changing the welding process, moving to a different base metal P-number group, changing the filler metal F-number or A-number, welding outside the qualified thickness range, deleting or significantly reducing preheat, and adding, deleting or changing post-weld heat treatment. Non-essential variables do not affect mechanical properties and can be changed by simply revising the WPS without requalification — examples include groove design details, back gouging method, and cleaning technique. Supplementary essential variables, such as position and heat input, become essential only when the construction code requires impact toughness testing.

Q: What is a welder qualification (WPQ)?
A: A WPQ (Welder Performance Qualification) demonstrates that an individual welder is capable of depositing sound weld metal while following a qualified WPS. The welder produces a test coupon which is examined by bend testing or radiography, and the resulting certificate states the ranges that welder is qualified to work within: welding process, base metal P-number range, filler metal F-number range, material thickness range, welding position, whether backing is used, and pipe diameter range where applicable. Welder qualifications also require continuity — if the welder does not use the process within a period defined by the code, commonly six months, the qualification lapses and must be renewed. Welds made after a qualification has expired are not acceptable.

Q: What are P-numbers and F-numbers?
A: P-numbers and F-numbers are grouping systems in ASME Section IX that make qualification economically practical by allowing one test to cover a range of materials instead of requiring a separate qualification for every grade. P-numbers group base metals with similar weldability, mechanical properties, and composition — for example P-No. 1 covers carbon steels, P-No. 8 covers austenitic stainless steels, P-No. 10H covers duplex stainless, and the P-No. 4 and 5 groups cover Cr-Mo alloy steels. F-numbers group filler metals by their usability characteristics, governing welder qualification ranges. A-numbers classify the chemical composition of the deposited weld metal. A welder qualified on carbon steel is not automatically qualified on stainless or duplex.

Q: What extra welding requirements apply to sour service?
A: NACE MR0175 / ISO 15156 requires that the maximum hardness limit of 22 HRC (250 HV10) for carbon and low-alloy steels applies not only to the parent metal but also to the weld metal and the heat-affected zone. The heat-affected zone is frequently the hardest region in a fabricated component and is where sulphide stress cracking failures concentrate. Consequently, welding procedures for sour service must be qualified with hardness surveys across the weld and HAZ, and production hardness testing may also be specified. A procedure qualified without hardness testing is not qualified for sour service, regardless of whether the base material meets the limit. Cold work restrictions and stress relief requirements may also apply.

Q: Are ASME Section IX and ISO 15614 interchangeable?
A: No. ASME Section IX governs welding procedure and performance qualification for ASME code construction, while ISO 15614 (procedure qualification) and ISO 9606 (welder qualification) are the European counterparts used for PED and EN-code work. The underlying concepts are parallel — a written procedure supported by a qualification record, plus individual welder qualification — but the variables, their classification, and the ranges each qualification covers differ between the systems. A project that specifies ASME Section IX cannot be satisfied by ISO 15614 documentation without explicit agreement between the parties. Fabricators working internationally often hold qualifications under both systems, and the required standard should be stated explicitly in the enquiry and verified at document review.