What Is a Welding Procedure Specification (WPS)? A Railway Guide

A welding procedure specification (WPS) is the written instruction that tells a welder how to make one kind of production weld: the process, base and filler metals, joint detail, preheat and electrical settings. Under AWS D1.1/D1.1M:2025, a WPS is either prequalified under Clause 5 or qualified by testing under Clause 6.

The point of a WPS is repeatability. Two qualified welders following the same WPS on the same joint should produce welds with the same properties, and an inspector can check the work against a document instead of against someone's habits.

This guide covers what a WPS contains, how one is built and qualified, and which welding code governs which railway structure. It ends with a worked example of reading a prequalified D1.1 WPS. The companion question, how a WPS differs from the test record that supports it, is covered in our WPS vs PQR guide.

What does a welding procedure specification contain?

A welding procedure specification lists every variable the governing code says controls weld quality, with a value or a permitted range for each. The code decides which variables must appear. A D1.1 WPS, an ASME Section IX WPS and an ISO 15609-1 WPS cover much the same ground but use different terms and ranges.

The American Welding Society's terminology standard, AWS A3.0, describes a WPS as a document providing the required welding variables for a specific application "to assure repeatability by properly trained welders and welding operators" (wording as quoted in secondary sources; check the current A3.0).

The checklist below is what a reviewer expects to find on a structural steel WPS. If a line is blank, the welder has to guess, and a guess is not a qualified procedure.

Key takeaway: a WPS is complete when a qualified welder could set up and weld the joint from the document alone.

Which welding code governs a WPS on railway work?

No single code covers "railway welding." The structure being welded decides the code, and the contract names the edition. That's the first question to settle before anyone writes a WPS, because a D1.1 procedure is not automatically acceptable on a bridge, a freight car or a rail joint.

AWS describes the scope of D15.1 this way:

"This specification establishes minimum welding standards for the manufacture and maintenance of railcars, locomotives, and their components, intended for North American railroad service." — American Welding Society, D15.1/D15.1M:2025 product page

The product page says nothing about rail in track, so citing D15.1 for a field rail weld is a mistake. Per the same page, Clauses 4 through 16 cover general railroad welding and Clauses 17 through 22 cover base metals thinner than 1/8 in. (3 mm).

For bridges, the AWS product page states that D1.5 "covers the welding requirements for welded bridges made from carbon and low-alloy constructional steels and designed to AASHTO or AREMA requirements." D1.5 is stricter about procedures than D1.1. AWS forum discussions of past editions report that only SMAW procedures with listed electrodes are prequalified under D1.5, and that FCAW and SAW procedures must be tested; confirm this against the D1.5 edition in your contract.

Key takeaway: identify the structure, then the code, then the edition, before drafting a WPS.

What is a prequalified WPS under AWS D1.1 Clause 5?

A prequalified WPS is one that D1.1 exempts from procedure qualification testing because every variable sits inside limits the code has already proven. It is not exempt from being written down. A study guide to the 2025 edition puts it plainly: a written WPS is always required, and prequalification only removes the PQR test.

Prequalification is all or nothing. If one variable falls outside Clause 5, such as an unlisted steel, a joint detail not in the figures, or a non-prequalified process, the WPS must be qualified by testing under Clause 6.

The table summarizes the Clause 5 limits as described in a 2025-edition study guide. Table and figure numbers moved between the 2015, 2020 and 2025 editions, so check them against the edition in your contract.

Reading the prequalified preheat table

The minimum preheat depends on a category (set by process and steel group) and the thickness of the thicker part at the point of welding. These are the values published for the 2025 edition by a CWI study guide (Med confidence; CONFIRM against the code):

The same guide states that steel below 32°F (0°C) must be preheated to at least 70°F (20°C) before welding. Quenched and tempered Group IV steels and the A913 grades have their own categories; read them from the code, not from a summary.

Key takeaway: a prequalified WPS saves the cost of a PQR test, but only if every line stays inside Clause 5.

How is a WPS qualified by testing under AWS D1.1 Clause 6?

When a WPS can't be prequalified, the contractor welds a test coupon, records the actual variables on a procedure qualification record (PQR), and tests the coupon under Clause 6, Part B. If the coupon passes, the WPS may be written to the ranges that the code allows from that PQR.

For a CJP groove weld in plate, a 2025-edition study guide describes the sequence as follows:

1. Weld the test plate (at least 15 in. long, with at least 1 in. discarded from each end).
2. Visually examine the full length, then examine the coupon by RT or UT before cutting it.
3. Cut and test six specimens: two reduced-section tension tests plus four bend tests.
4. Add Charpy V-notch tests if the contract specifies toughness.

The thickness ranges are as quoted on the AWS forum from D1.1:2015 and repeated in later study guides. The same forum quote adds that square-groove welds qualified without backgouging are limited to the test thickness. Treat all of these as CONFIRM items for the 2025 edition.

Changing a variable beyond the code's limits means a new PQR. D1.1's Clause 6 tables of essential variables cover items such as process, base metal group, filler classification, shielding gas, position, current type and polarity, and amperage, voltage and travel speed beyond set percentages. When toughness is required, heat input becomes tightly controlled as well. The detailed comparison of what the WPS carries and what the PQR records is in our WPS vs PQR article.

Key takeaway: a tested WPS is only as wide as its PQR, so choose the test plate thickness with the production range in mind.

How do ASME Section IX variables differ from AWS D1.1?

ASME Section IX sorts every welding variable into three classes, essential, supplementary essential and nonessential, and lists them per process in the QW-250 tables (QW-253 for SMAW, for example). D1.1 doesn't use that three-way vocabulary in the same way, so the terms shouldn't be borrowed from one code into a WPS written to the other.

QW-401 assigns each variable in Article IV to one of these classes for a given process, as referenced in QW-250. Section IX is a qualification code only. Production acceptance for an ASME weld comes from the construction code, such as Section VIII or B31.3.

The 2025 edition of Section IX is current. Walter Sperko's summary in the AWS Welding Journal (July 2025) notes that its changes "will become mandatory for new qualifications on January 1, 2026." In railway work, Section IX mostly appears in shop systems such as air receivers and pressure piping, not in structural steel or rail.

Where does ISO 15614 fit?

ISO 15614-1 is the international standard for qualifying arc and gas welding procedures for steels (and arc welding of nickel alloys) by a welding procedure test. The WPS document itself follows ISO 15609-1, and ISO 15607 sets the general rules.

The current edition is ISO 15614-1:2017 with Amendment 1:2019; a third edition was at draft (DIS) stage in 2025. TWI notes that the 2017 edition offers two test levels. Level 1 is based on ASME Section IX, and Level 2, the default when no level is specified, is more extensive with narrower ranges. US railway projects rarely call for ISO qualification, but imported rolling stock components and European-built equipment can arrive with ISO-qualified procedures.

How are thermite and flash-butt rail welds qualified?

Rail welds in track are not qualified to AWS D1.1, D1.5 or D15.1. Thermite and flash-butt welds join rail steel, which none of those codes covers. Qualification normally comes from the railroad's or agency's own welding specification, the equipment or kit supplier's procedure, and AREMA recommended practice. AWS publishes a separate document for rail, AWS D15.2/D15.2M:2022, Specification for Joining Railroad Rail and Related Rail Components (4th edition; earlier editions were recommended practices), which has its own essential-variable tables for arc, thermite and flash welds; owners may adopt it in their specifications. The WPS vs PQR guide covers how those rail qualifications are recorded.

The details vary by owner, so treat the following as typical practice to confirm against your contract:

• Flash-butt welds. AREMA's 2022 Manual for Railway Engineering Chapter 4, Part 3 (Joining of Rail) lists a "Specification for the Quality Assurance of Electric-Flash Butt Welding of Rail" (3.10) and a "Specification for Fabrication of Continuous Welded Rail" (3.11). AREMA publishes this material "as a guide to railways in establishing their individual policies and practices", so it applies only where the owner adopts it.
• Thermite welds. The weld kit, mold, preheat time and crucible procedure come from the supplier, and the railroad's specification sets welder training and acceptance. The 2022 Chapter 4 table of contents lists no separate thermite welding specification; some owner specifications still reference an AREMA thermite specification, so check which document your contract cites.
• European parallel. EN 14730-1:2017 approves aluminothermic welding processes from workshop test welds on new rails of the same profile and grade, and states that compliance doesn't by itself prove a process suits specific track and traffic conditions.

Once in service, a cracked or porous rail weld is a "defective weld" under 49 CFR 213.113, with remedial action set by the size of the defect. That rule governs disposition, not how the weld was qualified. Inspection of these welds is covered in our guides to flash-butt weld inspection and aluminothermic weld inspection.

Key takeaway: for rail in track, ask for the owner's welding specification, not a D1.1 WPS.

Worked example: reading a prequalified D1.1 WPS for a CJP groove weld

This example checks whether one prequalified WPS covers a planned repair, and what preheat it requires. The values come from the 2025-edition study guide tables above and should be confirmed against the code edition in the contract.

The job. A transit station's new platform canopy needs a CJP groove butt splice in a built-up column flange, a statically loaded member. One plate is 1-1/4 in. thick and the other is 3/4 in., both ASTM A572 Grade 50. The work is outdoors in winter and the steel reads 40°F. (A cyclically loaded member, such as a crane runway girder, adds backing and inspection rules that this example doesn't cover.)

The WPS on file. It is marked "Prequalified, AWS D1.1/D1.1M:2025". It specifies SMAW with E7018 electrodes, joint designation B-U2a (a CJP single-V butt joint with steel backing), flat and horizontal positions, and the Category B preheat values.

What would change the answer.

• If the thicker flange were 2 in., the row becomes 1-1/2 < T ≤ 2-1/2 in., and the Category B minimum rises to 150°F.
• If the welder switched to E6010 (non-low-hydrogen), the job would leave Category B. Category A covers only Group I steels, so E6010 on A572 Grade 50 has no prequalified preheat row and would need a WPS qualified by testing.
• If the same joint were qualified by test instead, the test plate thickness would set the range. A 3/4 in. PQR qualifies 1/8 in. to 2T = 1-1/2 in., which covers this 1-1/4 in. flange but not a 2 in. one. A 1 in. PQR qualifies 1/8 in. to unlimited.

The answer. The prequalified WPS covers the splice. The welder must preheat to at least 50°F over at least 2-1/2 in. (3 in. is the practical choice) and keep the interpass temperature at or above that minimum. The inspector verifies preheat with a temperature-indicating crayon or contact thermometer before the arc is struck.

How do you write a WPS, step by step?

Start from the code and the joint, not from the welding machine. The order below follows how a reviewer reads a WPS, so writing it this way avoids most rejections.

1. Name the code and edition from the contract, and decide whether the WPS can be prequalified (D1.1 Clause 5) or must be qualified by test.
2. Define the base metals by specification and grade, look up their group, and state the thickness range you need.
3. Choose the process and filler metal from the code's listed combinations for that group, including electrode size and shielding.
4. Select the joint detail by its code designation and copy its root opening, groove angle, root face, backing and backgouging requirements.
5. Set preheat and interpass temperatures from the prequalified table, using the thickness of the thicker part, or from the PQR for a tested WPS.
6. Enter the electrical settings and technique: current, polarity, amperage or wire feed speed, voltage, travel speed, bead type and pass sequence.
7. Cite the basis. Write "prequalified" or list the PQR number(s), and check that every range falls inside what the PQR supports.
8. Sign and control the document. Issue it with a revision number, and make sure the welders who use it are qualified for the process and position.

What should a WPS review check?

A review confirms that the WPS is technically valid under the named code and that the shop will actually follow it. It is a document check against the code, not a simulation or a new test.

Under D1.1, the contractor writes the WPSs and qualifies the welders as part of its own inspection. Verification inspection for the owner audits WPSs, welder records, mill test reports and NDT reports, with results "promptly reported to the owner and contractor to avoid delays in the work". That quote comes from D. Peterson in AWS Inspection Trends, February 2023, describing D1.1:2020 8.1.2.

A practical review checks:

• the code and edition match the contract;
• prequalified WPSs use only listed processes, steels, fillers and joint details;
• tested WPSs reference a PQR whose test thickness, position and variables support every range claimed;
• preheat matches the table row for the thickest part the WPS allows;
• welder qualification records cover the process, position and thickness on the WPS;
• low-hydrogen electrode storage and exposure controls are in place.

Common faults found in review are a WPS copied from another job with the wrong steel group, a thickness range wider than the PQR supports, and GMAW-S labeled "prequalified".

What a WPS can't do

A WPS sets how a weld should be made; it doesn't prove that any particular weld was made that way. Production quality still depends on inspection.

• It doesn't qualify the welder. Welder performance qualification is separate (D1.1 Clause 6, Part C). A good WPS in unqualified hands produces nonconforming welds.
• It doesn't replace inspection. Visual inspection under D1.1 Clause 8, and any RT, UT, MT or PT the contract specifies, still decide acceptance. D1.1 itself doesn't require UT unless the contract does. See our AWS D1.1 weld inspection criteria guide and our overview of weld defect types.
• It doesn't carry across codes. A D1.1 prequalified WPS isn't automatically valid under D1.5, D15.1 or ASME IX.
• It doesn't cover rail in track. Thermite and flash-butt welds follow owner and supplier procedures, not structural welding codes.
• It can't fix bad fit-up or field conditions. Wind on gas-shielded processes, wet electrodes and joints outside tolerance all take the work outside the WPS, even if the document is perfect.

How EMA Industries applies this

EMA Industries supports WPS/PQR work and the inspection that follows it from Southern California, mobilizing nationwide. On structural jobs, EMA's AWS D1.1 structural weld UT, MT and PT confirms that welds made to the WPS meet the contract's acceptance criteria, and its NDT Level III services cover procedure review. For rail in track, EMA's rail weld inspection works to the owner's specification. Contact EMA Industries to discuss a WPS review or weld inspection scope.

FAQ

Is a prequalified WPS still a written document?

Yes. Prequalification under AWS D1.1 Clause 5 removes only the PQR test, not the need for a written WPS. The welder, the inspector and the owner's verification inspector all need the same controlled document to check the work against.

Can one WPS be used on several projects?

A WPS can be reused wherever the code, edition, base metals, thickness, joint details and positions all fall within its ranges. Check each new contract: a different code edition, an owner who requires toughness testing, or a change of steel group can all make a valid WPS unusable on the next job. AWS also publishes Standard Welding Procedure Specifications (SWPSs) backed by AWS B2.1 PQRs; confirm that the governing code and the contract accept one before using it.

Who is responsible for writing and approving a WPS?

Under AWS D1.1 the contractor is responsible for its WPSs and for qualifying its welders. The owner's engineer or verification inspector reviews them, and a contract may require approval before welding starts. A welding engineer or CWI usually prepares the document.

Does AWS D1.1 apply to railroad track welding?

It doesn't. D1.1 covers structural steel made from carbon and low-alloy constructional steels, not rail. Rail welds in track follow the railroad's specification and supplier procedures, railcars and locomotives follow AWS D15.1, and steel railway bridges follow AASHTO/AWS D1.5.

When does a WPS need to be revised or requalified?

Revise the WPS whenever a variable changes, and requalify it with a new PQR when the change crosses the governing code's limits. Examples are moving outside the prequalified limits of Clause 5, or changing an essential variable under ASME IX. Also check the WPS when the contract moves to a new code edition, because table numbers and some limits change.

Sources

• American Welding Society, AWS announces release of D1.1/D1.1M:2025, Structural Welding Code – Steel, April 2025
• American Welding Society, D15.1/D15.1M:2025 Railroad Welding Specification for Cars and Locomotives (product page and scope)
• American Welding Society, AASHTO/AWS D1.5M/D1.5:2025 Bridge Welding Code (product page and scope)
• D. Peterson, contractor and verification inspection under D1.1, AWS Inspection Trends, February 2023
• W. J. Sperko, Summary of Changes to ASME BPVC Section IX, AWS Welding Journal, July 2025
• AWS, Evaluating AWS Standard Welding Procedure Specifications, Inspection Trends
• TWI, What's new in ISO 15614-1:2017?
• AREMA, Manual for Railway Engineering 2022, Chapter 4 table of contents (titles only)
• CEN, EN 14730-1:2017 Aluminothermic welding of rails – Approval of welding processes (scope listing)
• 49 CFR 213.113 Defective rails, on eCFR
• OpenExamPrep, AWS D1.1:2025 Clause 5 prequalification study guide and Clause 6 WPS qualification study guide (secondary; table numbers and values to be confirmed against the code)