API 1104 vs AWS D1.1 WPS: what changes for pipeline work

Most welding codes share the same underlying logic — run a test weld, verify mechanical properties, write a procedure bounded by those qualified ranges — but the specifics of AWS D1.1 and API 1104 diverge sharply enough that a structural WPS package will not satisfy a pipeline inspector. Understanding the differences before bidding mixed work prevents expensive surprises.

What each code governs

AWS D1.1 is the structural steel welding code, published by the American Welding Society. It governs weld fabrication and erection of steel structures: buildings, bridges, crane runways, industrial equipment, and offshore platforms. The authority having jurisdiction (AHJ) — an engineer of record, building department, or owner specification — invokes it on a project.

API 1104 is the welding standard for pipelines, published by the American Petroleum Institute. It covers butt welds and fillet welds on pipelines used in the petroleum and natural gas industry: onshore and offshore gathering lines, transmission lines, and distribution systems.

Both codes require written WPSs supported by qualification records and maintained by the contractor, but the paths diverge at almost every detail.

The biggest structural difference: API 1104 has no prequalified route

Under AWS D1.1, a contractor using a prequalified WPS can write a procedure without running a test weld — provided the base metal, filler metal, joint geometry, and process parameters all fall within the prescriptive limits of Clause 5. For common processes on listed structural steels this is a major time and cost advantage, as explored in the PQR vs prequalified cost comparison.

API 1104 has no equivalent provision. Every welding procedure must be qualified by test before production welding begins. For a fabricator crossing into pipeline work for the first time, this means budgeting procedure qualification test welds — and the associated mechanical testing — before the first production joint is made.

Essential variables: different structure, different content

Under D1.1, essential variables for base qualification are listed in Table 6.6 with columns indicating which processes each row applies to. API 1104 structures its essential variable list differently. For pipe butt welds, the key essential variables include:

Outside diameter (OD) range. Pipe diameter is a qualification variable under API 1104. The OD of the test pipe sets a range of production diameters the procedure can support. AWS D1.1 uses plate or section thickness as the dimensional variable; there is no analog for diameter.

Wall thickness range. Similar in concept to D1.1 thickness qualification ranges, but the bands and their calculation differ.

Base metal specification and grade. API 1104 base metal grades follow API 5L pipe specifications — X52, X65, X70, X80, and higher grades — rather than ASTM structural grades. Qualifying on X65 does not automatically cover X80 material.

Direction of welding for vertical joints. Vertical-uphill vs vertical-downhill is an explicit essential variable. Downhill (vertical-down) progression is common in pipeline work for speed on root and fill passes, but it requires its own separate qualification and is not interchangeable with uphill-qualified procedures.

Shielding gas type and composition. A change in gas mixture triggers requalification under API 1104, similar to the D1.1 treatment.

Position. The position qualification ranges differ in how they are structured and applied.

The practical result: even if a D1.1 PQR was run on the same process and filler classification, the essential variable list an API 1104 inspector uses is different from Table 6.6, and the qualification record won't have the required API 1104 fields.

Mechanical test requirements

Both codes require tensile and bend specimens from the procedure qualification weld. The differences are in detail and in what additional test types are required. API 1104 commonly requires:

Nick-break tests. A nick-break specimen is notched at the weld centerline and fractured to expose internal discontinuities. The fractured surface is examined visually for slag, porosity, and fusion quality. AWS D1.1 does not use nick-break testing for procedure qualification.

Reduced-section tensile specimens. The required specimen geometry, count, and acceptance criteria differ from D1.1. The tensile strength requirement is tied to the API 5L base metal grade.

Impact testing (CVN) for higher-grade applications. API 1104 addresses CVN testing requirements for higher-strength grades and fracture-critical applications in a separate appendix, with its own test temperature and absorbed energy requirements. The concept is similar to D1.1's Table 6.8 supplementary essentials, but the triggering conditions and test criteria are different.

The testing lab needs to be scoped for the correct code's specimen types. API 1104 results on D1.1-format test reports — or vice versa — will fail document review.

Position designations: a different vocabulary

API 1104 position designations describe the orientation of the pipe:

AWS D1.1 positions describe the weld joint in space:

A welder performance qualification on 5G or 6G under API 1104 demonstrates broad positional capability for pipe, but does not transfer to a D1.1 WPQ. Code, position designation system, and essential variable list are all different. See the welding position qualification limits under AWS D1.1 article for the D1.1 side of this.

Acceptance criteria

Visual acceptance criteria exist under both codes, but radiographic and ultrasonic acceptance criteria differ. AWS D1.1 provides detailed tables for RT and UT acceptance based on discontinuity type, size, and location. API 1104 has its own acceptance criteria for RT and visual inspection, and separate criteria apply if the owner elects to use the alternative acceptance criteria (engineering critical assessment) option.

For a CWI working across both codes: never apply D1.1 visual acceptance limits to a pipeline weld. The criteria are different, and the inspection record must cite the correct code.

When a shop handles both

Shops that take both structural and pipeline work maintain parallel WPS libraries — one set qualified and administered under D1.1, another under API 1104. The qualification records belong to the employer, and the record retention obligations differ by code.

Managing both code families in a single system — with separate rule engines, separate essential variable checklists, and clear code attribution on every document — prevents cross-contamination errors. A D1.1 essential variable check running against a pipeline WPS will produce the wrong pass/fail result if the system doesn't distinguish the governing code.

Rule library based on AWS D1.1:2025; verify against your governing edition. For API 1104 work, consult the current edition of API 1104 directly.

Shops that need to manage WPS qualification records across multiple welding codes can review plan options at /pricing.