Weld Maps, PWHT Records & Heat Number Traceability: The Complete Guide

A welding procedure specification tells the welder what to do. A welder qualification record proves the person doing it is competent. But neither document tells you what actually happened at weld joint J-047 on vessel V-203, who made it, what material it joined, whether it was heat treated, and what the RT result was. That is the job of the weld map, the weld traveller, the heat number record, and the PWHT chart — the downstream documentation that turns a qualified procedure and a qualified welder into a traceable, inspectable, defensible record of physical work.

This guide covers the second half of the welding documentation chain. For WPS, PQR, and welder qualification records, see our companion article on managing welding traceability from procedure to production weld. Here we go deeper on the records that fabricators most commonly get wrong: weld maps that aren't kept current, PWHT charts filed without links to weld numbers, and heat number traceability that breaks at the cut-and-mark stage.

What Is a Weld Map and Why Is It Required?

A weld map is a drawing or isometric diagram that assigns a unique identifier — a weld number — to every weld joint on a structure, pressure vessel, pipeline, or piping system. The purpose is straightforward: every physical weld must be traceable to a documented record, and to find that record you need a systematic reference system. The weld map is the index that connects the physical joint to its paper trail.

At its simplest, a weld map is the fabrication drawing marked up with weld numbers at each joint location. In practice, the weld map also carries — or references — the key attributes of each joint: the joint type (butt, fillet, nozzle, socket, branch), the welding process used, the WPS reference number, the welder's ID, and the NDT method required. Some weld maps include the material specification of each piece being joined and whether PWHT applies to that joint.

Who requires weld maps? The requirement is present, in some form, in every major fabrication code:

• ASME Section VIII (Pressure Vessels) requires identification of each weld joint and the qualified welder or welding operator who made it, traceable through the Manufacturer's Data Report
• ASME B31.3 (Process Piping) requires weld identification numbers and records linking each joint to the WPS and qualified welder
• API 1104 (Pipeline Welding) requires a weld map or isometric marking system so each weld can be identified and its records retrieved
• AS/NZS 2980 (Quality Requirements for Welding) requires weld identification records as part of the welding quality plan
• DNV and Lloyd's Register marine and offshore specifications routinely require as-built weld maps as a condition of certification
• Client specifications for major oil and gas, power generation, and infrastructure projects almost universally require a weld register with numbered joints

The weld traveller. Closely related to the weld map is the weld traveller — sometimes called a job card, history card, or inspection release document. While the weld map shows where each joint is on the structure, the traveller accompanies the joint through fabrication, recording what happens at each stage. A traveller for a pressure piping joint might capture: fit-up dimensions and fit-up inspector sign-off; the WPS number and welder ID for the root pass; the WPS number and welder ID for fill and cap passes (which may differ); the preheat temperature measured before welding; any interpass temperature readings; whether PWHT applies and the PWHT record reference number; the NDT method, operator, and result; and final visual inspection acceptance. When fabrication is complete, the traveller becomes the in-process record for that joint and forms part of the as-built weld package.

As-built weld records. The initial weld map is a plan — it shows what joints are to be made. The as-built weld record is what actually happened. Welds get repaired. Joints get re-numbered when the design changes. A welder falls ill and a second welder completes the joint. Each of these events must be captured in the as-built record. The single most common weld map failure in fabrication audits is an as-built weld map that doesn't reflect actual construction — repair welds that were assigned new numbers but not added to the map, or joints that were re-welded under a different WPS after a failed NDT result, with only the original record surviving.

Heat Number Traceability — From MTC to Weld Record

Every batch of steel produced at a mill is assigned a heat number — the unique identifier for that specific melt of steel. The mill issues a Mill Test Certificate (MTC) against that heat number, documenting the chemical composition (carbon content, manganese, chromium, and other alloying elements) and the mechanical properties (yield strength, tensile strength, elongation, Charpy impact results) of the material as it left the mill. The MTC is the birth certificate of the material.

Heat number traceability in fabrication means maintaining an unbroken chain from that MTC, through every handling and cutting operation, to the specific weld joint where the material was used. The chain looks like this:

1. Mill heat number — stamped or stencilled on the material at the mill, referenced on the MTC
2. Receiving inspection — material received at the fabrication shop is checked against the MTC, and the heat number is verified as present on the material
3. Material identification — before cutting, the heat number is marked on every piece that will be cut from the parent plate or pipe (paint pen, low-stress stamp, adhesive label — depending on the material and code requirements)
4. Cut piece marking — when a piece is cut, the heat number is transferred to every resulting cut piece before the original marking is removed
5. Fit-up and weld record — when two pieces are joined, the heat numbers of both base material pieces are recorded against the weld joint number in the weld register
6. Weld record to MTC link — the weld register entry references the heat numbers, which can be looked up against the MTC file to confirm the material met the required specification

Why it matters. If a pipe joint fails in service — through cracking, leaking, or structural failure — the investigation will ask: what material was in that joint? Was it to specification? The answer to those questions requires heat number traceability. Without it, the failure investigation cannot determine whether the material used was compliant. In pressure equipment regulated under jurisdictional requirements (Australian pressure equipment regulations, ASME Code, PED in Europe), the inability to demonstrate material traceability can require the entire affected section to be quarantined, inspected by alternative methods, or replaced — at potentially enormous cost.

Where traceability breaks. The most common point of failure is the cut stage. A fabricator receives a 6-metre length of pipe with the heat number stencilled at one end. They cut it into three pieces. The first piece retains the stencil. The second and third do not — unless someone transferred the marking before cutting. In a busy workshop, this step is routinely skipped. The result is two untraceably marked pieces that cannot be linked to an MTC. When an inspector or assessor asks for heat number traceability for a joint made from those pieces, the fabricator cannot provide it.

The fix is procedural, not technological: a material control procedure that requires heat number transfer before cutting, verified by the inspector who signs off fit-up. The digital traceability system records the result — but the physical marking is the foundation.

Consumable traceability. Heat number traceability applies to consumables as well as base materials. For each weld joint, the weld record should capture the electrode or wire classification and the batch number (or heat number, for wire), the flux lot number for submerged arc welding, and the shielding gas certificate reference. For pressure-critical joints, the consumable certificate must be on file confirming the consumable meets the filler material specification required by the WPS. This is separate from the base material heat number traceability but equally required under ASME, AS/NZS, and ISO welding quality standards.

For a broader treatment of how traceability applies across testing and inspection workflows, see our guide on improving traceability in laboratory testing. For material test certificate management — including how to store, link, and retrieve MTCs against material records — see our article on MTC management for fabrication and inspection.

Post-Weld Heat Treatment: Records and Compliance

Post-Weld Heat Treatment is a controlled thermal process applied to a completed weld and its surrounding heat-affected zone. The purpose is to relieve residual tensile stresses introduced by welding (which can promote stress corrosion cracking and fatigue crack initiation), to temper any hard microstructural regions formed in the heat-affected zone (improving toughness and reducing the risk of hydrogen-induced cracking), and to restore hardness to values within the limits specified by the applicable standard or client specification.

When PWHT is required. The threshold for mandatory PWHT depends on the applicable code, the base material P-number (ASME grouping) or material group (ISO grouping), the wall thickness or heat input, and the service conditions:

• ASME B31.3 (Process Piping) — Table 331.1.1 specifies mandatory PWHT thresholds by P-number and nominal wall thickness. P-No. 1 carbon steel typically requires PWHT above 19 mm; Cr-Mo steels (P-No. 4, 5) have lower thresholds or are always required.
• ASME Section VIII (Pressure Vessels) — UCS-56 through UCS-58 cover carbon and low-alloy steel; equivalent tables exist for other material groups
• AS/NZS 4458 (Pressure Equipment Manufacture) — references the applicable welding standard for PWHT requirements, typically consistent with ASME thresholds for equivalent materials
• Client specifications — frequently impose PWHT requirements more stringent than the code minimum, particularly for hydrogen service, sour service (H2S environments), or high-temperature applications

What a PWHT record must contain. A PWHT record is not just a temperature reading. It is a complete documented account of the heat treatment cycle, and it must contain enough information to demonstrate that every requirement of the applicable PWHT procedure was met:

1. Weld numbers or joint numbers covered — the PWHT record must be unambiguously linked to the specific weld joints that were heat treated. A chart that shows a temperature-time profile but cannot be linked to specific weld numbers is not a traceable PWHT record.
2. Date and time of heat treatment — the start and completion time of the treatment, including the time at which the soak temperature was first reached
3. Heating method — furnace (batch or continuous), induction heating, resistance heating blankets, or flame heating with temperature control — each method has different documentation requirements
4. Thermocouple positions and readings — for furnace treatment, the thermocouple configuration in the furnace; for local PWHT, a sketch or photograph showing where thermocouples were attached to the pipe or vessel, and the temperature recorded at each thermocouple throughout the cycle
5. Temperature-time chart — a continuous record (from a chart recorder or digital data logger) showing the temperature at each thermocouple position against time, with the heat rate, soak temperature, soak duration, and cool rate all visible. This chart is the core evidence document — it shows that the material was heated and cooled at the rates specified, and that it was held at the required soak temperature for the required duration.
6. Witness signature — the QC engineer, welding inspector, or third-party inspector who witnessed and accepted the PWHT must sign the record. For code-stamped pressure vessels, this may need to be the Authorised Inspector (AI).
7. PWHT procedure reference — the document that specified the heat treatment parameters (heating rate, soak temperature, soak time, cooling rate) must be referenced on the record so an assessor can verify the parameters used were as specified

Chart recorders and digital records. Historically, PWHT records consisted of a circular or strip chart recorder trace, with the weld numbers, date, and inspector signature written on the chart. These paper charts are still commonly used and are legally acceptable — but they must be filed in a way that allows retrieval by weld number. A chart filed by date, with weld numbers noted only on the chart itself, requires manual searching to answer the question "show me the PWHT record for joint P-112."

Modern data loggers produce digital temperature-time records that can be archived as files, named by job number and weld number, and stored alongside the weld traveller. Digital records make retrieval faster, but they introduce new risks: data file formats that become unreadable as software changes, and the temptation to modify digital records after the fact. An approved digital PWHT system should produce tamper-evident records — locked files with an audit trail — that carry the same legal weight as a paper chart.

Local PWHT documentation. Local PWHT (heating blankets or induction coils applied directly to the joint rather than placing the item in a furnace) requires additional documentation to demonstrate that the heated band covered the required width around the weld. ASME B31.3 and Section VIII specify minimum band widths — typically the greater of 25 mm or the wall thickness on each side of the weld. The PWHT record for local treatment should include a sketch or photograph showing the heating element arrangement, the thermocouple positions (which must be within the heated band), and the temperature gradient measurements if required.

Post-PWHT hardness testing. For certain material groups — particularly Cr-Mo steels used in high-temperature service — hardness testing after PWHT is required to confirm that the tempering effect has been achieved. The hardness results (typically Vickers or Brinell) must be recorded against the weld number and referenced to the PWHT record. A PWHT that produces a correct temperature-time profile but leaves the weld metal or HAZ above the specified hardness limit is not an acceptable PWHT. The hardness results complete the PWHT compliance record.

The Complete Welding Documentation Package

When a fabrication project is complete, the client, the owner's inspector, or the certification authority will request the welding documentation package — the set of records that collectively demonstrate the fabrication was performed by qualified people, following qualified procedures, with traceable materials, inspected by qualified inspectors, and accepted against defined acceptance criteria. The package is not a bundle of whatever records happen to be available — it is a structured, linked set of documents where every gap is a potential finding.

A complete weld package for a pressure vessel, piping system, or structural project contains the following elements:

1. WPS library — all Welding Procedure Specifications used on the project, each showing the WPS number, revision, applicable standard, base material group, filler material, welding process, and essential variable ranges. See our guide on WPS and PQR management for a full treatment of procedure qualification.
2. PQR records — each PQR supporting the WPS documents used on the project, with original test results: tensile, guided bend, impact (where required), hardness (where required), and the NDT result on the qualification test coupon
3. Welder qualification records — WQRs for every welder who worked on the project, showing each welder's scope of approval, the qualifying standard, the test date, and the continuity evidence confirming the qualification was current throughout the project
4. Weld map (as-built) — the final weld map showing all joint numbers, joint types, welding processes, welder IDs, and WPS references as actually constructed — including repairs and any joints that were re-welded
5. Weld register / travellers — the joint-by-joint record of fit-up inspection, welding (including welder ID, date, and WPS reference), PWHT (with record reference number), and NDT (with report reference number and result)
6. Material traceability records — heat numbers recorded against each weld joint for both base material pieces, with MTCs on file for each heat number used on pressure-critical joints
7. Consumable records — electrode and wire batch or heat numbers, filler material test certificates, flux lot certificates, and storage records (demonstrating electrodes were kept in heated quivers or dry storage as required by the WPS)
8. PWHT records — temperature-time charts or digital records for every joint that required PWHT, with thermocouple positions, witness signatures, and the PWHT procedure reference — each record linked to the specific weld numbers covered
9. NDT records — radiographic, ultrasonic, magnetic particle, and liquid penetrant test reports for every weld subjected to NDT, with the result (accept or reject) recorded against the weld number. NDT personnel certifications should also be on file — see our guide on NDT personnel certification tracking.
10. Repair records — where a weld was rejected by NDT and repaired, the repair record must show: the original defect finding (NDT report reference), the repair procedure used (repair WPS), the repair weld number (typically the original joint number with a suffix — J-047-R1), the welder who made the repair, and the post-repair NDT result confirming acceptance
11. Final inspection certificate — dimensional verification records, final visual inspection sign-off, hydrostatic or pneumatic pressure test records, and the Manufacturer's Data Report or Inspection Certificate issued by the certification authority

The critical feature of a complete weld package is that the records are linked — not merely co-located. A PWHT chart that is filed in the same box as the weld register but not cross-referenced to specific weld numbers is not linked. An NDT report filed by report number rather than by weld number requires manual searching to answer "what was the RT result on joint P-112?" A complete package allows an assessor to start with any weld number and trace forward and backward through every associated record without leaving gaps.

5 Common Welding Documentation Failures

These are the five failures that appear most frequently in fabrication audits, client reviews, and certification assessments — each one capable of calling the acceptability of an entire weld package into question.

1. Weld map not updated after repairs. A weld fails RT. The defect is excavated and re-welded. The repair is assigned the number J-047-R1. The original weld map, however, still shows J-047 with the original welder's ID and the original WPS — and no indication that the joint was repaired. An assessor reviewing the weld map alongside the NDT reports finds the RT rejection report for J-047 and the acceptance report for J-047-R1 — but the weld map only reflects the original construction. The as-built weld map is no longer as-built. This discrepancy raises the question of what else in the map doesn't reflect actual construction.

2. Heat number tracking lost at the cut stage. Material arrives with heat numbers stencilled on one end. When it is cut into multiple pieces, the shop fails to transfer heat number markings to every cut piece before the original marking disappears. The result is material in fabricated joints that cannot be traced to an MTC. When the assessor requests heat number traceability for joints P-204 and P-207, the fabricator can provide MTC records for the heat numbers they received — but cannot demonstrate that those heat numbers correspond to the specific pieces used in P-204 and P-207. This is a direct traceability failure under ASME, API, and AS/NZS codes for pressure-critical joints.

3. PWHT charts filed separately from weld records. The heat treatment contractor files temperature-time charts by date of heat treatment — each chart is labelled with the job name and the furnace batch number. The fabricator's weld register records which joints required PWHT, with a note "PWHT completed 14 March" — but no reference to a specific PWHT record number. When the assessor asks for the PWHT record for joint V-031, the fabricator must search through heat treatment charts by date, find the March 14 batch, and identify which chart covers joint V-031. If the chart covers a batch of 12 joints, V-031 may or may not be clearly listed. The link between the weld record and the PWHT evidence is broken — and an assessor has grounds to treat it as a non-conformance.

4. Welder ID not recorded on the weld map or weld register. The weld map was created from the fabrication drawings and shows joint numbers, joint types, and WPS references — but the welder column was never filled in consistently. Some joints have a welder stamp; others have initials that are illegible; others have no attribution at all. When a defect is found in service and the failure investigator asks who welded joint J-083, the answer is not in the records. Under most fabrication codes, the requirement to identify the welder who made a joint is not administrative — it is a code requirement. Joints that cannot be attributed to a qualified welder may be un-demonstrably compliant under the applicable standard.

5. NDT results filed by date rather than by weld number. The NDT contractor submits radiographic test reports numbered sequentially by the date of examination — RT-2026-0047, RT-2026-0048, and so on. Each report covers multiple joints examined on that day. The weld register records "RT complete" against each joint but does not record the RT report number. When the assessor asks for the RT report for joint J-047, the fabricator must search through chronological RT reports to find the one that includes J-047. In a large fabrication with hundreds of joints and dozens of RT reports, this can take hours. For the assessor, a documentation system that cannot retrieve a specific NDT result by weld number in under two minutes is a symptom of inadequate record management — even if every result is ultimately findable.

Managing Weld Records Digitally

Paper weld packages are vulnerable in ways that digital systems are not. Documents are misfiled. Handwriting is illegible under photocopying. Charts are lost in floods, fires, or office moves. After ten years, paper records that were meticulously assembled at project completion may be missing critical elements — and the project team has long since dispersed. For pressure equipment with design lives of 20 to 40 years, paper is a fragile foundation.

Beyond the durability problem, paper systems fail at retrieval. A paper weld package can contain every required document and still take hours to navigate when an assessor asks for the complete traceability chain for a specific joint. The documents exist — they are just not cross-referenced in a way that allows rapid retrieval. Digital systems that enforce links between records at the point of data entry solve this problem at the source.

A digital welding records system that genuinely supports the full documentation workflow should do the following:

• Assign weld numbers and build the weld map — creating the numbered joint register at project setup, with fields for joint type, welding process, WPS reference, and required NDT type
• Record welder IDs against joints at the time of welding — with validation that the welder is currently qualified for the applicable process, material group, and position
• Record heat numbers against each weld joint — for both base material pieces, with the system linking those heat numbers to the MTC record on file
• Attach PWHT records to weld numbers — whether as scanned charts, digital logger exports, or structured data entries — so that retrieving a PWHT record by weld number is a single search operation
• Record NDT results against weld numbers — with the method, the operator's certification reference, the report number, and the accept/reject decision, all retrievable by weld number
• Track repair records — creating repair weld numbers as children of the parent joint, with the repair WPS, the welder, and the post-repair NDT result all linked
• Generate the as-built weld register automatically — as data is entered, the register updates, so the as-built document is always current rather than assembled retrospectively
• Alert when welder qualifications are approaching expiry — so that continuity windows are managed proactively rather than discovered when an assessor flags an expired qualification
• Produce a complete weld package for client or assessor on demand — a structured export of all records linked to a project, organised by weld number, with every required element retrievable in under two minutes

OMS covers the full welding documentation workflow. The welding inspection module manages weld maps and weld registers, linking each joint to its WPS, qualified welder, heat numbers, PWHT record, and NDT results. The WPS/PQR library maintains the procedure foundation, and the welder qualification register tracks continuity with automatic alerts. When a client or assessor requests the documentation package for a project, the complete chain — from procedure qualification through to final NDT acceptance — is available in a single, structured, searchable record.

The difference between a good weld and a demonstrably good weld is the documentation. Every fabrication shop that has lost a contract, faced a rejection, or spent days assembling an audit package under pressure understands this distinction. Digital weld record management doesn't change how welds are made — it changes how reliably the evidence of good work survives the project.