Interpass temperature is one of the most audited parameters in a welding quality system — and one of the most frequently underdocumented. Quality managers reviewing procedure compliance routinely find weld maps with temperature columns left blank, or filled with a single value copied from the first pass and repeated across twenty subsequent ones.
This guide provides the reference tables, measurement method comparison, and documentation framework your team needs to bring interpass temperature control from a checkbox exercise into a robust quality gate — with direct traceability back to your Welding Procedure Specification.
What Is Interpass Temperature?
In multi-pass welding, interpass temperature (Ti) is the temperature of the base metal or previously deposited weld metal measured immediately before the next electrode or wire is struck. It is distinct from preheat temperature (measured before the first pass) and post-weld heat treatment (applied after all passes are complete).
The Welding Procedure Specification specifies Ti as either a maximum, a minimum, or both — depending on the material group being welded:
- Maximum Ti → prevents overheating of heat-sensitive materials (austenitic stainless steels, duplex grades, non-ferrous alloys)
- Minimum Ti → prevents cold cracking in susceptible ferritic and low-alloy steels (same logic as preheat)
Most WPS documents derived from EN ISO 15614-1 or ASME IX list a single maximum value. Multi-material fabrications — common in pressure vessels, chemical plant piping, and rolling stock — may require both limits to coexist within the same work order.
Interpass Temperature Limits by Material Group
The values below are derived from EN 1011-2 (ferritic steels), EN 1011-3 (stainless and nickel alloys), and common practice from EN ISO 15614 procedure qualifications. Always verify against your approved WPS — these are typical ranges, not replacements for procedure-specific data.
| Material Group | Typical Ti Minimum | Typical Ti Maximum | Governing Standard |
|---|---|---|---|
| C-Mn structural steel (S235–S355) | Preheat value (50–100 °C) | 250–300 °C | EN 1011-2, EN 1011-1 |
| Fine-grain HSLA (S420–S690) | 80–150 °C | 200–250 °C | EN 1011-2 |
| Cr-Mo pressure vessel steel (15CrMoV) | 150–200 °C | 300 °C | EN 1011-2 |
| Austenitic stainless (304, 316) | None specified | 150 °C | EN 1011-3 |
| Duplex stainless (2205, 2507) | None specified | 100–150 °C | EN 1011-3 |
| Nickel alloys (Inconel 625, 718) | None specified | 100 °C | EN 1011-3 |
| Aluminium alloys | None specified | 60–80 °C | EN 1011-4 |
Audit-ready tip: When a WPS lists “interpass temperature: 150 °C max”, that value must appear verbatim in your welder’s production records — not estimated, not assumed, not left blank. Auditors under EN ISO 3834-2 and EN 1090-2 will cross-reference WPS values against weld traceability records for every qualified procedure used in scope.
Measurement Methods: A Practical Comparison
Four measurement technologies are in common use on fabrication floors. Each has a distinct place in the compliance workflow.
Contact Thermometers (Thermocouple Probes)
The traditional method. A calibrated contact pyrometer with a thermocouple tip — typically K-type — is pressed against the metal surface adjacent to the weld zone, usually at a distance of 4× the material thickness as specified in EN ISO 13916:2017.
Advantages: Direct metal contact gives a reliable conductive reading. Instruments are inexpensive and widely familiar to inspectors.
Limitations: Single-point measurement; requires interruption of welding; probe tip wears and must be re-calibrated. Emissivity is not a factor, so stainless steel surfaces are handled correctly without surface preparation.
Infrared Pyrometers (Spot Pyrometers)
Non-contact, aimed at a defined spot. Fast and weld-safe, but highly sensitive to surface emissivity — an oxidised carbon steel surface (emissivity ~0.85) and a brushed austenitic stainless surface (emissivity ~0.15) read very differently for the same actual temperature.
Advantages: No contact required; no interruption of arc; distance-to-spot ratio (D:S) allows measurement from a safe working distance.
Limitations: Emissivity must be configured correctly and verified against a contact measurement before each use. EN ISO 13916 permits pyrometer measurement but requires documented emissivity setting.
Thermal Imaging Cameras
Area measurement rather than spot measurement. A thermal camera for interpass temperature measurement captures the full temperature field across the joint, joint preparation, and adjacent base metal simultaneously — giving the inspector a spatial map, not a single reading.
This approach is particularly valuable for:
- Long weld seams where temperature gradients are significant along the joint length
- Components with complex geometry where the measurement point per EN 13916 is difficult to reach manually
- Continuous monitoring during automated or robotic welding cycles
For in-depth guidance on continuous monitoring implementation, see our guide on preheat and interpass temperature monitoring with thermal cameras.
Digital Contact Thermometers (Magnetic / Roller Type)
Slower response than probe thermocouples but useful for curved or irregular surfaces where a flat-tip probe cannot achieve full contact. Roller-type thermocouples maintain contact while moving along a seam and are used in continuous pass sequences on pipe joints.
Measurement Point and Timing per EN ISO 13916
EN ISO 13916:2017 defines three requirements that every measurement record must satisfy:
- Location: measured on the base material at a distance of 4× the material thickness from the fusion line (or as specified in the WPS if different).
- Timing: measured immediately before striking the arc for the next pass — not after a cooling wait.
- Instrument calibration: pyrometers and contact thermometers must be calibrated to a traceable standard at defined intervals (typically 12 months for contact instruments, more frequently for pyrometer emissivity checks).
Quick compliance checklist:
- WPS states Ti limit (max, min, or both) for the material group
- Measurement point documented in the welding traveller (distance from fusion line)
- Instrument calibration certificate current and on file
- Ti reading recorded per pass (not per joint)
- Out-of-limit readings trigger NCR and documented corrective action
Interpass Temperature in Welder Qualification Testing
When a welder qualifies under ISO 9606-1 or ASME IX, the test piece is welded following a defined WPS that includes the interpass temperature range. The range of approval for the qualified procedure does not extend to wildly different Ti values — EN ISO 15614-1 §8.4.5 specifies that a change in specified minimum or maximum interpass temperature by more than 25 °C outside the range used in the procedure qualification record (PQR) requires re-qualification.
In practice, this means a procedure qualified with Ti max 150 °C cannot be applied to a joint where the WPS has been revised to Ti max 250 °C without a new PQR — a point frequently missed in small fabrication shops when procedure documents are updated informally.
Documentation: What a Traceable Weld Record Must Contain
A weld traceability record compliant with EN ISO 3834-2 or EN 1090-2 must capture, per pass:
- Ti reading (numeric value, not “OK” or a tick mark)
- Time of measurement
- Measurement method (contact / pyrometer / thermal camera)
- Instrument ID (serial number, calibration due date)
- Welder ID and weld joint reference
For high-volume production, manual paper-based records are a compliance risk: a welder completing twenty passes per shift cannot accurately recall temperatures retroactively, and supervisors counter-signing records hours after the fact are a routine finding in third-party audits.
The most reliable solution integrates temperature measurement directly into the production data stream. A thermal camera for interpass temperature monitoring connected to the welding cell generates timestamped thermal records automatically — each pass gets a documented temperature profile without requiring the welder to break arc, reach for a probe, and log a value manually.
For guidance on calculating the complementary heat input parameter — which alongside interpass temperature controls overall heat balance in the joint — see our welding heat input calculation guide.
Common Control Failures and How to Prevent Them
| Failure Mode | Root Cause | Prevention |
|---|---|---|
| Interpass too hot (austenitic) | Production pressure, no real-time feedback | Continuous thermal monitoring or timed inter-pass hold |
| Interpass too cold (ferritic, HSLA) | Ambient temperature drop, long arc breaks | Minimum Ti alarm linked to welding cell interlock |
| Missing records | Manual logging burden | Automated data capture from measurement instrument |
| Wrong emissivity setting | Pyrometer reused across material types without reset | Per-material emissivity cards at workstation |
| Measurement point deviation | Welder estimates rather than measures from fusion line | Marked measurement fixtures on run-off tabs |
Summary
Interpass temperature control is not a single measurement — it is a documented, per-pass discipline that connects your WPS to your production records through calibrated instruments and structured logging. The measurement method matters less than the completeness of the record: a contact thermocouple with a full per-pass log beats an advanced thermal camera whose output is never archived.
If your current workflow relies on welder judgment or retroactive temperature estimates, the risk is not just a failed audit — it is undetected metallurgical damage in joints that look visually acceptable but carry hidden degradation. Establishing consistent interpass temperature control is one of the highest-leverage quality interventions available to a fabrication operation.
Integrate interpass temperature data into your weld quality records
Therness thermal monitoring systems capture interpass temperature automatically on every pass — timestamped, calibration-linked, and ready for EN ISO 3834-2 audit. Talk to our engineers about your application.
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