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Interpass Temperature in Welding — Control Methods per AWS D1.1 & ASME

Published 17 February 2025 · Fast Group Engineering · 8 min read

Of the three core thermal control concepts in technical welding — preheat, interpass, and PWHT — interpass temperature is the one most frequently misunderstood. Many engineers confuse interpass with preheat because both involve measuring base metal temperature before welding. The critical distinction: preheat is a minimum threshold required before the first weld pass; interpass temperature is the maximum threshold that must not be exceeded between subsequent passes. Failing to control interpass — especially overheating — causes weld metal property degradation that is invisible to the naked eye and undetectable by routine VT or PT inspection.

Welding inspector measuring interpass temperature between weld passes using Tempilstik® crayon on base metal adjacent to the weld
Measuring interpass temperature between weld passes: mark the Tempilstik® crayon on the base metal surface at least 25 mm from the weld edge — instant result, no power required.

Precise Definition — What is Interpass Temperature?

Interpass temperature is the temperature of the base metal in the weld zone, measured immediately before depositing the next pass in a multi-pass weld. Two points in this definition are critical:

The measurement point is the base metal surface approximately 25 mm (1 inch) from the weld toe, or as specified in the WPS. Do not measure on the just-deposited weld bead itself.

Stage Parameter Type of Limit Purpose
Before welding Preheat ≥ Tmin MINIMUM Heat base metal to minimum — confirm before first pass
Between passes Interpass ≤ Tmax MAXIMUM Measure before each subsequent pass — stop if limit exceeded
After completion PWHT 580–760°C SEPARATE CYCLE Stress relief and HAZ improvement — independent heat treatment cycle after weld cools

Why Maximum Interpass Matters — Metallurgical Consequences of Overheating

When the temperature between passes is too high, the base metal and previously deposited weld passes experience an additional, uncontrolled thermal cycle. The specific metallurgical consequences depend on the steel type, but the common mechanisms include:

HAZ grain coarsening

Extended time at elevated temperature promotes grain growth in the heat-affected zone. Coarse grains reduce impact toughness (Charpy V-notch energy) — particularly critical for steels with low-temperature CVN requirements.

Temper embrittlement (Cr-Mo steels)

For alloy steels such as P11 and P22, elevated interpass temperature over extended periods can cause brittle-phase precipitation at grain boundaries — a form of property degradation not detectable by standard non-destructive examination methods.

Distortion and dimensional deviation

For thin-section structures or small components, accumulated heat causes thermal distortion that is harder to control and correct.

Creep resistance degradation (P91)

P91 steel is particularly sensitive — interpass overheating affects carbide distribution and long-term creep resistance, with consequences that may only manifest after thousands of hours of service at elevated temperature.

Key point: Property degradation from interpass overheating is invisible — the weld looks completely normal on the surface, with no cracks or discontinuities. It is only detectable through destructive mechanical testing (Charpy, tensile) or after equipment failure in service. This is why on-site interpass control — before depositing each subsequent pass — is a non-negotiable preventive step.

Maximum Interpass Limits by Material Type

Maximum interpass temperature limits are not published in a single universal table — they are determined in the WPS based on material type, welding process, and mechanical property requirements. The following typical values are commonly encountered in Vietnamese industrial practice:

Material / Steel Type Typical Grade / Spec Typical Max Interpass Technical Reason Tempilstik® Part No.
Carbon structural steel SS400, A36, SM490 250–300°C (482–572°F) Prevent HAZ grain coarsening; control distortion #28039 (232°C) / #28053 (300°C)
Cr-Mo-V alloy (P91) A335 P91, A182 F91 ≤ 300°C (572°F) Protect martensite tempering structure; prevent adverse carbide precipitation #28053 (572°F / 300°C)
Cr-Mo alloy (P11, P22) A335 P11, A335 P22 250–300°C (482–572°F) Control temper embrittlement; maintain toughness #28039 (232°C) / #28053 (300°C)
Duplex stainless steel UNS S31803, S32205 ≤ 150°C (302°F) Prevent sigma phase precipitation — causes severe embrittlement and loss of corrosion resistance #28318 (302°F / 150°C)
Austenitic stainless steel TP304, TP316, TP321 ≤ 175°C (347°F) Prevent sensitization (chromium carbide precipitation at grain boundaries) causing intergranular corrosion (IGC) #28318 (150°C) or #28025 (177°C)
Pipeline steel API 5L Gr.B, X52, X65 250°C (482°F) — WPS dependent Similar to structural carbon steel — refer to API 1104 #28039 (232°C)
Duplex stainless — the 150°C limit is strict: Engineers accustomed to carbon steel often underestimate interpass control when working with duplex. Sigma phase begins to precipitate when duplex is held above 300°C for extended periods — but even at 150–300°C, the accumulated thermal exposure during multi-pass welding is sufficient to cause degradation. A WPS limit of ≤ 150°C is not conservatism — it is a technically-based requirement supported by test data.

The Two-Crayon Technique — Simultaneous Preheat Min & Interpass Max Control

This is standard practice in high-quality welding programs: using two Tempilstik® crayons at different ratings to define the permitted welding window — confirming both the minimum preheat has been reached and the maximum interpass has not been exceeded before each pass.

Crayon #1 — Minimum Preheat

Mark the surface before welding begins.

  • If not melted → surface below minimum → continue heating
  • If melted → minimum temperature reached → permitted to weld

Example: #28006 (66°C) for SS400 structural steel; #28318 (150°C) for P11

Crayon #2 — Maximum Interpass

Mark before each subsequent pass.

  • If not melted → temperature within limit → permitted to continue
  • If melted → too hot → stop, allow cooling, re-measure

Example: #28039 (232°C) or #28053 (300°C) depending on WPS

The field logic: before depositing each pass, the inspector marks both crayons on the base metal surface adjacent to the weld. The condition for welding to proceed is:

Crayon #1 has melted (minimum preheat reached) AND Crayon #2 has not melted (maximum interpass not exceeded)

If either condition is not met, welding must stop.

Common Crayon Pairs — Offshore and Industrial Applications in Vietnam

ApplicationCrayon #1 (Preheat Min)Crayon #2 (Interpass Max)
Structural carbon steel (SS400 / SM490)#28006 — preheat ≥ 66°C#28039 — interpass ≤ 232°C
P91 alloy pipe welding#28327 — preheat ≥ 200°C#28053 — interpass ≤ 300°C
Duplex stainless steel pipe#28009 — preheat ≥ 79°C#28318 — interpass ≤ 150°C
Two Tempilstik crayons used simultaneously to control minimum preheat and maximum interpass temperature during multi-pass welding
The two-crayon technique: one crayon confirms minimum preheat has been reached; the second confirms the maximum interpass temperature has not been exceeded. Both conditions must be satisfied before the next pass.

Correct Field Measurement Procedure

  1. Complete the weld pass. Stop welding and remove or extinguish the electrode/wire.
  2. Remove slag from the deposited pass using a chipping hammer and wire brush. Slag is an insulator — leaving it in place will produce an artificially high surface temperature reading.
  3. Wait 30–60 seconds before measuring. Allow heat from the weld bead to redistribute into the surrounding base metal. Measuring immediately after the arc stops gives an inflated reading that does not represent the true HAZ temperature.
  4. Mark the interpass crayon (e.g. #28053) on the base metal surface 25 mm from the weld toe.
    • Mark stays solid (chalky, dry): surface temperature is below the crayon's rated temperature → within the interpass limit → check the preheat crayon to confirm the joint has not cooled below minimum, then proceed with welding.
    • Mark liquefies immediately: surface temperature is at or above the rated temperature → above the maximum interpass limit → stop, allow natural cooling, re-measure until within range.
  5. Record in the QA/QC file: pass number, time of measurement, result (pass/fail), action taken.
Measurement location for pipe butt welds: Measure at a minimum of 3 evenly distributed positions around the circumference (e.g. 12 o'clock, 4 o'clock, 8 o'clock). Temperature is not uniform — the top position is typically hotter than the bottom due to convection. Record the highest reading as the controlling value.
Measuring preheat and interpass temperature on a pipe weld at an industrial site
Measuring preheat and interpass temperature on a pipe weld at an EPC project site — results recorded in the welding inspection report.

Quick-Reference Part Numbers for Interpass Temperature Control

Part Number Temperature (°F / °C) Used to verify maximum interpass for Notes
#28318 302°F / 150°C Duplex stainless (≤ 150°C); austenitic SS (reference) Lowest interpass limit — most common for duplex
#28025 350°F / 177°C Austenitic SS (≤ 175°C per some WPS) —
#28039 450°F / 232°C Carbon structural steel (≤ 250°C); pipeline steel API 5L Most common interpass check for carbon steel
#28053 572°F / 300°C Carbon steel (≤ 300°C); P11; P22; P91 — mandatory Most important part number for pressure piping interpass

For minimum preheat part number selection by material and thickness, see the preheat temperature reference table — AWS D1.1, ASME B31.3, API 1104. For the full range of 116 Tempilstik® part numbers, visit the Tempilstik® product page.

Frequently Asked Questions

Yes. AWS D1.1 and ASME both require interpass temperature to be checked and recorded as part of the welding control procedure. The minimum record includes: pass number, measurement time, temperature result (pass/fail), and corrective action if the limit was exceeded. Third-party inspectors routinely request this record during project audits. Absence of a record does not mean the measurement was not taken — it means it cannot be demonstrated.

Stop welding and allow the joint to cool naturally to within the permitted range before continuing. If the exceedance was sustained or significant, the applicable WPS and code will specify whether additional mechanical testing is required. The final disposition decision belongs to the Welding Engineer, based on the severity of deviation, material type, and project requirements — it cannot be made from a single data point alone.

Yes — particularly for SAW (Submerged Arc Welding), which typically has higher heat input than SMAW or GTAW and accumulates heat faster. In mechanized welding, interpass is usually checked at intervals defined in the WPS (by pass count or elapsed time) rather than every single pass. Tempilstik® remains the standard spot-check tool even in automated operations.

It can be used, but requires correctly configured emissivity settings for the specific surface condition — polished steel and oxidized steel have substantially different emissivity values, and an incorrect setting produces significant measurement error. In practice, many workshops use an infrared gun for continuous monitoring and Tempilstik® for spot-check confirmation at critical points during the weld. Tempilstik® has no emissivity-related error because measurement is by direct contact.

Use #28039 (450°F / 232°C). The selection rule: choose the part number with a rating below but closest to the maximum limit. When #28039 has not melted, you have confirmed the surface is below 232°C — comfortably within the 250°C interpass limit with a conservative safety margin. Using a crayon rated exactly at the limit would give a pass signal only at the boundary — no safety margin.

Need Tempilstik® for preheat and interpass control?

Fast Group Engineering is the authorized Tempil® distributor in Vietnam — direct import from the USA, with C/O, C/Q, and VAT invoice. Stock in Ho Chi Minh City and Vung Tau. Available as a box of 10 or individual crayons.

Request a Quote Browse All Part Numbers

📞 +84 938 888 958  |  ✉ sales@tempil.vn

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