What Is Interpass Temperature?
Interpass temperature is the temperature of the weld zone measured immediately before depositing the next weld pass in a multi-pass weld. It is a maximum limit—welding must not proceed until the weld area cools below the specified interpass temperature. Controlling interpass temperature prevents overheating of the weld and HAZ, which can degrade toughness, promote excessive grain growth, cause hot cracking, or alter the ferrite-austenite phase balance in duplex stainless steels.
The interpass temperature is recorded on the welding procedure specification (WPS) and verified by the welding inspector before each pass on critical welds.
Why Interpass Temperature Matters
| Material | Effect of Exceeding Max Interpass |
|---|---|
| Carbon steel | Reduced toughness from grain coarsening; may fail impact testing requirements |
| Cr-Mo alloy steel | Grain growth, reduced creep properties, risk of reheat cracking |
| Austenitic stainless steel | Sensitization (carbide precipitation at 500-800 deg C), reduced corrosion resistance |
| Duplex stainless steel | Excessive ferrite, sigma phase precipitation, loss of toughness and pitting resistance |
| Nickel alloys | Hot cracking, reduced corrosion resistance, ductility-dip cracking |
Maximum Interpass Temperature by Material
| Material Group | Max Interpass (deg C) | Source |
|---|---|---|
| Carbon steel (P-1) | 250-315 (varies by spec) | WPS per ASME IX; project specification |
| Low-alloy steel (P-3, P-4) | 250-315 | WPS per ASME IX |
| Cr-Mo steel (P-5A, A335 P22) | 315 (max typical) | WPS; API RP 582 |
| P91 (P-5B, A335 P91) | 315 | WPS; EPRI guidelines |
| Austenitic SS (P-8, 304/316) | 150-177 | ASME IX WPS; NORSOK M-601 specifies 150 deg C |
| Duplex SS (2205) | 150 | NORSOK M-601 (mandatory) |
| Super duplex SS (2507) | 100-150 | NORSOK M-601; some specs limit to 100 deg C |
| Nickel alloy (Inconel 625, 825) | 150 | NORSOK M-601; filler-specific |
| Copper-nickel (90/10, 70/30) | 100-150 | AWS/project specification |
Interpass Temperature vs Preheat
| Parameter | Preheat | Interpass Temperature |
|---|---|---|
| Control type | Minimum temperature | Maximum temperature |
| When measured | Before root pass | Before each subsequent pass |
| Purpose | Slow cooling to prevent hydrogen cracking | Prevent overheating and property degradation |
| Carbon steel | 10-200 deg C minimum (thickness dependent) | 250-315 deg C maximum |
| Duplex SS | None (or ambient) | 150 deg C maximum |
| Cr-Mo steel | 150-250 deg C minimum | 315 deg C maximum |
For carbon steel and Cr-Mo steels, the welding temperature window is bounded by both: preheat sets the floor, interpass sets the ceiling. For stainless steel and duplex, the interpass ceiling is the critical control.
Measurement Methods
| Method | Accuracy | Application |
|---|---|---|
| Digital contact pyrometer | +/- 1-2 deg C | Standard method; probe placed on weld surface |
| Thermocouple (type K) | +/- 2 deg C | Continuous monitoring; required for critical Cr-Mo welds |
| Temperature-indicating crayon | +/- 1% of rated temp | Quick check; melts at calibrated temperature |
| Infrared pyrometer | +/- 2-5 deg C | Non-contact; affected by emissivity variations on shiny surfaces |
Measurement location: on the weld metal or within 25 mm (1 in.) of the weld edge, on the same side as the next pass.
Impact on Weld Properties
| Interpass Control | Toughness | Hardness | Ferrite (Duplex) | Corrosion Resistance |
|---|---|---|---|---|
| Within limits | Meets specification | Within range | 35-65% (balanced) | Full pitting resistance |
| Exceeded (too hot) | Reduced (grain growth) | May be low (soft) | <30% (excessive austenite) | Degraded |
| Too cold (below preheat) | Risk of cracking | Too high (hard HAZ) | >70% (excessive ferrite) | Degraded |
Interpass temperature is documented on the weld log and verified per the inspection and test plan. Records are archived alongside PWHT charts, mill test certificates, and pipe inspection reports.
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