Interpass temperature in multi-run welding
Interpass temperature is the temperature of a multi-run weld and the metal next to it just before the next run is struck (ISO 13916). It has two limits: the bottom one is the preheat, the top one protects toughness and corrosion resistance. For S355 the usual window is from the preheat up to 250 °C, for austenitic stainless up to 150 °C, for 6xxx aluminium about 120 °C.
Short answer
Measure on the weld metal or right beside it, immediately before the next run. Below the minimum — heat again; above the maximum — wait. Typical upper limits: non-alloy and fine-grained steels 250 °C, quenched and tempered steels 200 °C, austenitic stainless 150 °C, duplex 150 °C, aluminium 6xxx about 120 °C. The figure that binds you is the one in your WPS; the table below shows where those figures come from.
Maximum interpass temperature by material group
The minimum is the preheat, and it holds for every run, not just the first. The maximum depends on what heat does to the material.
| Group ISO/TR 15608 | Materials | Minimum | Maximum | Why the upper limit |
|---|---|---|---|---|
| 1, 2 | S235, S355, S460N/M | preheat by EN 1011-2 from the preheat calculator; metal below +5 °C is always warmed | 250 °C | toughness of the weld and HAZ; t8/5 gets longer with every run |
| 3.1 | S690Q and other quenched and tempered steels | preheat from the steel maker’s data | 200 °C | the HAZ loses the strength of the quench-and-temper treatment |
| 5.2 | 10CrMo9-10 (P22) | 200 °C | 350 °C | creep-resisting Cr-Mo: kept hot against hydrogen cracking, range from the electrode datasheet |
| 8.1 | 1.4301 (304), 1.4404 (316L) | none | 150 °C | sensitisation, hot cracking, distortion |
| 10 | duplex 1.4462 (2205) | none | 150 °C | intermetallic phases, loss of the ferrite–austenite balance |
| 10 | super duplex 1.4410 (2507) | none | 100–150 °C | as above, more sensitive; many project specifications set 100 °C |
| 22, 23 | aluminium 5xxx with Mg above 3 %, 6xxx (6060, 6082) | none; thick sections warmed to drive off moisture | about 120 °C, no longer than 15 min | the HAZ of heat-treatable alloys softens for good |
Where the figures come from: 250, 200 and 150 °C are the defaults of the pWPS generator and the preheat calculator on this site; 200–350 °C for 10CrMo9-10 is the Böhler datasheet for its CrMo2 electrode; 150 °C for duplex is the guidance of the International Molybdenum Association (IMOA); 100 °C for super duplex is the limit found in offshore specifications based on NORSOK; 120 °C for aluminium is the AWS D1.2 limit as quoted by Hobart and ESAB. Your WPS and the steel or filler datasheet take precedence over all of them.
Why there is an upper limit at all
- Structural steel. Each run reheats the joint. The hotter the starting point, the slower the cooling from 800 to 500 °C: grain grows and impact energy drops. The example below shows by how much.
- Quenched and tempered steel. Its strength comes from heat treatment; too much heat in the HAZ undoes it.
- Austenitic stainless. Time at 450–850 °C precipitates chromium carbides on the grain boundaries (sensitisation, see welding stainless steel); a hot joint also distorts more and is prone to hot cracking.
- Duplex. Heat input is limited on both sides: too cold gives too much ferrite, too hot gives brittle intermetallic phases.
- Aluminium 6xxx. The HAZ of a heat-treatable alloy loses strength above about 120 °C and does not get it back without a new heat treatment (welding aluminium).
How to measure interpass temperature (ISO 13916)
- Where. On the weld metal or the parent metal immediately next to it. Preheat is checked at a distance of 4·t from the joint edge, but no more than 50 mm; above 50 mm thickness the parent metal must be at temperature over at least 75 mm.
- When. Right before the next run, not after a coffee break.
- Which side. Where practicable, the face opposite the heating. If only the heated face can be reached, take the heat away and wait about 2 min per 25 mm of thickness before reading.
- With what. Temperature-indicating crayons, a contact thermometer, a thermocouple or an infrared thermometer. With crayons use a pair: the lower one (preheat) must melt, the upper one (maximum) must not. An infrared reading on bright stainless or aluminium comes out low unless the emissivity is set for that surface.
Interpass temperature and t8/5: worked example
S355J2, butt weld, MAG 135, 28 V, 250 A, travel 280 mm/min. Heat input Q = k·U·I·60 / (v·1000) = 0.8·28·250·60 / (280·1000) = 1.2 kJ/mm (the same formula as the heat input calculator). With CET 0.34 and a low-hydrogen wire (HD 4) the EN 1011-2 formula gives a preheat of 80 °C at 20 mm, so the window is 80–250 °C. The cooling time from the t8/5 calculator formula, with the interpass temperature as T₀:
| Interpass T₀ | t8/5 at 10 mm, s | t8/5 at 20 mm, s |
|---|---|---|
| 20 °C | 14.7 | 5.7 |
| 80 °C | 19.2 | 6.8 |
| 150 °C | 27.5 | 8.5 |
| 200 °C | 37.2 | 10.3 |
| 250 °C | 53.1 | 13.3 |
At 20 mm the time goes from 6.8 s at the preheat to 13.3 s at 250 °C — still inside the usual 5–25 s for structural steel. At 10 mm the heat can only flow in the plane of the plate: 19.2 s at the preheat, 27.5 s at 150 °C and 53.1 s at 250 °C. On thin plate with high heat input the real limit is therefore lower than the WPS maximum — either wait for the joint to cool or reduce Q. The cooling time of the first run uses the preheat as T₀; the time of later runs uses the interpass temperature.
Interpass temperature in the WPS and WPQR
The WPS gives a maximum (and with preheat, a minimum). Under EN ISO 15614-1 the interpass temperature recorded on the test piece qualifies up to 50 °C more: 250 °C on the test gives 300 °C on the WPS. For austenitic, duplex and nickel alloys there is no allowance — the WPS keeps the value used on the test. The WPQR range calculator applies both rules, and the pWPS generator fills the line in with the defaults from the table above.
Staying inside the window on the shop floor
- Work on two or three joints in turn instead of waiting at one.
- Measure before each run, not by the clock: a 6 mm fillet and a 30 mm butt cool at very different rates.
- Forced cooling (compressed air, water) only if the WPS allows it.
- If the joint falls below the minimum between runs, heat it back up before carrying on — common on thick steel and in cold weather (cracks in welds).
- Write it down. The measured interpass belongs in the welding record alongside the preheat.
Frequently asked questions
What is interpass temperature?
The temperature of a multi-run weld and the adjacent parent metal immediately before the next run (ISO 13916). In a WPS it normally has a maximum; the minimum is the preheat.
What is the maximum interpass temperature for S355?
250 °C is the usual WPS value for non-alloy and fine-grained steels, and the one the preheat calculator uses. The minimum is the calculated preheat, 80 °C in the 20 mm example on this page.
What is the maximum interpass temperature for stainless steel?
For austenitic grades such as 304 and 316L, 150 °C; for duplex 2205, 150 °C; for super duplex 2507 often 100 °C. The upper limit protects corrosion resistance — carbide precipitation in austenite, intermetallic phases in duplex.
Where do I measure interpass temperature?
On the weld metal or the parent metal right next to it, just before the next run. Preheat is checked 4·t from the joint, no more than 50 mm away; on plate over 50 mm the parent metal must be at temperature over at least 75 mm.
Does interpass temperature change t8/5?
Yes: it is the T₀ of every run after the first. In the example on this page, 10 mm plate at 1.2 kJ/mm cools from 800 to 500 °C in 19.2 s from 80 °C and in 53.1 s from 250 °C — check it in the t8/5 calculator.
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