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Welding preheat temperature calculator
Enter the parent metal composition, the plate thickness, the hydrogen content of the consumable and the heat input, and read the minimum preheat temperature from EN 1011-2 equation C.8 — the same number that is also the minimum interpass temperature for the rest of the weld. The tool returns CET and CEV from the same composition, so one certificate feeds both the equation and the diagram method.
Nothing is sent anywhere: the calculation runs in your browser, and the shareable link carries only the numbers you typed.
Two routes in Annex C, one of which is an equation
CET [%] = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40 — EN 1011-2 equation C.2
Tp [°C] = 697 × CET + 160 × tanh(d/35) + 62 × HD0.35 + (53 × CET − 32) × Q − 328 — equation C.8
CEV (IIW) [%] = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 — the diagram method's carbon equivalent
d = plate thickness in mm · HD = diffusible hydrogen in ml/100 g to ISO 3690 · Q = heat input in kJ/mm
Method B of Annex C is the equation above, and it is the one a calculator can honestly run. Method A is a family of nomograms keyed on CEV, combined thickness, a hydrogen scale and heat input; those charts are part of the standard and are not reproduced here, so the Method A block on this page computes CEV and the combined thickness and records which hydrogen scale you are working to. You read the temperature off the diagram in your own copy.
How to calculate a preheat temperature
Four inputs, in the order the paperwork gives them to you.
- Take the composition from the mill certificate. Enter carbon, manganese, molybdenum, chromium, copper and nickel in weight percent. The tool returns CET for the equation and CEV for the diagram method.
- Enter the governing plate thickness. Use the thickness of the plate itself, and where the two plates differ take the thicker one. Fill the combined-thickness block separately if you are working to the diagram method.
- Enter the hydrogen content of the consumable. Diffusible hydrogen in ml per 100 g of weld metal, determined to ISO 3690 and printed on the consumable datasheet. A basic low-hydrogen electrode and a rutile one differ by a factor that changes the answer by tens of degrees.
- Enter the heat input from the WPS. In kJ/mm, after the process thermal efficiency factor if your WPS states arc energy instead. Read the resulting temperature as the minimum preheat and the minimum interpass temperature together.
- Check the warnings before you use the number. The tool reports when an input falls outside the range the equation is stated for. Outside that band the result is an extrapolation and needs a metallurgist, not a calculator.
- The equation has a stated range and the tool says when you leave it. CET from 0.20 to 0.50 %, plate thickness 10 to 90 mm, hydrogen 1 to 20 ml/100 g, heat input 0.5 to 4.0 kJ/mm, parent metal up to 1000 N/mm² yield. Outside those bounds the arithmetic still produces a number and the number stops meaning what it says.
- A negative result is not a licence to weld cold. Where the equation returns a temperature at or below ambient it is saying no preheat is needed for hydrogen cracking on those inputs. Moisture on the joint, frost, and a cold shop floor are separate problems, and they are usually why a fabricator warms a joint that the equation says needs nothing.
- Preheat is a floor for the whole weld, not an event at the start. The same number is the minimum interpass temperature. On a multi-run joint the risk sits in the pass welded after a break, when the plate has dropped back below the minimum and nobody re-measured — the failure mode hydrogen-induced cracking feeds on.
- Heat input has to be the right quantity. If the WPS quotes arc energy, apply the process thermal efficiency factor before entering it — the heat input calculator does that conversion, and the heat input guide explains which convention each standard means.
- Recording beats calculating. The number above is what the joint must reach. HeatCam IR-C measures what it actually reached, pass by pass, which is the part an auditor can check.
For measurement practice rather than arithmetic, read preheat and interpass temperature monitoring to ISO 13916.
Questions engineers ask about this calculation
How is the minimum preheat temperature calculated in EN 1011-2?
Annex C of EN 1011-2 offers two routes. Method B is an equation: Tp = 697 × CET + 160 × tanh(d/35) + 62 × HD^0.35 + (53 × CET − 32) × Q − 328, with the temperature in °C, the plate thickness d in mm, the diffusible hydrogen HD in ml per 100 g of weld metal and the heat input Q in kJ/mm. Method A instead reads the temperature off diagrams keyed on the carbon equivalent CEV, the combined thickness, a hydrogen scale and the heat input. This page runs the equation and prepares the four values the diagram needs.
Does the preheat calculation use plate thickness or combined thickness?
The equation uses the plate thickness, and where the two plates differ the thicker one governs. Combined thickness — the sum of the material thicknesses meeting at the joint, measured a short distance from the joint line — belongs to the diagram method, where a T-joint of three 20 mm members behaves thermally like a much heavier section than any one of them. Feeding a combined thickness into the equation inflates the answer, because the tanh term is already saturating by 90 mm.
Why does more heat input lower the required preheat temperature?
Both preheat and heat input act on the same variable: how fast the joint passes through the temperature range where hydrogen becomes trapped in a hard, crack-sensitive microstructure. A larger heat input slows the cooling on its own, so less external heat is needed to reach the same cooling time — which is why the heat-input term carries a negative coefficient for any CET below about 0.60. It is a trade, not a discount: raising heat input to avoid preheating lengthens t8/5 and can push the heat-affected zone out of the toughness window from the other side.
Is the calculated preheat temperature enough on its own to prevent hydrogen cracking?
No. It addresses one leg of the problem, the cooling rate. Hydrogen cracking needs susceptible microstructure, dissolved hydrogen and tensile stress together, so consumable handling, joint cleanliness and restraint matter as much as the number. The other half of the job is proving the joint actually reached and held the temperature: a chalk stick touched to the plate says nothing about the metal 100 mm along the seam, and the temperature has to hold as the minimum interpass value for the whole of a multi-run weld.
A preheat number is a claim until something records it.
HeatCam thermal cameras watch the joint continuously and log the surface temperature for every pass, so the preheat and interpass minima in the WPS become a record with a timestamp instead of a chalk mark someone remembers making.
