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Carbon equivalent formula and preheat temperature for welding A36, 4140 and carbon steel

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 — the IIW formula. Below 0.40, most steel needs no preheat at all. Run the numbers for 4140 chromoly, though, and it lands at 0.772, which WeldMaster puts in the 400 – 700 °F band. Trust this only with the mill cert in hand; the Preheat / CE calculator (Pro) works the formula from that certificate and the thickness you give it.

3 min read · Updated 2026-09-15

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What does carbon equivalent tell you about weldability?

Carbon and alloying elements make steel harder as it cools after welding. Hard, brittle zones plus hydrogen cause cold cracking — that combination is the thing to avoid. The carbon equivalent rolls the whole composition into one number. Higher number, more preheat, slower cooling.

What is the carbon equivalent formula?

WeldMaster uses the IIW formula, with each element in weight percent from the mill cert:

  • CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
  • A36 preset (C 0.26, Mn 0.75): 0.26 + 0.75/6 = 0.385
  • 4140 preset (C 0.40, Mn 0.85, Cr 0.95, Mo 0.20): 0.40 + 0.85/6 + (0.95 + 0.20)/5 = 0.772

What preheat band does each carbon equivalent need?

WeldMaster preheat bands (°C are exact conversions)
Carbon equivalentPreheatIn °C
under 0.40None required—
0.40 to under 0.4570 – 200 °F21 – 93 °C
0.45 to under 0.60200 – 400 °F93 – 204 °C
0.60 and up400 – 700 °F204 – 371 °C

Thick sections cool faster, so for material over 1" the app moves the light and medium bands up one level and shows a warning. With no preheat needed, it still says to weld above 50 °F (10 °C).

What are worked carbon equivalent examples?

Preheat / CE results
SteelThicknessCarbon EquivalentPreheat
A36 Mild Steel preset1/4"0.385None required
C 0.22, Mn 1.201/2"0.42070 – 200 °F
C 0.22, Mn 1.201 1/2"0.420200 – 400 °F (bumped for the section over 1")
C 0.30, Mn 1.001/4"0.467200 – 400 °F
4140 Chromoly preset1/2"0.772400 – 700 °F

What is the preheat temperature for welding 4140?

With the 4140 Chromoly preset, the CE is 0.772, well above 0.60, so the app shows 400 – 700 °F (204 – 371 °C) at any thickness, with the advice to control interpass temperature and consider post-weld heat treatment. For real 4140 work, the heat-treat condition and the procedure matter as much as the number, so check the WPS.

What is the preheat temperature for welding carbon steel?

Plain carbon steel depends on its carbon and manganese. The A36 preset lands at 0.385, so no preheat at 1/4". A carbon steel with C 0.30 and Mn 1.00 reaches 0.467 and moves into the 200 – 400 °F band. That is why the mill cert matters more than the grade name.

How do you use the Preheat / CE calculator?

  1. Open Preheat / CE from Calculators (Pro).
  2. Under Composition Presets, tap A36 Mild Steel or 4140 Chromoly, or type your own values under Composition (weight %): Carbon (C), Manganese (Mn), Chromium (Cr), Molybdenum (Mo), Vanadium (V), Nickel (Ni) and Copper (Cu).
  3. Set the Thickness, from 0.06" to 3".
  4. Read Carbon Equivalent and Preheat under Preheat Guidance, plus the recommendation text.

What does this estimate leave out?

  • Hydrogen level of the consumable: a low-hydrogen process can allow less preheat, a damp rod needs more.
  • Restraint: a stiff, locked-in joint cracks more easily than a free one.
  • Heat input: a hot, slow pass cools more slowly than a fast, cold one.
  • Other elements such as boron are not in the IIW formula.

Questions and answers

What is a good carbon equivalent for weldability?
Under 0.40 is generally easy to weld with no preheat, and that is where WeldMaster shows None required. From 0.40 the app starts suggesting preheat, and at 0.60 and up it suggests 400 – 700 °F.
Where do I get the composition?
From the mill test certificate of the heat of steel you are welding. The presets use typical values for A36 and 4140, which may differ from your material.
Does thickness change the carbon equivalent?
No. Thickness only matters over 1", where the app raises the light and medium preheat bands one level.
Is the Preheat / CE calculator free?
No, it is part of WeldMaster Pro, a one-time purchase.

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