Weld Preheat Temperature Calculator
Estimate a minimum preheat temperature for carbon steel from its carbon equivalent and section thickness, as an educational guide.
How it works
CE = C + Mn ÷ 6 + (Cr+Mo+Ni+Cu…) ÷ 5 · Preheat = table lookup by CE band and thickness band
Preheating slows the cooling rate after welding, which lets hydrogen escape and prevents the hard, brittle martensite that causes cold cracking. The higher a steel’s carbon equivalent — a single number that rolls carbon and alloying into one hardenability index — and the thicker the section (which pulls heat away faster), the more preheat you need. This tool gives an educational starting estimate; a qualified welding procedure to a code such as AWS D1.1 or EN 1011 is the authority for the actual value.
Worked example
A steel with 0.18% C, 0.80% Mn and no extra alloying: CE = 0.18 + 0.80 ÷ 6 = 0.31. At 20 mm thick that falls in the low-CE, thin-section band, so the guide suggests about 20 °C (68 °F) — essentially just working above freezing and dry. A 0.45% carbon steel at 50 mm jumps to roughly 150 °C.
Frequently asked questions
What is carbon equivalent?
It is a single number that combines carbon with alloying elements (each weighted by how much it raises hardenability) to indicate how prone a steel is to hard, crack-sensitive microstructures. Above about 0.40–0.45 it usually signals the need for preheat.
Why does thicker steel need more preheat?
A thick section acts as a heat sink, pulling warmth out of the weld quickly. Faster cooling promotes crack-prone microstructures, so more preheat is used to slow the cooling rate.
Is this a substitute for a welding procedure?
No. This is an educational estimate. The governing value comes from a qualified Welding Procedure Specification to a code such as AWS D1.1 or EN 1011, which accounts for hydrogen level, restraint and the exact steel grade.
Related calculators
Educational estimate only — follow a qualified Welding Procedure Specification (e.g. AWS D1.1, EN 1011) for actual preheat requirements.