Preheat Temperature Calculator

Carbon equivalent (CE) rolls a steel's full alloy chemistry into one number that predicts how susceptible it is to hydrogen-induced cracking during welding, and this calculator uses the IIW (International Institute of Welding) formula — carbon plus manganese divided by 6, plus chromium and molybdenum combined divided by 5, plus nickel divided by 15 — to compute it from the percentages you enter. The IIW formally adopted this formula in 1967, building on Dearden and O'Neill's original 1940 hardenability work; the complete published version also carries a vanadium term alongside chromium and molybdenum, and a copper term alongside nickel, both of which this calculator omits since they're typically negligible in ordinary carbon and carbon-manganese structural steel and rarely appear on a standard mill certificate. Higher-alloy steels harden more readily in the heat-affected zone next to a weld, and that hardened, brittle microstructure is exactly what cracks under the residual stress and trapped hydrogen a weld leaves behind; preheating the base metal slows the cooling rate enough to avoid forming that brittle structure in the first place. The calculator sorts CE into three weldability bands — good below 0.45, fair up to 0.6, and difficult above that — and scales the recommended preheat temperature upward within each band based on both CE and material thickness, since thicker sections pull heat away from the weld faster and need more preheat to achieve the same cooling-rate protection.

The result is capped at 600°F, since beyond that point other precautions (such as post-weld heat treatment) typically become part of the welding procedure rather than preheat alone. The interpass temperature — the maximum the joint should reach between weld passes — is set 200°F above the preheat minimum as a practical ceiling to avoid degrading mechanical properties from excessive heat buildup. This tool applies a general engineering heuristic tying CE and thickness to preheat, not a substitute for a qualified Welding Procedure Specification, which should always be verified against the applicable code and the base metal's actual specification.