Carbon Equivalent & Preheat
Hydrogen cracking needs three things at once: a hardened heat-affected zone, hydrogen, and restraint. The carbon equivalent measures how readily the steel hardens. Preheat works on all three — it slows the cooling, and gives hydrogen time to diffuse out.
Codes set preheat mainly by thickness, because the thickest part at the weld is the heat sink that sets the cooling rate. Chemistry decides which category a steel sits in; thickness decides how hot within it.
Carbon equivalent (weight %)
CE(IIW) = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B. Take the chemistry from the mill certificate, not the grade's maximums.
Prequalified minimum preheat — AWS D1.1
The preheat table and its notes are the AWS D1.1 prequalified values as reproduced in UFC / TI 809-26 (2000); later editions have revised and renumbered it, so confirm against the edition your contract names. Prequalified minimums do not cover high restraint, wet consumables or steels outside the listed categories — use a qualified procedure or EN 1011-2 there. The carbon equivalent reading is screening only. A helper, not a substitute for engineering judgment or the governing code.