The carbon equivalent used as a guide to the weldability of steel is Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 (mass %). Which value is closest to Ceq for a steel with 0.20% C, 1.20% Mn, 0.30% Cr, 0.10% Mo, 0% V, 0.30% Ni and 0.15% Cu?

Answer: B. About 0.51%

Calculating each term gives C = 0.20, Mn/6 = 0.20, (Cr + Mo + V)/5 = 0.40/5 = 0.08 and (Ni + Cu)/15 = 0.45/15 = 0.03, for a total of Ceq = 0.51%. About 0.40% stops after the C and Mn terms, and about 2.25% simply adds up the element contents without dividing by the coefficients. The carbon equivalent converts the effect of each alloying element on hardenability into an amount of carbon and sums them. The higher the Ceq of a steel, the more readily the weld heat-affected zone is quenched into hard martensite and the greater the risk of cold cracking. As a guide, steels with Ceq up to about 0.4% can often be welded without preheating, while above that, preheating becomes necessary depending on plate thickness, restraint and hydrogen content.