Arc welding is carried out at a voltage of 25 V, a current of 200 A and a welding speed of 300 mm/min. With an arc thermal efficiency of 0.80, which value is closest to the heat input per mm of weld, Q = ηVI/v?

Answer: B. About 0.80 kJ/mm

Converting the welding speed to per second gives v = 300/60 = 5.0 mm/s. The arc power is VI = 25 × 200 = 5000 W, so Q = 0.80 × 5000/5.0 = 800 J/mm = 0.80 kJ/mm. About 1.0 kJ/mm forgets to multiply by the thermal efficiency, and about 13 J/mm divides by the per-minute value of 300 as is. A larger heat input slows the cooling of the weld, making the heat-affected zone less likely to harden, but it lengthens the time at high temperature, so grains can coarsen and toughness can fall. Upper and lower limits on heat input are therefore set according to the type of steel to control the cooling rate.