The rate at which designated points in a weld cool through well-defined temperature bands where metallurgical changes can occur. The rates of cooling depend on a number of factors, such as the thickness of the material being welded, its thermal conductivity, the preheat or interpass temperature, and the energy input used to make the weld. The effects these variables have on the cooling rate of steel are generally measured at 590 or 700°C (1 100 or 1300″F), temperatures at which significant metallurgical changes are expected to occur while cooling. The maximum cooling rates occur in heavy sections, generally thicker than about 25 mm (1 in.). In thinner sections the cooling rates will be less because the heat sink provided has been reduced.
In very thin sheet, the cooling rate due to a heat sink will be very low and most of the cooling will be due to radiation or convection. See CRITICAL COOLING RATE, PREHEAT, and INTERPASS TEMPERATURE, Welding.