Steel balls 12 mm in diameter are annealed by heating to 1150 K and then slowly cooling to 400 K in an air environment for which T? = 325 K and h = 20 W/m2/K. Assuming the properties of the steel to be k = 40 W/m2K, ? = 7800 kg/m3, and c = 600 J/kg/K, estimate the time required for the cooling process.