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/m ∙ K, p = 7800 kg/m³, and c = 600J/kg ∙ K, estimate the time required for the cooling process.