Steel balls 12 mm in diameter are annealed by heating to 1100 K and then slowly cooling to 380 K in an air environment for which [infinity] 325 K and 20 W/m2·K. Assuming the properties of the steel to be 40 W/m·K, 7800 kg/m3, and 600 J/kg·K, estimate the time required for the cooling process.
