Steel balls with a 8 mm diameter are treated by heating in a furnace. The balls are first heated up to 900 oC and then cooled slowly to 100 oC in ambient air at 35 oC. If the average heat transfer coefficient is 75 W/m2.oC, determine how long the treatment will take? (Properties for steel ball: = 7833 kg/m3, k = 54 W/m.oC, Cp = 0.465 kJ/kg. oC).