Stainless steel ball bearings ( = 8085 kg/m 3 , k = 15.1 W/mC, c p =

Stainless steel ball bearings (ρ = 8085 kg/m³, k = 15.1 W/m·°C, c_p = 0.480 kJ/kg·°C, and α = 3.91 × 10^-6 m²/s) having a diameter of 1.2 cm are to be quenched in water. The balls leave the oven at a uniform temperature of 900°C and are exposed to air at 30°C for a while before they are dropped into the water. If the temperature of the balls is not to fall below 850°C prior to quenching and the heat transfer coefficient in the air is 125 W/m²·°C, determine how long they can stand in the air before being dropped into the water.