Stainless steel ball bearings (ρ = 8085 kg/m3, k = 15.1 W/m · °C, cp = 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 e×posed 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.
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