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
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 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.