Stainless steel ball bearings [ρ = 8085 kg/ m 3 , k = 15.1 W/m.K, C = 480 J/kg.K], 1.2 cm in diameter are taken from an oven at a uniform temperature of 900°C and are exposed to air at 30°C with h =...

Stainless steel ball bearings [ρ = 8085 kg/ m³, k = 15.1 W/m.K, C = 480 J/kg.K], 1.2 cm in diameter are taken from an oven at a uniform temperature of 900°C and are exposed to air at 30°C with h = 125 W/ m².K, for a short period and then they are dropped into water for quenching. If the temperature of balls does not fall below 850°C prior to quenching, calculate, how long they stand in air before being dropped into water ?