A solid steel ball (k = 35 W/m.K), 300 mm in diameter is coated with a dieletric material (k = 0.04 W/m.K), 2 mm thick. The coated sphere is initially at a uniform temperature of 500°C and is suddenly...

A solid steel ball (k = 35 W/m.K), 300 mm in diameter is coated with a dieletric material (k = 0.04 W/m.K), 2 mm thick. The coated sphere is initially at a uniform temperature of 500°C and is suddenly quenched in a large oil bath at 100° with h = 3300 W/ m².K. Calculate the time required for coated steel sphere to reach 140°C.