In heat treating to harden steel ball bearings (C = 500 J/kg.K, ρ = 7800 kg/ m 3 , k = 50 W/m.K) initially at 27°C is desired to increase the surface temperature for a short time without significantly...

In heat treating to harden steel ball bearings (C = 500 J/kg.K, ρ = 7800 kg/ m³, k = 50 W/m.K) initially at 27°C is desired to increase the surface temperature for a short time without significantly warming the interior of the ball. This type of heating is obtained by sudden immersion in the molten salt bath at 1027°C with h = 5000 W/ m².K. Calculate the time required to reach the surface temperature of 20 mm diameter ball to 727°C.