BLID 5.71 (5.57 in 5th edition) In heat treating to harden steel ball bearings (c=500J/kgK,=7800kg/m3,k=50 W/mK ), it is desirable to increase the surface temperature for a short time without significantly warming the interior of the ball. This type of heating can be accomplished by sudden immersion of the ball in a molten salt bath with T=1300K and h=5000 W/m2K. Assume that any location within the ball whose temperature exceeds 1000K will be hardened. Estimate the time required to harden the outer millimeter of a ball of diameter 20mm, if its initial temperature is 300K