Acontrolled quenching process has been devised for a certain material. the material is a sphere of radius r0 = 5 mm and is initially at a uniform temperature of 400°c. it is subjected to a two-step cooling process as follows: step 1: cooling in air at 20°c until the centre of the sphere reaches critical value of 335°c, with a convective heat transfer coefficient of ha=10 w. m-2.k-1. step 2: cooling in a water bath maintained at 20°c until the centre of the sphere is at least 50°c, with a convective heat transfer coefficient of hw = 6000 w. m-2.k-1. the thermo-physical properties of the material are: rho = 3000 kg. m-3 k = 15 w. m-1.k-1 cp = 1000 j. kg-1.k-1 α=7.4x10-7m2.s-1 i) calculate the time required to complete the first step of the cooling process. /2 ii) calculate the minimum time required for step 2.