A short cylindrical metal billet of radius R and height h is heated in an oven to a temperature of 600C throughout and is then taken out of the oven and allowed to cool in ambient air at T = 65C by convection and radiation. Assuming the billet is cooled uniformly from all outer surfaces and the variation of the thermal conductivity of the material with temperature is negligible, obtain the differential equation that describes the variation of the temperature in the billet during this cooling process. Appropriate assumptions should be explicitly stated.