Calculations are presented for the temerature history of points in a 61S-T6 aluminum plate under varying connditions of source velocity and heat input. These conditions were subsequently duplicated by experiments in which a tungsten or a consumable electrode arc became the hypothetical point source of heat. The decrease in hardness at any point in the plate is predicted from a temperature modified time versus transformation curve@ developed from isothermal reaction rates. The predicted hardness values are compared with the measured hardness values as a check on the applicability of the analytical method.