Abstract: In a method of treating a coil spring to improve its performance and reliability when operating at elevated temperatures, the spring is first stressed by applying a load of preselected magnitude and then, while the load is maintained, the spring is heated until it relaxes or collapses to a predetermined extent at which stage the heating is automatically discontinued and the applied load is afterwards removed. Apparatus for carrying out the method in relation to compression springs includes a spring supporting base, an overhead operating head with a vertically movable ram or plunger for applying the predetermined load, means for passing an electric heating current through each spring while beneath the operating head, and limit switch means which responds to the position of the ram or plunger and terminates the supply of heating current when the loaded spring has reached the required predetermined value of relaxation or collapse.