A New Look at Subcritical Quenching (SCT) for Fatigue Strength Improvement

A comprehensive summary of in-house information concerned with thermal prestressing of large forgings for fatigue strength improvement is presented. Seven commercial grades of carbon and alloy steel railway forgings with and without a subcritical quenching treatment (SCT) are compared. Conventional designs of full-size press-fitted assemblies and a filleted shaft were tested in rotating bending to evaluate the treatment. Fatigue strength improvement values ranging from 45 to over 130 percent have been established for breaking off in the wheel fit, depending upon the yield strength of the shaft material and the configuration at the press fit. Compressive residual stresses induced by the treatment were measured for most materials, and in some instances the loss of surface compression due to cyclic stressing was evaluated. An example of practical use of the information summarized is cited by a discussion of the application of SCT in the development of a tubular axle for transit car service. Information being presented in this paper updates published literature with current technology pertinent to the application of thermal prestressing to a variety of large forging materials. It provides a comprehensive reference background to substantiate and stimulate greater use of the treatment by others confronted with the design of fatigue resistant assemblies.