On the Changes of Residual Stresses Resulted from Carburizing and Quenching due to Aging and Repeated Stressing

Residual stresses resulted from carburizing and quenching were measured dy the etching method, using plate specimens of two kinds of steels. The microstructure after quenching is likely to include retained austenite due to high carbon content of the carburized layer, which may decompose into martensite in time at room temperature. To investigate the effect of aging on the distribution of residual stress, stress measurements were made on the specimens which were kept in rest at room temperature for 1, 7, 15, 23, and 30 days after quenching separately. The effect of repeated stressing on the residual stress was also examined by applying the stress amplitude below the endurance limit. Fatigue tests for the carburized specimens and those which were subjected to subsequent quenching were made to investigate the improvement of fatigue strength by quenching process. The results obtained were as follows; (1) Residual stresses which result from carburizing and quenching are simple thermal stress type, being compression at the surface and tension in the core. The stress reversal occurs neas the boundary between case and core. (2) Tempering process reduces the initial residual stresses considerably. (3) surface residual stresses tend to decrease during the aging period of seven days after quenching and then turn to increase noticeably for following fifteen days, thenceforward turning again to diminish gradually. (4) Surface residual stresses subjected to repeated stressing decrease during the first few fraction of ten-thousands of cycles and thence turn to increase untill the most severe distribution of residual stress is developed, being followed by gradual diminution. After 107 cycles they do not fade completely and have the values about 60% of the initial values. (5) Among the fractured specimens of the fatigue test, fish eyes were recognized. Crack seems to initiate at the region slightly inside of the carburized layer, where tension peak occurs as the combined result of the bending and the residual stresses. (6) Numbers of cycles to fracture have remarkable difference between the carburized and quenched specimens and as-carburized specimens, and accordingly surface compressive residual stresses could not be neglected in regard to their role in increasing fatigue strength. (Received June 5, 1958)