浸炭焼入れを施したSCM420H 鋼の疲労過程における残留オーステナイトおよび残留応力の変化挙動

The transformation behavior of retained austenite in carburized SCM420H steel and its effect on the change in residual stress on a surface are investigated. The retained austenite on the carburized steel is transformed significantly in the first cycle of fatigue loading with a stress ratio of −1, and the transformation is less significant thereafter. Tensile loading significantly affects the transformation of retained austenite as compared with compressive loading. This is because the mechanical driving force that contributes to the martensitic transformation of retained austenite differs under tensile and compressive stresses. To investigate the transformation behavior of retained austenite and residual stress more comprehensively, an in situ X-ray measurement of retained austenite and residual stress is conducted. In the measurement, retained austenite or residual stress on a specimen surface is measured under loading conditions. Results show that the transformation of retained austenite under tensile loading involves a threshold stress value at which the transformation begins. The transformation of retained austenite proceeds when the applied stress exceeds the threshold value; however, no transformation occurs regardless of the number of loading cycles when the applied stress does not exceed the threshold. The compressive residual stress on the specimen surface increases as retained austenite transforms to martensite due to stress loading.