炭素鋼の焼入による残留応力の時効および応力繰り返えしによる変化について

Abstract

Residual stresses greatly affect the fatigue strength of metallic materials, in such a way that compressive residual stress improves fatigue life and vice versa. However, the existence of residual stress means instability of structure and therefore natural trends of the change of residual stress must be considered when the influence of residual stress on fatigue strength is discussed, since the application of repeated stressing is to give an exitement to the unstable material structure.Following the study of authors on the change of residual stress caused by cold working due to stress repetition, the behaviour of residual stress initiated by heat treatment was studied in the same view point. The character of residual stress by heat treatment was classified in two categories:thermal residual stress-embedded in materials by heat treatment without transformation.transformation residual stress-embedded in materials by heat treatment accompanied by transformation.A high carbon steel with 0.88per cent carbon content was used for the material of specimens, which were 3.2mm in thickness. Specimens with thermal residual stress were prepared by heating at 500°C and subsequent quenching in water. Those for transformation residual stress were heated at 850°C and then quenched in oil. The distributions of residual stresses were observed by means of the corrosion method with 30 percent solution of nitric acid.A series of tests were carried out for the study of change of residual stress due to aging effect, and another series of tests were for the study of change due to the repetition of stresses in alternating bending.The conclusions obtained are as follows;1) The thermal residual stress changes in monotonous decrease with time. The same trend is observed in greately accelerated feature when alternating bending stresses are applied.2) The transformation residual stress remarkably decreases in the course of one week after quenching and then tends to increase slightly. The increase ceases around the end of the second week and then decreases monotonously. The character of residual stress variation was interpreted by the transformation process of retained austenite to martensite.3) The application of alternating bending stress to the structure with transformation residual stress accerelates and magnifies the trends of change described in the foregoing item. The magnitude of residual stress at the occasional increase was almost the same as the initial value.