Study on Micro-Strain and Residual Stress of Austempered Ductile Iron

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Austempered ductile iron (ADI) has recently attracted special interest for its excellent strength and toughness, and has received practical application in some industry. The matrix structure of ADI is composed of bainite and retained austenite, which are not equilibrium phases in normal ductile irons at room temperature, so that two phases are estimated to be subject to a large strain. In this work, the micro-strain and the residual stress of ADI were examined by analyzing X-ray reflections of ferrite and austenite ; then, the rolling fatigue process and the sliding wear one were investigated by X-ray diffraction.Results obtained were summarized as follows ;1. A system was developed to measure and analyze X-ray reflection by using a general X-ray diffractometer and a personal computer. This system effectively operated to digitally measure X-ray reflections and calculate the Bragg angle, the half-value breadth, the integrated breadth and the integral intensity, and the lattice constant, … More the volume fraction of retained austenite, the micro-strain and the residual stress could be exactly obtained from analysis of the reflections.2. As to ADI treated at relatively low temperature, there was a isothermally held time range where the volume fraction and the dissolved carbon content of retained austenite did not vary with time. In this time range, however, the sub-structure and the micro-strain varied with holding time.3. The fatigue process and the the remainder of the fatigue life in the rolling contact of ADI could be estimated by tracing variations of X-ray residual stress, the psi-split and the half-value breadth or the integrated breadth of reflections of ferrite and austenite on the rolling surface.4. The rolling fatigue life and the sliding wear resistance of ADI were superior to ductile irons with pearlitic matrix structure of which hardness were similar to ADI. The volume fraction of the retained austenite on the surface decreased with rolling and sliding. These excellent properties were considered to be due to large volume fraction of the retained austenite. Less