Abstract
In the previous study on the fatigue crack propagation characteristics of ductile cast iron, it was found that the propagation resistance could be improved by austempering from (α+γ) phase region (partial-austempering) without decreasing static strength, especially in the low ΔK region. In this paper, to improve the static strength in addition to the propagation resistance without decreasing ductility, the influence of treating temperture in (α+γ) phase region was examined. Microstructure inspection, tensile tests, fatigue tests (stress ratio, R=0.1) and fractography were carried out on the ductile cast iron specimens which were partial-austempered at four different treating temperatures (800°C, 815°C, 830°C and 840°C). The results obtained were compared with those of the as-cast material whose matrix-structure consisted of ferrite (about 60%) and pearlite (about 40%) and the materials whose matrix-structures were changed into ferrite (about 100%) or bainite (about 100%) by heat-treating.
The matrix-structure of the partial-austempered materials consisted of ferrite and bainite. When partial-austempered at 800°C, 815°C, 830°C and 840°C, the ratio of bainite became approximately 20%, 40%, 80% and 90%, respectively. The static strength (0.2% proof stress and tensile strength) was improved with an increase of the ratio of bainite, but the ductility and propagation resistance decreased except the propagation resistance in the material whose matrix-structure was ferrite. The reasons were discussed in terms of fractography. The most effective treating temperature to improve the static strength and propagation resistance without decreasing ductility was 830°C for the ductile cast iron used in this study.
