Toughening and its Application of Austempered Ductile Cast Iron by Thermomechanically Treatment

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In the fiscal year 1998, effects of rolling reduction, austempering temperature and its time during thermo-mechanical treatments on toughness of the ADI were investigated. In the fiscal year 1999, in order for the optimization and efficiency of the thermo-mechanical treatment conditions, effects of graphite diameter and austempering temperature on toughness were investigated. In the fiscal year 2000, effects of rolling reduction and stability of Austenite on strength and toughness were evaluated.It has been certified that the effects of the graphite size on the fracture toughness of the ADI is almost negligible.. On the other hand, according to the analysis of the relationship between the gamma pool and toughness, the gamma pool is predominant over the fracture toughness properties of the ADI.Although shear bands which are formed in-between the graphite nodules act as a nucleation site for the beinitic ferrite effectively, it sometimes causes fracture there. It has been concluded that … More large graphite nodules are preferable for the practical application of the thermo-mechanical treatment. As for the absorbed energy, the maximum value was obtained for the rolling reduction of 40% and austempering time of 14.4ks, while the fracture toughness reached maximum for the rolling reduction of 60%. The tensile strength exceeds 1000MPa when the rolling reduction ranges 0 to 4O%, and the elongation reached the maximum for the 4O% reduction. The residual Austenite decreased and its stability increased within the range and deformation-induced Martensite transformation disappeared in the high rolling reduction range.Comprehensively speaking, the rolling reduction of 40% is concluded as the best condition from a viewpoint of the balance between strength and toughness. Strength becomes higher in the lower rolling reduction side, while toughness becomes superior in the higher side. The optimization of characteristics of respective portion becomes possible together with near-net-shape forming by utilizing the knowledge obtained above. By performing microstructural and component designs of the ductile cast irons on the basis of the above-mentioned concepts, predominance over the traditional steel can be recognized which enables substitution from the steels. Less