オーステンパー処理した球状黒鉛鋳鉄の衝撃特性

抄録

  Austempered bainitic ductile iron exhibits a favorable combination of strength and toughness, and has been successfully applied to parts like chain sprockets, gears and pinions. However, since there have been only few studies on the mechanism of bainitic transformation in cast iron, the desired structure can not be reliably obtained. In this study ductile irons with various silicon content were austempered and the relationship among impact properties, isothermal transformation diagram and quantity of retained austenite was investigated. It was found that the isothermal holding time giving the maximum impact values can be predicted from the B_f point detected by dilatometer at each temperature, and there is a positive correlation between the impact value and the amount of retained austenite. As the silicon content increases, the precipitation of carbide during bainite transformation is suppressed, and the maximum impact value and amount of retained austenite increase. Silicon also reduces the rates of their decrease from the maxima after an excessively long holding time. The retained austenite partially transforms to martensite by holding below −75 degree C, decreasing impact value and increasing hardness.