Effect of Raw Materials, Mn and P on the Impact Properties of ADI

Austempered ductile iron (ADI) exhibits a favorable combination of strength and toughness, and has been used as a substitute material for steel, for example, ball joint sockets and sliding parts of air-conditioner for automobile. ADI contains a large amount of retained austenite which contributes to the improvement of ductility and toughness. The content, distribution and morphology of the retained austenite are affected by the contents of alloying and trace elements. However, the effects of the raw materials and trace and alloying elements have not been clear. In this study, ductile irons were made from high purity pig iron, ordinary pig iron or high purity steel scrap. Manganese up to 0.7% and phosphorous up to 0.1% were added to the irons. Some of them were annealed in order to homogenize alloying and trace elements before austempering. The content, distribution and morphology of the retained austenite and the impact properties were measured. Following results were obtained. By using high purity ductile iron with low Mn content which made from high purity pig iron or high purity steel scrap, the rate of transformation of austenite to bainite is accelarated, and high impact property is obtained in a short period. In ADI with high Mn content, the rate is lowered by the segregation of Mn and the impact value is decreased by the large untransformed Mn-segregated area. High impact values are obtained by Mn-homogenizing anneal and by decreasing of untransformed austenite. In ADI with high phosphorous content, the impact property decreases a little.