Effects of Successive-stage Austempering on the Structure and Impact Strength of High-Mn Ductile Iron

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Abstract
In the austempering of ductile cast iron, it is necessary to alloy the metal to prevent pearlite transformation. The addition of alloying elements delays the pearlite transformation and at the same time, causes a similar delay in the beginning of the stage I reaction. In general, alloying addition levels greater than a specified amount are not desirable in the conventional austempering process due to the decrease in toughness and ductility. In this experiment, 1% manganese alloyed ductile iron was used in the successive-stage, High-Low Austempering Temperature Process (HLAT Process) in different time combinations of high and low temperature austempering to improve the toughness of high manganese ductile cast iron. The impact strength provided by the HLAT Process is a function of high temperature austempering time as well as low temperature austempering time. The impact strength at the best process combination is much higher than conventional high manganese ADI and well comparable to low alloy austempered ductile iron. The retained austenite volume fraction resulting from the HLAT Process is higher than that from the high austempering temperature process and the volume of untransformed austenite is much lower. When intergraphite nodule distance is small, manganese segregation is less and the reaction rate of austempering in that area is more than in the high Mn segregated region, thus reducing the volume of untransformed austenite.

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