Influence of Transformation Time on Fatigue Properties of Austempered Ductile Iron

Abstract

Mechanical properties determined by tensile and fatigue tests of ADI transformed at the temperature of 400 °C during 11 various transformation dwells in the range from 2 minutes to 25 hours are confronted with obtained matrix structure. Maximum content of retained austenite is obtained for the dwell of 40 minutes for which also the maximum of elongation to fracture was observed. For shorter dwells UTS substantially increases with transformation time while for longer dwells the value of UTS is nearly constant. Yield stress increases in the whole range of transformation time. Fatigue limit is the mechanical characteristic relatively closely connected with the content of retained austenite: the higher its content, the higher fatigue limit. Optimal combination of tensile properties is obtained in rather narrow range of transformation time from 40 to 60 minutes where the maximum of elongation to fracture is observed. The range of the highest values of fatigue limit is wider (10 to 100 minutes with fatigue limit of 260 to 270 MPa) because its dependence on transformation time is weaker. This rather wide technological window means that ADI is structural material not very sensitive to keeping technological conditions and, therefore, its good fatigue properties can be obtained also in mass production or in plants with average level of technology.