Abstract
Microstructural observations and measurements of the retained austenite content, hardness, austenite C content, and unreacted austenite content are reported during austempering at 400, 375, 320, and 285°C after austenitising at 920°C for a ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0·007%P, 0·013%S, 0·25%Mo, 0·25%Cu, and 0·04%Mg. The segregation of solute during solidification to the intercellular areas is shown to result in the stage I reactions in the eutectic cell and intercellular areas being separated in austempering time. Evidence is provided of the occurrence of the stage II reaction before the completion of the stage I reaction. The consequence of this sequence of changes is that the processing window as defined for unalloyed irons and known to correspond to optimum mechanical properties is closed for all austempering temperatures.
MST/3053
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