Effect of Microstructural Features on the Austempering Heat Treatment Processing Window

Abstract

Microstructural changes during the austempering heat treatment of alloyed ductile iron are described. The effect of heat treatment cycle and casting structure on mechanical properties of Austempered Ductile Iron, ADI, of composition 3.52%C, 2.64%Si, 0.25%Mo, 0.25%Cu, 0.67%Mn are studied. Comparison of EDX analysis with mechanical properties show that the optimised properties for an ADI are achieved when an ausferritic structure of high carbon austenite and ferritic plates with minimum martensite and carbides is formed. Processing Window is the period of austempering time over which the structure is fully ausferritic The results show that for a high Mn DI, single austempering process does not provide a fully ausferntic structure and usually further austempering process, double or stepped austempering process, is required. The experimental results demonstrate that such an ADI still might satisfy the ASTM standard with careful control over austempering parameters. This study confirms that some casting features like low nodules count and preferential segregation of alloying elements narrow the processing window This study also shows that the microstructural defects like microporosity and poor modularity have a detrimental effect on properties of ADI.