Austempering heat treatments of ductile iron using molten metal baths

摘要

This work aims to evaluate the use of two different zinc-tin and zinc-aluminum molten metal baths on austempering heat treatments performed in ductile cast iron. Samples were extracted from as-cast standard Y-blocks for austempering heat treatments. The samples were heated for austenitization at 910 degrees C for 90min and further cooled in two different molten metal baths for austempering: zinc-tin and zinc-aluminum alloys at 370 and 400 degrees C, respectively, for 30, 60 and 90min. The Zn-50wt% Sn hypoeutectic alloy and the Zn-5wt% Al eutectic alloy were chosen for the molten metal baths. After heat treatments, the samples were analyzed by optical and scanning electron microscopy, Brinell hardness, Vickers microhardness, Charpy impact and tensile tests, and fracture mode analysis. The results indicated the viability of using Zn-Al and Zn-Sn molten metal baths as a substitute of molten salts. When the austempering temperature was increased from 370 to 400 degrees C, the hardness, tensile strength, and elongation decreased, while impact energy increased. The ideal processing parameters were obtained for austempering at 370 degrees C for 60min, where the austempered ductile cast iron presented a microstructure completely formed by finer ausferrite.