INFLUENCE OF HEAT-TREATMENTS ON MICROSTRUCTURE OF AUSTEMPERED DUCTILEIRON

The influence of temperature and time of austempering on a nodular cas t iron austenitised for 30 min at 900 degrees C has been investigated using optical and transmission electron microscopy. The carbon content of the austenite has been evaluated by measuring the lattice paramete r via X-ray diffraction. The structure consisted of bainite containing retained austenite, the amount of which increased, and the carbon con tent of,which decreased, with increasing austempering temperature. No carbides were detected in the materials austempered at 300 and 370 deg rees C for up to 2 h, which contained only bainite and austenite. Afte r austempering at 410 degrees C and cooling to room temperature a cert ain amount of martensite was always detected. After long times at this temperature the austenite decomposed to ferrite and complex carbides. Martensite formation seems to be promoted by the decrease in the carb on content of austenite due to the precipitation of epsilon carbide ho mogeneously within it. During long austempering treatments at 410 degr ees C complex carbides are formed, probably from the epsilon carbides precipitated in the austenite.