Phase transformation study of aluminium-containing ductile cast irons by dilatometry

The temperature for the start of martensite formation (MS) for three Al-alloyed ductile cast irons containing 0.48, 4.88 and 6.16 wt.%Al were determined by high-speed dilatometry. Microscopic observations were also used to indicate the effects of aluminium addition on the martensite transformation during quenching. Various heat treatment cycles, with different continuous cooling and isothermal heat treatments, were used for the experimental irons to indicate the phase transformation in the solid state. The examination confirmed that an austenite/martensite transformation occurs for irons containing 0.48 and 4.88 wt.%Al. The results indicate that martensitic transformation occurs at about 200 °C for 0.48 wt.%Al and about 190 °C for 4.88 wt.%Al. In contrast, there was no evidence of the martensitic transformation for 6.16 wt.%Al. It was further found that for the 4.88 wt.%Al experimental alloy used, isothermal transformation at 350 °C for different soaking times gave a typical bainitic microstructure that increased with increasing time of austempering.