Transformation process of weld heat affected zone in solution strengthened ferritic ductile cast iron

The purpose of this study was to examine the weldability of Si-solution strengthened ferritic ductile cast iron (SSFDI: EN-GJS-500-14) compared with a traditional ferrite-pearlite type ductile cast iron of similar strength (JIS-FCD500-7). The α+graphite→γ reverse transformation characteristics on heating process and the transformation characteristics on cooling were investigated by synthetic weld thermal cycle method. Heating SSFDI at 300°C/s (200-800 °C), the reverse transformation started at 837 °C and finished at 1149 °C. The austenite arose around graphite nodules and the area fraction in matrix moderately increased to 100% as the temperature became higher. In the ferrite-pearlite type ductile cast iron FCD500 with 2.27%Si, the reverse transformation occurred in the temperature range from 763 °C to 1105 °C. The pearlite rapidly transformed to austenite and then the ferrite moderately transformed. The austenitization rate of SSFDI was also lower than the ferritepearlite type ductile cast iron at 1000 °C. In case of the cooling parameter t8/5=25s, the martensite area fraction increased as the maximum temperature rose. Less martensite appears in SSFDI, indicating arc welding would give rise to less brittle heat affected zone (HAZ) in SSFDI.