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

Abstract

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 the synthetic weld thermal cycle method. Heating SSFDI at 300°C s− 1 (200–800°C), the reverse transformation started at 837°C and finished at 1149°C. The austenite arose around graphite nodules and the fraction in the 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 ferrite-pearlite type ductile cast iron at 1000°C. In the case of the cooling parameter t8/5 = 25 s, 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 un SSFDI.