The formation of microstructure at the surface

The microstructure found in the surface and the center area is different. Figure 5 shows that the microstructure formation on specimen S4 is average and consists of a fully ferritic-matrix microstructure. Therefore, with the rapid cooling process, eutectic carbide formation and a microstructure containing cementite, ferrite, and pearlite are very uniform and compact on the surface area. As explained by Riposan L. et al. that the graphite carbide structure is formed at the end of the solidification step in the ductile iron casting process [34,35].
The microstructure results for Sample S1 are shown in Figure 6, using a preheating temperature of 500OC, with very uniform Ferro carbide in the edge area. Also, the phase structure is randomly formed around the graphite nodule, which is visible because the main material used for all cast specimens is ductile iron. Therefore, chill uses SS 304 material dominated by the composition elements of 19.34%Cr and 4.9%Ni (Table 1). The coating area is found to be a primary eutectic carbide or Cr-carbide structure and is defined as (FeCr)3C, where more details are explained in the SEM-EDX analysis results. According to Vander Voort and Baldwin, chromium carbide is formed with the cooling rate of solidification and the 5%Cr composition. This carbide structure characteristic is very good for high wear-resistant [36].
Figures 7 describe the sample S2 and the parameters using a preheating temperature of 700OC. The type of microstructure formed in the edge area is similar to the S1 sample, which is dominated by Ferro carbide. The phase structure found near the coating area, and the perlite phase around the graphite nodule was acicular ferrite. A ledeburite phase forms in areas far from the diffusion region, while a structure eutectic carbide, also known as a white cast iron layer, is formed in the coating area. The eutectic carbide structure is called (FeCr)7C3, and this structure is formed due to the diffusion of the Cr and Ni structures contained in the chill material at a preheating temperature of 700OC.
The microstructure formation on the surface of the S3 specimen using the preheating temperature of 900OC as shown in Figure 8, that the average microstructure consists of ferrous carbide and acicular ferrite near the diffusion zone with the small number of martensite phases. Meanwhile, Figure 8c shows that the diffusion layer formed on the surface has been shifting inward, this layer is the ledeburite phase or the so-called white cast iron layer which is dominant in the surface layer. These results as discussed by J. A. Pero-Sanz Elorz et al. [37].