The figure shows the graphite morphology of vermicular graphite cast iron with different wall thickness. Among them, figures (a) – (c) are graphite morphology diagrams in samples with thickness of 20,40 and 60mm respectively.
It can be seen from the figure that the wall thickness of the sample has a great influence on the creep rate of vermicular graphite cast iron. With the increase of sample wall thickness, the amount of vermicular graphite in cast iron alloy increases, that is, the vermicular rate of vermicular graphite cast iron increases. In the sample with 20mm wall thickness, the graphite morphology is mainly spherical graphite, and the size of spherical graphite is small and the number is large, while the content of vermicular graphite is small and the length is short. This is because the wall thickness of the sample is small, the solidification speed of the sample is fast, the solidification undercooling is large, and the rare earth, magnesium and other alloy elements in the vermicularizing agent have less burning loss and more residues. The residual magnesium and rare earth elements are adsorbed on the a-axis of graphite, which inhibits the accumulation of graphite atoms on the a-axis and reduces the growth rate in the a-direction. At the same time, the diffusion and accumulation of carbon atoms in the metal melt to the c-axis are accelerated, which increases the growth rate in the c-axis direction and makes the graphite morphology in the alloy change from vermicular to spherical. When the wall thickness of the sample increases, the residence time of the metal melt at high temperature increases, and the vermicularizing agent is burned seriously, which reduces the content of residual magnesium and residual rare earth in the solidified cast iron alloy. During the solidification process, the growth of stone and plastic in the metal melt is mainly symbiotic growth in C and a directions, and finally vermicular graphite and spherical graphite are generated, Vermicular graphite is the main graphite form of vermicular graphite cast iron.