Influence of Melting Conditions on Graphite Morphology in Ni–C Alloy and Grain Number of Matrix

抄録

The influence of the melting conditions on the graphite morphology and the grain number of the matrix in a Ni–C alloy was studied. Two groups of parent alloys were prepared using high purity materials, one group containing the spheroidizing element of Ce, Mg or Ca, while in the other group, S was added as an anti-spheroidizing element.
The spheroidizing element-added sample of 0.5 g was melted and held for 15 minutes at 1673 K in an Ar atmosphere and then cooled. At the high cooling rate of 1000 K/min, an irregular graphite appeared. When an Ar+3%H₂ atmosphere and a 10-min holding time at 1673 K were used, the formation of the spheroidal graphite was confirmed at the cooling rate of 1000 K/min in both groups.
The atmosphere and the holding time were then independently changed at the cooling rate of 1000 K/min. It was found that the 3%H₂ did not significantly affect the spheroidal graphite formation. Moreover, the holding times of 1 min and 20 min also did not significantly affect the spheroidal graphite area fraction in the Ni–C alloy, while they affected the graphite morphology of the samples containing the spheroidizing elements like Mg.
The grain number of the matrix around the graphite phase was also investigated in this study based on EBSD observations. The results showed that the matrix around the massive graphite morphology is polycrystalline, but is mainly single crystalline around the spheroidal graphite. The mechanism of the difference in the crystal structure of the matrix in the two different graphite morphologies was also discussed.