Solute concentration and carbides formation for steel milling rolls

The selection and the formation of carbide govern quality and the service life of hot steel milling rolls. Thus the microsegregation and the formation of the carbides and the graphite during the solidification have been investigated for high speed steel type cast iron and Ni-hard type cast iron. The crystallization of high speed steel type cast iron proceeds in the order of primary austenite (γ), γ+MC and γ+M₂C eutectic. On the other hand, in Ni-hard type cast iron, eutectic graphite flakes can crystallize after the formation of primary γ, and γ+M₃C eutectic by controlling the content of Ni and Si in spite of containing strong carbide formers such as Cr. As γ+carbide eutectic grows, the residual liquid among eutectic cells becomes rich or poor in carbide formers according the partition coefficient between residual liquid and eutectic cell. It is realized that the change of composition of carbide formers during the solidification is estimated with Scheil-Gulliver equation and computed phase diagrams in both of cast irons. The graphite forming tendency is also evaluated by applying the parameter, which expresses the solubility limit of carbon to the molten iron for Ni-hard type cast iron.