1995 Volume 35 Issue 4 Pages 402-408
The morphology of MnS inclusions in steel formed during solidification and the modification of this morphology by additions of alloying elements Al, Si, C and Ti have been investigated using optical and electron microscopy techniques.
In sulfur-lean melts the morphology of the secondry MnS inclusion formed after the primary crystallization of the Fe phase during solidification can be classified under the following categories: i) Globular or droplet shaped MnS resulting from a monotectic reaction, ii) Rod like MnS formed through an eutectic reaction and iii) a fish-bone type MnS formed as a result of an irregular eutectic reaction. In sulfur-rich melts the primary crystallization phase is MnS and depending on the nature of additional elements present in the melt and the melt atmosphere, the morphology of these primary MnS inclusions can be classified as iv) spherical, v) dendritic or vi) angular.
The formation mechanism of the different MnS morphologies is discussed on the basis of phase diagram information. It sis shown that the spherical morphology of the primary and secondary MnS inclusions is a result of metastable reactions, while the eutectic, dendritic and anglular morphologies are the products of stable reactions, with the high melting dispersed particles like TiN and Al2O3 acting as nucleants for the MnS crystals. It is also shown that the addition of C and Si increases the probability of stable reactions by increasing the temperature interval between the eutectic and the monotectic points.
