It is well-accepted that the life of the roll begins with the chemical composition, casting and solidification processes. In general, solidification is a heterogeneous transformation, where the growth of the crystallizing phase is controlled by heat transport. In case of horizontal centrifugal casting, this phenomenon becomes more complex, because of additional forced mixing of the molten metal while it is being crystallized. The convection heat transfer becomes predominant, where the local mass turbulence is driven by thermal gradients and centrifugal forces. As a consequence, the solid phase migrates, segregates and precipitates from the liquid to the outer or inner diameter of the shell depending upon its density. Possible chemical inhomogeneity through the shell layer in this case is due to so called “gravitational liquation” controlled by density difference between solid and liquid phases, also including insoluble liquid phases during crystallization [42, 43].
One of the fundamental issues of relatively large cast products is the formation of eutectic brittle phases and carbide networks. These are formed by the rejection of high alloy content into
the liquid between the growing and colliding dendrites. Such interdendritic segregations are very brittle, because of the carbides formed inside them, having crack-like defects and lattice misfit by default. As the matter of the fact, it considerably lowers the fracture toughness of the material and reduces alloying content in the matrix. The grown columnar dendritic structure is very dense, but because of the weak boundaries, the equiaxed eutectic austenite cells are more preferable [44].
As an example of developing eutectic carbides during solidification, A. Bedolla-Jucuinde et al. [45] conducted a detailed three-dimensional investigation of M7C3 (Cr-rich) carbide and discussed its nucleation and growth from the surface of primary dendrites.
This sort of a classical dendrite formation decorated with interdendritic eutectic carbides has even been observed even for rapid cooling with the rates about 106 Ks-1 [2, 46].
A simple description of centrifugal horizontal spin casting process to produce HSS rolls is illustrated in Figure 12. In the top row of figures, the material is melted and poured into cast, laid horizontal and spun. The HSS shell tube is produced by spinning the HSS melt inside the molds until solidification is completed. The middle row of pictures shows the monitoring of the process along with transferring the completed shell. In the bottom figures, the shell is placed in a mold and the ductile iron core is cast.
Figure 12. Main stages of horizontal centrifugal casting with stationary vertical core pouring
Figure 13 shows a schematic of a particle (i.e., precipitate in a melt), which is subjected to forces resulting from gravity and the centripetal acceleration caused by the shell rotation during centrifugal horizontal spin casting. The applied centripetal acceleration varies between 40-100 G, depending on the specifics of the process and related manufacturing parameters.
During horizontal centrifugal casting segregation takes place either on the inner and/or the outside surface of the shell tube. The segregation behavior is a function of precipitate density and applied angular velocity. The Gibbs free energy of formation dictates the precipitation sequence over time, i.e. first the appearance of solids in the liquid, with subsequent crystallization (growth of dendrites). At the same time, the solidification mechanism is driven by the thermal gradient and solute segregation (i.e., a type of microsegregation due to the diffusion of excess solute content into the residual liquid during propagation of the solidification front). The centrifugal forces break the solidification front, and new nucleuses appear in the liquid, providing more nucleation sites.
Therefore, there are several factors, affecting the segregation: a) different densities of atom clusters, b) high rotation speed and precipitation sequence, c) insufficient solidification rate, d) possibility of two solidification fronts.
Figure 13. Free body diagram of a particle in centrifugal field
The major factors, promoting segregation, are strongly related to two solidification fronts
unidirectional solidification has to be the preferred solidification mechanism which needs to be controlled [132].
Several numerical and computational models of the phenomenological dendrite growth behavior are discussed in the literature [47, 48, 49, 50]. This information shows that the mechanism of dendrite development depends considerably on thermal gradients (under static conditions).
Modelling and simulation of the solidification structure during centrifugal casting are also discussed where prediction of grain structure and columnar-to-equiaxed transition are shown to be controlled by the superheat of the liquid (stationary case). Macro-segregation is a function of particle clustering and agglomeration formation under the centrifugal field [51].
In addition to macrosegregation, another deleterious factor that produces casting cracks is liquation, which is produced by the complex tearing forces present at the mixed solid-liquid interface. It is shown that the formation of casting cracks can be controlled and suppressed by proper simultaneous selection of the mold material, protective coloration and oxide preventing fluxes, pouring temperatures, cooling rates and rotational speed [17, 42, 43, 51].
Microstructural refinement of the as-cast structure has been studied in several works [52, 53, 54, 55, 56, 57, 58, 59, 60, 61]. In summary, high levels of refinement can be achieved by: 1)
addition of rare earth (RE) elements; 2) controlling cooling rates and rotational speeds while regarding critical temperatures; 3) combination of the former two; 4) application of external electro-magnetic fields.
In summary, the review of the literature seems to clearly indicate that the casting method, has important implications in the final as-solidified microstructure and hence the mechanical properties of HSS rolls. In addition, it appears that there is a dearth of information regarding kinetics data for carbide formation, precipitation sequence and volume fraction of carbides.
Similarly information regarding phase transformations during solidification under a centrifugal force field is also limited in the literature. One of the objectives of this thesis is to fill the gaps of understanding regarding microstructure, carbide formation and precipitation during the solidification that takes place in the horizontal spin casting of HSS rolls.