Effects of High Solidification Rates on Segregations and Solid Phase Transformations in High Speed Steels

High Speed Steels (HSS) represent high alloyed steels that belong to the complex Fe-Cr-C-X system whit X being a strong carbide forming element such as V, Nb, Mo or W. During the solidification of HSS, segregations corresponding to preferential distribution of elements can occur. Segregations are influenced both by the initial chemical composition and by the casting route, which is the case with conventional spin casting process. But the same phenomenon should also exist with the expanding additive manufacturing techniques that involved higher thermal gradients and higher cooling rates compared to classical processes.

The solid phase transformations are controlled by the local concentrations of elements, especially the supersaturation which may exist in certain regions. That is the reason why the segregations inherited from the solidification of the HSS alloys lead to unexpected solid state phase transformations during the ongoing cooling stage. These phase transformations may occur either in the moderate temperature range or subsequently in the lower temperature range which is close to the ambient temperature, as for the martensitic transformation.

In the present work a general review of the literature dealing with the fundamentals of solidification is reviewed aiming at justifying the need to search for tools that will be more adapted to simulate or study the microstructures obtained by manufacturing processes such as laser cladding, which refer to conditions far away from equilibrium. The new approach to understanding non-equilibrium structures also requires consideration of related studies with materials that are different from the HSS alloys, such as stainless steels, in order to take advantage of relevant analyzes that have already been achieved in other areas. The study is open, because the scope of the analyses is large and it can be extend to the phase transformations and also to the properties. Some relevant results obtained on two HSS processed by laser cladding are discussed related to the solidification mode and the solidification sequence.