Microstructure of two centrifugal cast high speed steels for hot strip mills applications

High speed steels (HSS) present excellent hardness, wear resistance and high-temperature properties. These mechanical properties are due to the presence of a great amount of hard carbides in the martensitic matrix. In the last 10 years, Japanese rollmakers have developed HSS grades and introduced them into hot strip mills.The Marichal Ketin society (Liège, Belgium) has developed two grades of HSS: Kosmos and Aurora. Both grades present interesting properties but Aurora shows overall better performance than Kosmos, mainly because of a better distribution of harder (MC and M2C) carbides in the martensitic matrix. Moreover, the hardness of the Aurora grade stays constant in depth and can be strongly improved by heat treatment, due to secondary hardening.The purpose of this work is to describe the microstructure and the mechanical properties of the Kosmos and Aurora grades by various techniques such as optical microscopy, scanning electron microscopy (SEM), and macrohardness measurements.

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► The structure of two HSS grades (Kosmos and Aurora) was characterized.
► The effects of heat treatments on the structure were studied.
► The Aurora grade presents secondary hardening due to the presence of very fine M2C carbide.
► The hardness of the Aurora grade is not influenced by depth in the part.
► The intrinsic hardness of MC carbides was measured by nanoindentation.