Microstructure and Nanochemistry of Carbide Precipitates in High-Speed Steel S 6-5-2-5

Abstract

Depending on the steel composition several types of carbides are precipitated in high-speed steels: mainly MC, M 2C, M 6C and some others with minor importance. During hot working, the primary carbides formed during solidification change their as-cast structure to a more spherical one. They have an incoherent interface to the matrix and are a few micrometers in size. In the finished tool, the primary carbides together with the secondary, and the high hardness of the matrix are responsible for the high wear resistance. For the production of tools it is necessary that the steels can be machined, which is enabled by soft annealing. During this heat treatment some additional carbides of 50 to 300nm in size are precipitated as demonstrated by high-voltage electron microscopy (HVEM). After machining, the tools get their desired properties from hardening and tempering. The examination under these conditions shows the existence of nanometer-sized secondary hardening carbides, precipitated during this heat treatment and consisting mainly of vanadium and carbon as proven by energy filtered transmission electron microscopy (EFTEM). The high red hardness up to temperatures of approximately 550°C is caused by these nanometer-sized carbides. High resolution electron microscopy (HREM) revealed a completely coherent transition of the lattice planes from these carbides to the matrix-, without any irregularities.