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

摘要

Depending on the steel composition several typesof carbides are precipitated in high-speed steels: mainly MC,M_2C, M_6C and some others with minor importance. Duringhot working, the primary carbides formed during solidificationchange their as-cast structure to a more spherical one. They havean incoherent interface to the matrix and are a few micrometers insize. In the finished tool, the primary carbides together with thesecondary, and the high hardness of the matrix are responsible forthe high wear resistance. For the production of tools it isnecessary that the steels can be machined, which is enabled bysoft annealing. During this heat treatment some additionalcarbides of 50 to 300nm in size are precipitated as demonstratedby high-voltage electron microscopy (HVEM). After machining,the tools get their desired properties from hardening andtempering. The examination under these conditions shows theexistence of nanometer-sized secondary hardening carbides,precipitated during this heat treatment and consisting mainly ofvanadium and carbon as proven by energy filtered transmissionelectron microscopy (EFTEM). The high red hardness up totemperatures of approximately 550deg C is caused by thesenanometer-sized carbides. High resolution electron microscopy(HREM) revealed a completely coherent transition of the latticeplanes from these carbides to the matrix-, without any irregularities.