Microstructure and nanochemistry of carbide precipitates in high-speed steel S 6-5-2-5

Depending on the steel composition several types of carbides are precipitat ed in highspeed steels: mainly MC, M2C, M6C and some others with minor impo rtance. During hot working, the primary carbides formed during solidificati on change their as-cast structure to a more spherical one. They have an inc oherent interface to the matrix and are a few micrometers in size. In the f inished tool, the primary carbides together with the secondary, and the hig h 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, wh ich is enabled by soft annealing. During this heat treatment some additiona l carbides of 50 to 300nm in size are precipitated as demonstrated by high- voltage electron microscopy (HVEM). After machining, the tools get their de sired properties from hardening and tempering. The examination under these conditions shows the existence of nanometer-sized secondary hardening carbi des, precipitated during this heat treatment and consisting mainly of vanad ium and carbon as proven by energy filtered transmission electron microscop y (EFTEM). The high red hardness up to temperatures of approximately 550 de grees C is caused by these nanometer-sized carbides. High resolution electr on microscopy (HREM) revealed a completely coherent transition of the latti ce planes from these carbides to the matrix-, without any irregularities. ( C) Elsevier Science Inc., 1999. All rights reserved.