Precipitation of Secondary Carbides in M2 High-Speed Steel Modified with Titanium diboride

The precipitation of the secondary carbides in high-speed steel of AISI M2 type modified with titanium diboride has been investigated for both the cast and the heat-treated states. The primary focus was on the effect of austenitizing temperatures on the secondary carbide precipitation during tempering. Some differences in origin of the secondary carbides, as well as in their shape and size distribution, were found in the tempered microstructure for the different austenitizing temperatures. After austenitization at 1180 A degrees C and triple tempering at 560 A degrees C, the secondary carbide particles of a spherical shape up to 200 nm in size were identified by selected area electron diffraction as M23C6. After austenitization at 1220 A degrees C, two types of the secondary carbides were found in the tempered microstructure, M23C6 with a size up to 200 nm and M6C with a size up to 400 nm. In both the cases, the carbide particles were slightly angular. After austenitization at 1260 A degrees C, only M6C secondary carbides were revealed in the tempered microstructure, which occurred as the angular particles up to 350 nm in size. In addition, considerably finer M23C6 carbide particles with a size of 10-40 nm were found to precipitate in the tempered microstructure.