Decomposition of the Coarse Primary Carbides in the M2 High Speed Steels Containing Silicon

The carbides of high speed steels have a determinant influence on their properties, and the control of the carbides has been a hot topic in development of high speed steels. The decomposition of the coarse primary M2C carbide in the as-cast M2 high speed steels with Silicon addition and without Silicon addition was investigated by using optical microscope, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction analysis. Both the M2 high speed steel and the M2-1Si steel were heated at 1000 and 1150 ℃, the soaking time were 15 min, 30 min, 1 h, 2 h and 4 h, respectively. The results showed that the decomposition of M2C carbides to M6C and MC carbides occurred during heating at 1000 ℃,the degree of the decomposition became obvious with the increase of heating time or addition of silicon. After the tested steels were heated at 1000 ℃ for 1 h, the primary M2C carbides in the M2-1Si steel had decomposed into M6C and MC completely, but the decomposition of the primary M2C carbides in the M2 steel was incomplete. Silicon addition was found to accelerate the decomposition of the coarse primary M2C carbides during heating and to inhibit the growth and coarseness of the decomposition products.