The dissolution and precipitation during heating, cooling and tempering of a new kind of semi high speed steel were studied by means of X-ray diffraction analysis, TEM microstructure examination and electron diffraction. The results showed that in the annealed state there are MC,M_6C and M_7C_3 in the steel; M_6C and M_7C_3 are dissolved totally during heating for quenching, MC,M_2C, M_6C and M_7C_3 are precipitated during tempering. The peak hardness of the steel is reached after tempering at 520 ℃. The p...