BEHAVIOR OF THE CARBIDES AND HEAT-TREATMENT IN LOW-W AND LOW W-Co HIGH SPEED STEELS

The report deals with a study on the behavior of carbides in low-W high-speed steel and low W-Co high speed steel.The total amounts of carbides in the several heat-treated specimens of the high speed steels were determined by electrolytic isolation method.Chemical compositions and crystalline structures of the isolated carbides were determined by chemical analysis and X-ray diffraction, and the shapes of the carbides were observed by electron microscope, and the following results were obtained.(1) In annealed states, the amounts of carbides in these steels are about 20-22% wt%, and about 85-95% of the W and V in the steels are concentrated in the carbides, while the most portions of the Si, Mn and Co are dissolved in the matrix. The carbides precipitated in these annealed steels are M6C, MC and M23C6.(2) In quenched states, the insoluble carbides in the high speed steels are only M6C, and it amounts to about 10 wt%, which is the same about 40% of the carbides in annealed state.(3) The more rises the tempering temperature, the more precipitate the carbides from the austenite. And, especially, the concentration percentage of the special alloy elements in the carbides increases suddenly at the tempering of 575°C, and MC carbide appeares in the steel by tempering at above 300°C.