The carbide microstructure in several spray-formed high speed steels has been investigated , with particular attention on the comparison with ones processed by powder metallurgy and the effect of Co. The pseudo-binary phase diagram for RIST alloys, the driving forces for the nucleation of MC and M6C carbides, and the equilibrium mole fractions of the carbides at 1180℃ and 560℃ were calculated, respectively, through the Thermo-Calc, the calculation program for phase equilibrium in thermodynamics. As a result of the calculation, it was found that both MC and M6C carbides are stable at 1180℃ and 560℃, but the driving force for the nucleation of MC was much larger than that of M6C at 560℃. Therefore, it was expected that secondary hardening could be accomplished by the precipitation of very fine secondary MC carbide. This was verified with the observation of the carbide microstructure after hardening treatment which showed that the Thermo-Calc had been properly fulfilled. Furthermore, it was calculated that the equilibrium mole fraction of the carbides at tempering was larger than that at austenizing. The mole fraction of MC increased with the increase of carbon content in the composition range of RIST alloys, on the other hand, that of M6C decreased with the increase of carbon content.
The carbide microstructure in spray-formed high speed steels before hardening treatment was coarser and less homogeneous than that in powder metallurgy. The size of droplet in spray forming process had been measured to be 10∼300㎛, but in the case of powder metallurgy it had been in the range of 150∼200㎛. Therefore, It was supposed that the difference in particle size at processing step had caused the difference in the carbide microstructure. After hardening treatment, the carbide microstructure in spray-formed high speed steels was improved to be a little fine and homogeneous. The precipitation of fine secondary MC carbide was observed through TEM, and the size of which was so small, 12∼13nm, that the hardness and the toughness of high speed steels were enhanced by leaps and bounds. The addition of Co has also made the carbide microstructure so fine by retarding the growth of carbides precipitated that the physical properties of high speed steels were enhanced surprisingly.
It was shown that, despite the coarser and less homogeneous carbide microstructure, the physical properties of spray-formed high speed steels were better than those in powder metallurgy. The characteristics of carbide microstructure were various in each high speed steels. For example, after hardening treatment, the total volume fraction of the carbides containing that of MC was larger in spray-formed high speed steels, while the size of the carbide was smaller in powder metallurgy. Judging from this result, it has been thought that the amount or the distribution of the carbides plays more important role in determining the final physical properties of high speed steels than the size of the carbides.
