The effect of REM addition on the morphology of M2C type carbide during solidification, the heat treatment characteristics and the mechanical properties of the SKH 51 type high speed tool steel, has been investigated.The results obtained are as follows.(1) REM addition is very effective for the crystal nucleation during steel solidification. Accordingly it results in smaller grain diameter with equiaxed crystals.(2) REM addition is regarded to accelerate the transformation of primary carbide M2C to M6C+MC during ingot heating. Therefore it makes the size of carbide smaller.(3) Toughness, grindability and hardness after tempering are improved by REM addition, owing to its refining effect on the grain structure and the primary carbides.
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Yamauchi et al. (1986) studied this question.