The Development of High Hard and 
Tough Matrix Type High Speed Tool Steels 
By SHUN-SUKE NAKAHAMA*1, YUKINORI MATSUDA*2, KUNIO NAMIKI*3, KOZO OZAKI*4

3. The concept of the development (continued)

3.3 The concept of the development

The major failure modes are fatigue and impact fracture by high stress as mentioned previously. Furthermore, fracture surfaces of fatigue and Charpy impact specimens were observed. There observed carbides at the crack origin also for these kind of specimens. These carbides are decided as primary V carbides because of their high amount of V.

Kiyonaga et at. reported the effect of primary carbides on the fatigue strength and the toughness of high speed steels.^5)'6) Then in this study we have investigated the effect of the amount of primary carbides by using the steels with the same chemistry as shown in Fig.9 (below). Charpy impact values decrease with increasing in the amount of it. Ozaki et at. also reported that fatigue strength was increased by refining carbides⁷⁾ From these data it is concluded that the best way to obtain high toughness and fatigue strength is to reduce primary carbides. Furthermore, non-metallic inclusion is to be reduced as well.

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