In order to prevent heat cracks of tool steels in hot working, steels of high toughness and superior high-temperature strength has been developed. Typical hot-working tool steels, such as JIS SKD6, 61 and 62, contain about 1 mass% silicon. In this study, the influence of the silicon content was investigated on the 4 mass%Cr-MMo-V-Ni tool steel in the following points;
1) secondary hardening after tempering
2) high-temperature strength and room-temperature toughness with various cooling rates during quenching
The secondary hardening peak of the tool steel decreases with the decrease in its silicon content, due to the cementite precipitation during tempering. Both its high-temperature strength and room-temperature toughness are increased by decreasing its silicon content.
The extraction residuals analysis, the TEM observation and the EDX analysis are performed on the quenched and quenched-tempered specimens. The effects of decreasing the silicon content of the tool steel are summarized as follows;
A) The undissolved carbides (mainly MC type) are decreased at the quenching temperature, while these precipitates are increased after tempering.
B) The grain boundary carbides (M23C6) are decreased after tempering.
C) In the bainite structure, the silicon content does not affect the toughness, due to the precipitation of M2C type carbides. The high-temperature strength, however, is still dependent on the silicon content.
