1994 年 43 巻 488 号 p. 509-514
Silicon-modified 4340 (Si-4340) and 4340 steels have been studied to determine the effects of silicon addition on austempered structure and mechanical properties of ultrahigh strength low alloy steels. As a result of austempering Si-4340 steel in the bainitic temperature region (593-673K), the retention of a large amount of austenite (12-25vol.%) was observed in the carbon-free upper-bainitic ferrite region. Compared to the conventionally quenched and tempered steel (CQT), the austemper of Si-4340 steel increased fracture toughness (KIC) at 623K and below, owing to the increased Charpy impact energy and percent elongation, while the strength decreased. Compared to the CQT and 4340 steel austempered in the same temperature region, the austemper of the steel had detrimental effects on the KIC and other relevant mechanical properties at 643K and above. The results are described and discussed in terms of metallographic observations, X-ray measurements and fractography.