高力ボルトの遅れ破壊強度と熱処理

概要

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Concerning delayed fracture of high strength bolt used for friction jointing of steel construction, a discussion of chemical composition and heat treatme t process was made chiefly from the standpoint of metallurgy.Behavior of delayed fracture strength of quenched steel is divided into 4 characteristic regions according to its tempering temperature.By tempering it at the 4th high tempering temperature region, steel of the smallest delayed fracture sensitivity can be obtained, but in order to get steel of high strength and extraordinary consideration is necessary. In this series, high strength bolt up to F12T is being developed at present.The maximum temperature of 1st region is around 200°C and in this region steels containing low carbon like F13T with small delayed fracture sensibility are obtained.It is possible to reduce delayed fracture sensitivity by addition of Titanium, Boron etc. to steels, but they are still inferior to those tempered in the 4th domain.The practical character of steels treated in this region as bolt for friction jojnting is necessary to be more investigated. Since incomplete quenching which occurs in the industrial heat treatment of bolt causes low tempering temperature, it increases delayed fracture sensitivity.Carburization is especially dangerous for steels treated with lower tempering temperature.
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