Abstract
For developing the high strength bolts, for example, F15T bolt set which tensile stress is about 150Kgf/mm^2 (about 1.5GPa) and higher than that of the conventional strength bolt, the problem on delayed fracture must be solved. And it is evident from the past studies that the delayed fracture is concerned with the diffusible hydrogen in steel. In this paper, firstly from considering the past studies, the practical and easy method of the accelerated tests in which the diffusible hydrogen concentration is used for estimating the sensibility of the high strength bolts to the delayed fracture is proposed. And the results of this proposal accelerated tests on three types steels for high strength bolts are reported herein. From the consideration of these results, it is made up clear that the critical diffusible hydrogen concentration which is obtained by the proposal method can adequately estimate the sensibility of high strength bolt steels to the delayed fracture. Secondaly the results of this accelerated tests on four different notch shaped specimens are reported. And the influence of the notch shape on the critical diffusible hydrogen concentration in proposal method is discussed. Lastly the results of these tests on ten types specimens made from ten different F10T bolts set lots are reported. The F10T bolt set is the most popular and conventional high strength bolt in Japan. So from the consideration of these results of F10T bolts, the requirement on the critical diffusible hydrogen concentration for developing the F15T bolts sets is obtained.
