On of the problems in metallurgic technicality in Japan is concerning the delay in fracture, which is common in a hexagon bolt, when its high friction strength at the grip joint is desired, and the fact has been a source of great worry of technicians for improvement of bolt strength.
In our investigation, we dipped the V notched test specimen made of low carbon boron steel into 10% H2SO4 to introduce hydrogen, and observed the effect of hydrogen upon the steel by performing static bending test and fracture test with an electron microscope, noticing delay in fracture.
It has been found as the result of experimentary investigation that the range within which the hydrogen embrittlement occurs, in the case of the 10% H2SO4 dip, is limited to the thin surface layer of the specimen. The delay in fracture has been ascertained to be due to the effect of hydrogen, for the fracture surfaces both in the natural delayed fracture and in the acid are alike intercrystalline.
Under the delayed fracture limit no crack ever occurs at the notch root, and when a crack has once occurred, the specimen leads to final failure in short time. It is obvious in this way that no delay in fracture ever occurs until (a crack has been caused) by the gathering of hydrogen at the stress concentrated part.
