Evaluation of Delayed Fracture Property of High Strength Bolt Steels

Abstract

Experimental studies attempting to evaluate the delayed fracture property of high strength bolt steels by using slow strain rate tests (SSRT) of circumferentially notched round bar specimens are reviewed in this paper. The relationship between the notch tensile strength (NTS) of the hydrogen-precharged specimens and the hydrogen content was approximated by power law relationship. It has been found that the local stress and the local diffusible hydrogen concentration control the occurrence of delayed fracture. To take into account the effect of hydrogen uptake from the environment, the notched bar specimens were subjected to cyclic corrosion test (CCT) and outdoor exposure, and their NTSs were measured using SSRT. The susceptibility of high strength steels to delayed fracture estimated from the decrease of NTS was correspondent to the fracture ratio of high strength bolts of the steels obtained by outdoor exposure tests, and was successfully evaluated with consideration of hydrogen uptake. Hydrogen entry behavior under CCT was monitored by electrochemical hydrogen permeation test, and continuous increase in hydrogen entry with growth of rust layer was observed. It is suggested that the “delay” of delayed fracture is the time required for the enhancement of the hydrogen entry efficiency.