Evaluation of delayed fracture characteristics of high-strength bolt steels by CSRT

Abstract

Delayed fracture is affected by two parameters: stress and hydrogen. Hydrogen enters the material from the environment as a result of corrosion. Generally, it takes a relatively long time, so that fracture occurs with a time lag after loading. The delayed fracture occurs when the driving force (S-H) E becomes greater than the material resistance (S-H)C. Extensive research has been carried out to evaluate the material resistance against delayed fracture using the constant load test (CLT) and slow strain rate test (SSRT). Recently, the author has developed a test method called conventional strain rate test (CSRT). In CSRT, hydrogen, at a content corresponding to the accumulated hydrogen concentration in CLT and SSRT, is introduced uniformly into the specimen and loading is applied at a conventional strain rate with negligible hydrogen diffusion. The advantage of CSRT is that it requires no special testing equipment and the testing time is very much shorter than those of the previous test methods. The principle, test procedure and some experimental results of CSRT are discussed in this paper. © 2012 ISIJ.