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.KEY WORDS: delayed fracture; hydrogen embrittlement; high strength steel; slow strain rate test; atmospheric corrosion; electrochemical hydrogen permeation; cyclic corrosion test; exposure test.

Prevention of Hydrogen Embrittlement in Steels

“…The traps in these cases would not mitigate the embrittlement of the steel. Akiyama 143) pointed out that a particular bolting steel (Table 3) containing molybdenum carbides did not show exceptional resistance to hydrogen embrittlement in cyclic corrosion tests because of the high hydrogen uptake of the steel.…”

“…29,152,153) So the problem reduces to one in which the steel and its heat treatment are designed to maximise H C , possibly by using hydrogen trapping carbides. In the work described in the preceding paragraph, 143) laboratory experiments were performed in which samples were charged with hydrogen, rather than exposed to an environment. Therefore, the traps would have been saturated, and subsequently served as sources for hydrogen.…”

Numerical Analysis of Influence of Hydrogen Charging Method on Thermal Desorption Spectra for Pre-strained High-Strength Steel

“…1 oratories, and the latter is one method which simulates penetration of hydrogen into specimens in actual corrosive environment. 16,17) In addition, by simulating numerically thermal desorption spectra obtained in these experiments, we discussed the difference of them from the viewpoint of the hydrogen trap state. This paper is organized as follows.…”

Prevention of Hydrogen Embrittlement Using Ultra Rapid Cooling Thermal Spraying Gun

“…Delayed fracture of the high strength bolt is thought to be caused by diffusible hydrogen, which enters via the surface cathodic reactions. [8][9][10][11][12][13][14] Thus, hydrogen entry from the environment should be considered. An anti-corrosion-material coatings on a high strength steel is expected to make it possible to prevent delayed fracture due to the prevention of hydrogen entry.…”