Effect Of Nb And Ni On Delayed Fracture Behavior Of High Strength Bolt Steel

Hydrogen-induced delayed fracture (HIDF) is a major failure mode of high strength bolts, and thus has been one of the top issues in developing even higher strength level bolts. In this study, the influences of Nb and Ni on HIDF behavior of high strength bolt steels 42CrMo and 42CrMoVNb were investigated by using constant load tensile (CLT) testing, slow strain rate tensile (SSRT) testing, electrochemical hydrogen charging testing, thermal desorption spectrometry (TDS) analyzer and fracture surface analysis, in an attempt to further enhance their resistance to HE. The following results could be obtained:The research results of different content of Ni (0～1.02%) addition on delay fracture behavior of high strength steel 42CrMoVNb shows that HE index (HEI) decreases notably with increasing Ni content. SEM analysis of the fracture surface of HE specimen shows that there exhibits a mixing fracture mode of quasi-cleavage and intergranular fracture in crack initiation regions of hydrogen embrittlement fracture specimens. The percentage of intergranular fracture in the crack initiation region as well as the area of this region decrease with increasing Ni content. Further corrosion testing and electrochemical polarization testing reveals that corrosion rate, pit depth and absorbed hydrogen content tend to decrease, and pitting corrosion resistance tends to increase with the increase of Ni content. It is concluded that the addition of Ni could further improve the delayed fracture resistance of the tested steel mainly owing to enhanced corrosion resistance and decreased hydrogen absorption.The research results of different content of Nb addition on delay fracture behavior of 42CrMo steel shows that the addition of Nb can significantly refine the micro structure of the tested steel. During austenitizing treatment at high temperature, most of the NbC particles dissolve into austenite, and thus many fine plate NbC particles could precipitate during subsequent high temperature tempering treatment. These finely precipitated NbC particles causes significant secondary hardening, and thus notably higher strength level could be obtained. In the meantime, both these fine newly precipitated NbC particles and these undissolved coarse NbC particles could act as traps of hydrogen and thus delay the accumulation of hydrogen. Therefore, the HE susceptibility of the tested steel could be lowered with appropriate addition of Nb through suitable austenitizing and tempering treatment. This beneficial effect is only obliviously in the condition of higher austenitizing temperature. However, excessive high austenitizing temperature makes tested steel exhibits much coarse microstructure, which in turn presents comparatively high HE susceptibility.