Effect of direct-quenching and tempering on hydrogen-induced delayed fracture resistance of high-strength bolt steel

Abstract

This investigation attempts to explore the potential significance of controlled forging, i.e., direct-quenching at two different finish-forging temperatures of ∼930 °C (B8H sample) and ∼780 °C (B8L sample), and tempering at 600 °C in improving the hydrogen-induced delayed fracture (HIDF) resistance of a 1300-MPa-grade high-strength bolt steel by constant load tensile testing using circumferentially notched round bar specimens and hydrogen thermal analysis. The microstructural characteristics of the tested samples were analyzed and their influences on HIDF were discussed. The results showed that the B8L sample demonstrated a considerably increased HIDF resistance expressed by critical notch delayed fracture stress of 1825 MPa and a decreased hydrogen embrittled index of as low as 21% compared with those of 1675 MPa and 27% for the B8H sample, although the former exhibited slightly higher strength and absorbed hydrogen content. Scanning electron microscopy observation of the fracture surface demonstrated that the B8L sample prevented the brittle intergranular fracture with smaller quasi-cleavage facet and decreased area fraction of the brittle crack initiation zone compared with the B8H sample. The increased resistance to HIDF of the B8L sample is primarily because of its fine microstructure with increased number of finely distributed V-rich nanoscale MC precipitates and decreased dislocation density. It is proposed that controlled forging coupled with V-microalloying is a feasible and economical way to further increase the HIDF resistance of high-strength bolt steel.

Keywords

• High-strength bolt steel
• Controlled forging
• Hydrogen-induced delayed fracture
• Hydrogen embrittlement
• Hydrogen trap
• Microstructure