Effect of Ausforming on Hydrogen-induced Delayed Fracture of High-strength Bolt Steel

With the development of modern industry, there is an increasing demand for increasing the strength level of high-strength bolt steel without noticeably deteriorating its hydrogen embrittlement resistance, or hydrogen-induced delayed fracture (HIDF) resistance in the case of bolt. To explore the possibility of microstructure controlling on further improving the HIDF resistance of high-strength bolt steel, a V+Nb-microalloyed Cr-Ni-Mo high-strength bolt steel was subjected to low-temperature ausforming (low-temperature controlled forging with finish-forging temperature at ~625 ℃ followed by direct water quenching) and tempering at 450 ℃ and its HIDF behavior was studied by slow strain rate tensile (SSRT) tests using pre-electrochemically charged notched round bar tensile specimens. The steel subjected to conventional forging and quenching and tempering treatment (austenitized at 945 ℃, oil quenched and tempered at 450 ℃, air cooled) is also used for comparison. The results show that after low-temperature controlled forging, the experimental steel has a fine banded microstructure with obvious grain elongation along the forging direction and with grain refining by ~53%. The prior austenite grain boundary is serrated with no obvious coarse cementite films precipitated and a volume fraction of ~7.7% polygonal ferrite is formed along the grain boundary. Both the smooth and notch tensile strengths of the low-temperature controlled forged sample are significantly increased than those of the conventionally forged sample. Compared with the conventionally forged sample, the low-temperature controlled forged sample still shows excellent HIDF resistance, although its strength level is significantly enhanced. The HIDF resistance represented by notch tensile strength and the hydrogen embrittlement sensitivity index represented by relative notch tensile strength loss rate are increased by 62.1% and decreased by 27.6%, respectively after low-temperature controlled forging. The HIDF mechanism changes from brittle intergranular fracture along prior austenite grain boundaries of the conventional forged sample to transgranular quasi-cleavage fracture of the low-temperature controlled forged sample, and the proportion of the brittle zone area on the fracture surface of the latter is significantly reduced. The banded structure with fine grains, the formation of polygonal ferrite and the changes of cementite structure along the prior austenite grain boundaries are the main reasons for the excellent HIDF resistance of the low-temperature controlled forged sample compared with the conventionally forged one. Therefore, it is regarded that it is an effective way to further improve the HIDF resistance of high-strength bolt steel by adjusting the microstructure distribution and the prior austenite grain boundary characteristics based on low-temperature deformation.