Investigations into the delayed fracture susceptibility of 34CrNiMo6 steel, and the opportunities for its application in ultra-high-strength bolts and fasteners

The overall effort of the automotive industry to produce lightweight parts has led to a stronger focus on the development of high-strength and ultra-high-strength steels. The susceptibility of such steels to hydrogen embrittlement is often underestimated due to the delay of the damage, which can occur suddenly after a few weeks or even months. The purpose of this work is to investigate the influence of different tempering temperatures on the delayed fracture susceptibility of ultra-high-strength quenched steel. In order to test the hydrogen embrittlement, a device was built in which the notched samples were exposed to constant bending stress with a constant dropping of hydrochloric acid (HCl) and a 0.1 N concentration in the notch. The results demonstrate that tempering at elevated temperatures leads to a lower susceptibility to hydrogen embrittlement. The results of this paper lay the groundwork for the further development of advanced ultra-high-strength steels with an increased resistance to delayed fracture. The application of ultra-high-strength fasteners demonstrates great potential for a new generation of engines with reduced CO2 emissions.