2015 年 101 巻 1 号 p. 33-39
The susceptibility to hydrogen embrittlement for vanadium added spring steel (9254 V) with a tensile strength of 2 GPa class was evaluated by conventional strain rate tensile tests (CSRT) and torsion tests, using smooth specimens respectively. In the CSRT evaluation, the maximum tensile stress decreased with an increment in the diffusible hydrogen content, especially over 5.5 massppm the maximum tensile stress tended to fall off to the lower limit of 1.1 GPa and the fracture appearance changed to fully intergranular.
On the other hand, in the torsion tests, the maximum shear stress hardly exhibited any decrease until the hydrogen content reached 6 massppm, where the cracking trace changed from shear plane (transverse direction of the specimen) to a resolved tensile stress plane (45º against the shear plane); fractgraphically, from micro-void coalescence (MVC) to intergranular fracture, and the torsional strength began to decrease.
The resistance to hydrogen embrittlement as regards the CSRT properties of 9254 V was superior to that of vanadium-free SAE9254 but with the same tensile strength. Although the superior performance for 9254 V is partially attributable to the reduction of phosphorous and sulfur contents, it should be noted that the addition of vanadium causes refining the prior austenite grains followed by an effective reduction of the intergranular segregation in phosphorus and sulfur, and probably hydrogen trapping at the vanadium carbide interface. However, there was no difference between 9254 V and SAE9254 as regards the torsion properties insusceptible to hydrogen compared with CSRT.