2015 年 2015 巻 60 号 p. 43-50
Hydrogen embrittlement sensitivity of SAE9254 (tensile strength of 1.7 – 2.0 GPa class) and vanadium added spring steel (9254V) with a tensile strength of 2.0 GPa class was evaluated by conventional strain rate tensile tests (CSRT) and torsion strength tests, using smooth specimens respectively. In the CSRT evaluation, the maximum tensile stress of the both steels decreased with the increment of diffusible hydrogen content. On the other hand, in the torsion tests, the maximum shear stress hardly exhibited any decrease until the hydrogen content reached 6mass ppm, where the cracking trace changed from shear plane (transverse direction of the specimen) to a resolved tensile stress plane (45°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 of the CSRT properties of 9254V was superior to that of vanadium-free SAE9254 in the equivalent tensile strength level. Although the superior performance for the latter steel attributes partially to the reduction of phosphorous and sulfur contents, it should be noted that the addition of vanadium contributes the refining of prior austenite grains followed by the reduced intergranular segregation of phosphorus and sulfur. The hydrogen trapping effect at the vanadium carbide interface may contribute too. However, there was no difference between SAE9254 and 9254V in hydrogen embrittlement sensitivity when the torsion test is conducted.