号数 ばね論文集 60 号( 2015 年)
ページ数 pp.43-50
種類 共同研究
論文名 ばねの遅れ破壊に関する研究委員会報告 その2
―バナジウム添加ばね鋼の水素割れ感受性―
Title Evaluation of Delayed Fracture Properties in Spring Steels
Part II
− Susceptibility to Hydrogen Embrittlement for Vanadium Added Spring Steel −
著者 ばねの遅れ破壊に関する研究委員会 
Author The Research Committee on Evaluation of Delayed Fracture Properties in Spring Steels 
抄録 引張強度1.7〜2.0GPa級のSAE9254と,2.0GPa級のバナジウム添加鋼9254Vの水素割れ感受性をconventional strain rate tensile tests (CSRT)とねじり試験により評価した.いずれも平滑丸棒試験片を用いた.CSRT法では,両鋼とも最大引張応力は拡散性水素量の増加にしたがい,低下した.
 一方,ねじり試験では,両鋼とも最大せん断応力は,6mass ppmの水素濃度までほとんど低下することはなく,破面は試験片長手方向に対して垂直面(せん断応力面)の延性破壊でmicro-void coalescence(MVC)様相を呈した.最大せん断応力の低下がみられる高水素濃度域では,き裂は45°方向(引張応力面)に進展し,引張応力面の一部には粒界破面様相が現れた.
 CSRT法における耐水素割れ感受性に関しては,同一引張強度で比較して9254Vの方がバナジウムフリーのSAE9254より優れていた.9254Vにおける耐水素割れ感受性向上には,リンと硫黄の低量化による粒界偏析の減少と,バナジウム添加による旧オーステナイト粒の微細化による寄与が考えられる.さらに,バナジウム炭化物界面における水素トラップの効果も考えられる.しかしながら,ねじり特性の水素割れ感受性に関しては両鋼間で差はなかった
 
Abstract Hydrogen embrittlement sensitivity of SAE9254(tensile strength of 1.7 – 2.0 GPa class)and vanadium added spring stee(l 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 plan(e 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. 
著者の所属  
Belonging  
Key Words Hydrogen embrittlement, Conventional strain rate test, Torsion test, Spring steel, Vanadium carbide; Diffusible hydrogen, SAE9254, 9254V