含釩高強度鋼材之氫誘發破裂敏感性評估研究

Evaluation of Hydrogen-induced Cracking Susceptibility of High Strength Stress with Vanadium Additions

本研究利用對開槽圓棒試棒預充氫及一般拉伸試驗法，探討釩的添加與回火處理溫度對高強度鋼簧鋼材氫誘發破裂敏感性之影響。預充氫之試棒進行拉伸試驗後，利用升溫脫離法進行鋼材可擴散氫含量之測定，即可得氫含量-破斷應力值之關係圖。由圖中均可觀察到一明顯之”轉折點”，即為鋼材發生沿晶破壞之臨界氫含量值，當氫含量大於該臨界值後，其破斷應力隨著鋼材內之可擴散氫含量增加，而呈現冪次法則關係遞減，此臨界氫含量值之高低，可用以評估鋼材之氫誘發破裂敏感性。試驗結果顯示：含釩之彈簧鋼中，經550℃回火熱處理後，比經400℃熱處理之試片，具有較高之氫誘發破裂臨界氫含量值，此乃因前者具較低之抗拉強度，以及所生成VC析出物具有額外吸氫能力之雙重因素所致；然於相同550℃回火熱處理下，不含釩之鋼材，其氫誘發破裂臨界氫含量值反較含釩鋼材為高。研判含釩鋼雖擁有可額外吸氫之VC析出物，但析出量並不明顯，加上其抗拉強度亦較不含釩鋼為高，此時，含釩鋼因析出物造成氫誘發破裂敏感性之下降程度，仍不及不含釩鋼因抗拉強度減少所造成之影響，故使得含釩鋼氫脆發生所需之臨界氫含量值，仍較不含釩之鋼材為低。

The effects of vanadium addtions and tempering temperature on the hydrogen-induced cracking susceptibility of high stress strength spring steels was investigated by using conventional strain rate tensile tests (CSRT) of circumferentially notched round bar specimens(CNRBS) after hydrogen precharging in this study. The diffusible hydrogen content of prechaging specimens was measured by using thermal disorption spectroscopy (TDS), and thus the relationships between hydrogen content and rupture stress can be obtained. A obvious convert in hydrogen content on the relationship was observed, which is determined as the threshold for hydrogen-induced cracking. The rupture stress behind the point decreased in the power law relationship with increase in the diffusible hydrogen content. The value of the threshold is considered the criterion for the hydrogen-induced cracking susceptibility. The vanadium added steel with 550 ℃ tempering treatment owns higher threshold than that with 400 ℃ tempering treatment, which is due to both the lower strength stress and VC precipitation owning extra hydrogen trap ability. For the steels with 550 ℃ tempering temperature, the threshold of vanadium added one is lower than that without vanadium additions. It might be resulted from that the positive effect on the increase of threshold from the hydrogen trap ability due to VC precipitation is lower than the negative effect from higher strength stress of the vanadium containing steels with 550 ℃ tempering temperature.