回火熱處理對SAE 8625M高強度低合金鋼氫脆性質影響之研究

Abstract

8625M為鎳鉻鉬系高強度低合金鋼(HSLA steel)，於淬火態強度可達1600 MPa，在常溫下的衝擊值亦可達45 J，且為麻田散鐵鋼，可透過回火處理作機械性質上之調整。然而，高強度鋼種易受到氫脆影響，尤其又以麻田散鐵鋼最為嚴重，故本研究以陰極電化學法對8625M鋼材進行充氫，評估其在不同回火條件下充氫前後機械性質之變化，並透過SEM、TEM、TDS等等分析，釐清在不同回火條件下8625M鋼材之顯微組織、氫捕集位置與其氫脆現象之關聯性。 由實驗結果可知，在任何回火條件下，8625M鋼材最主要之氫捕集位置為差排，氫捕集活化能約為20 kJ/mol，屬於可擴散氫。在水淬態時，材料擁有最高密度之差排，其充氫後之吸氫量亦最多，大量可擴散氫造成水淬態之氫脆現象最為嚴重。200°C回火後則差排密度降低，吸氫量亦跟著降低，較少的吸氫量使得其有著本研究中次低的UTS loss以及最高的充氫後UTS。300°C以及400°C時，氫脆現象又開始變得嚴重，此時殘留沃斯田鐵分解為板片之間細長且硬脆的雪明碳鐵，其形貌容易使得應力集中並造成板片間強度下降，並在氫的作用下於更低的抗拉強度變造成氫致破壞。500°C回火後，這些細長硬脆的雪明碳鐵有明顯的球化，其形貌較不會造成其板片強度下降，且有著更低的差排密度與吸氫量，使得其有著本研究中最低的UTS loss以及最高的充氫後破斷應變。

8625M steel is a Ni-Cr-Mo high strength low alloy steel (HSLA steel). In quenching state, the ultimate tensile strength can reach 1600 MPa, and the impact energy at room temperature can reach 45J. Moreover, as a martensitic steel, the mechanical property of 8625M can be adjusted by tempering. However, high strength steel is prone to hydrogen embrittlement, especially the martensitic steel. In this study, we use the electrochemical method to charge hydrogen into the 8625M steel, and measure the ultimate tensile strength by the tensile test in order to evaluate the hydrogen embrittlement effect on the mechanical properties of 8625M steels. Meanwhile, the microstructure observation by SEM and TEM and the hydrogen trapping energy measurement by TDS are also conducted to clarify the relationship between the tempering temperature, microstructure, trapping site and the hydrogen embrittlement effect on mechanical property. Experimental results show that the dominant hydrogen trapping site of the 8625M steel at any tempering temperature is dislocation, of which trapping energy is about 20 kJ/mol, indicating that the hydrogen trapped in the dislocation is diffusible. In quenching state, 8625M has the highest dislocation density and the highest hydrogen content after hydrogen charged. The most diffusible hydrogen cause 8625M to exhibit severe hydrogen embrittlement in the quenching state. After 200°C tempering, the dislocation density drops, as well as the hydrogen content after hydrogen charged, and this make 8625M has lower UTS loss after 200°C tempering. After 300°C and 400°C tempering, the hydrogen embrittlement phenomenon becomes serious again. In these states, the retained austenite decomposes into sharp, needle-like inter-lath cementite, which may cause stress concentration and weak the strength of the lath, and make 8625M fracture at lower stress after hydrogen charged. After 500°C tempering, the 8625M has the lowest dislocation density, and the inter-lath cementite become spheroidized, making 8625M in 500°C tempering state has the lowest UTS loss and the highest elongation after hydrogen charged in this study.