- 學位論文
回火熱處理對PA500H耐磨鋼板之氫脆性質研究
The influence of tempering treatment on the hydrogen embrittlement of PA500H wear-resistant steel
摘要
PA500H鋼材為性質優良的耐磨鋼，屬於麻田散鐵系超高強度鋼，廣泛的使用在挖土機的鏟刀、自卸車的車斗、礦場的粉碎機，淬火態最大抗拉強度可達1700 MPa，180℃-400℃回火態最大抗拉強度皆大於1200 MPa，於前人的研究中，抗拉強度大於1000 MPa的超高強度鋼只需要1-3 ppm的氫便會發生氫脆，且麻田散鐵組織為對氫脆最敏感的組織，因此本研究利用電化學的方式對PA500H鋼材進行充氫，並利用拉伸試驗、定應力試驗、衝擊試驗，評估氫脆對PA500H鋼材機械性質之影響，此外利用OM、SEM、TEM對PA500H鋼材進行觀察，釐清PA500H顯微結構與氫脆現象之關連。 本研究使用PA500H-A及PA500H-B兩鋼材，前者鎳含量為0.46 %，後者鎳含量為0.92 %，淬火態為中鋼所熔煉軋延之DQ材，利用鹽浴爐對其進行不同溫度的回火熱處理，並進行後續之實驗。 實驗結果可以發現，DQ態有著最嚴重的氫脆現象，顯示麻田散鐵組織受到氫脆的影響相當嚴重，而180℃回火態可以大幅提升鋼材的抗氫脆能力，為本研究中最佳的回火參數，此時細小ε碳化物在麻田散鐵板條之內析出，為良好氫捕集位置，300℃及400℃回火態，氫脆現象再次變得嚴重，此時板條內的細小ε碳化物轉變為細小雪明碳鐵，且板條之間析出薄膜狀雪明碳鐵，板條內細小雪明碳鐵捕集氫能力不及ε碳化物，板條間雪明碳鐵又將氫捕集於板條界面，對於抗氫脆具有不良影響。 整體而言，鎳含量較高的PA500H-B具有較佳的抗氫脆能力，鎳在鋼鐵中以固溶原子存在，由於原子大小不同，固溶原子周圍會形成晶格扭曲，亦為氫捕集位置之一，鎳添加提升抗氫脆能力的現象在DQ態較為明顯，回火態由於碳化物的氫捕集能力較強，因此由碳化物主導，鎳添加所造成的抗氫脆差異便較不明顯。
並列摘要
PA500H steel is a high class wear-resistant steel. It belongs to martensitic type ultra-high strength steel. It is widely used in excavator, dump truck and crusher. In quenching state, the ultimate tensile strength can reach 1700 MPa. In 180 ℃ - 400 ℃ tempering state, the ultimate tensile strength can be higher than 1200 MPa. As reported in many early researches, martensitic type steels were very sensitive to hydrogen embrittlement. Only 1-3 ppm hydrogen could lead to hydrogen embrittlement for high strength steels with tensile strength higher than 1000 MPa, especially for the martensite structure. Hence, we study the hydrogen embrittlement phenomenon of PA500H by using the electrochemical method to charge hydrogen. Then, the general and constant-load tensile test and impact test are used to evaluate the hydrogen embrittlement effect on the mechanical properties of PA500H steels. Meanwhile, the microstructure analyses of OM, SEM and TEM are also used to clarify the relationship between microstructure and hydrogen embrittlement phenomenon of PA500H steels. In this research, PA500H-A and PA500H-B steels are studied. The former has lower nickel content, 0.46 %. The latter has higher nickel content, 0.92 %. The quenching state steels were prepared by CSC Company with DQ process. The temper treatments of these steels were executed by using the hot salt bath at various temperatures. Then, various tests were carried out for these tempered specimens. Experimental results show that the DQ state exhibits severe hydrogen embrittlement. This demonstrates that the martensite structure is quite sensitive to hydrogen embrittlement. After 180 ℃ tempering, the tempered martensite increases substantially the resistance of hydrogen embrittlement. In 180 ℃ tempering state, a lot of fine epsilon carbides precipitate inside the martensite laths. Epsilon carbides are good hydrogen trapping sites and are able to enhance the hydrogen embrittlement resistance. After 300 ℃ and 400 ℃ tempering, the hydrogen embrittlement phenomenon becomes serious again. In these tempering states, the fine epsilon carbides transfer to fine cementite. In addition, the film-like cementite also precipitates between the martensite laths. These fine cementite and film-like cementite have weaker hydrogen trapping ability than the epsilon carbide. Especially, the film-like cementite will trap the hydrogen around the martensite laths and this feature is harmful and leads to lower hydrogen embrittlement resistance. Overall, PA500H-B can exhibit better hydrogen embrittlement resistance because it has higher nickel content. These solid-solved nickel atoms in steel will induce the lattice distortion. These distorted lattice sites are also preferential hydrogen trapping sites. This feature will enhance the hydrogen embrittlement resistance. The improvement of hydrogen embrittlement resistance due to nickel addition is only obvious in DQ state. In tempering state, the carbides are stronger hydrogen trapping sites than the nickel induced lattice distortion sites. Hence, the carbides have a dominate effect on the hydrogen embrittlement resistance. As a result, the hydrogen embrittlement resistances in the tempering state are similar for both PA500H-A and PA500H-B steels.