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複合組織鋼의 劈開龜裂形成擧動에 미치는 微視的 內部應力의 影響
兪在煥 世明大學校 1992 世明論叢 Vol.2 No.-
In recent years, many studies have been made on the mechanical properties and fracture behavior of martensite-ferrite dual phase steel which is remarkably noticed as the structural materials for weight reduction. The macroscopic fracture behavior of this steel has been known to be closely related with the microscopic fracture mechanism which depends on microstructural prameters. But there still remain many problems to be solved, especially on the influence of internal stress which is formed in the microstructure by the heat treatment. In this study, special notice was made on the fracture ductility and the behavior of crack formation to clarify the influence of the micro-internal stress on the microscopic and macroscopic fracture behavior of this steel. The results obtained are summarized as follows; (1) The micro-internal stress of the dual phase steel is formed by the volume expansion accompanied by the martensite transmation during the heat treatment. (2) This internal stress is remarkably formed, when the volume expansion ratio of martensite surrounding ferrite and the carbon content of martensite is high. (3)) The cleavage cracks of ferrite is easily formed by this internal stress, so the fracture ductility is decreased.
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吳世旭,姜鎬珉,兪在煥,林東哲 東亞大學校 1988 東亞論叢 Vol.25 No.1
This study has been carried out to investigate the characteristics of fatigue crack of welded zone and heat affected zone of friction welded dual phase steel. The results obtained from the experiments are summarized as follows. Microstructure of welded zone is changed to ferrite and pearlite from ferrite and martensite by wel-ding heat. Micro-vickers hardness of welded zone is higher than that of base metal zone and heat affec-ted zone. Micro-vickers hardness of heat affected zone is lower than that of welded zone and microstructure of heat affected zone is changed to fine martensite and ferrite by heating cycle of welding. Fatigue limits of heat affected zone is a little higher than that of welded zone and base metal zone. There are no difference of fatigue limit of base metal zone and welded zone. Fatigue strength of welded zone is similar to that of base metal zone. At high △K region(△K>19MPa·??), fatigue crack propagation velocity of heat affected zone is slower than that of welded zone and base metal zone, and the difference of fatigue crack propagation velocity is smaller than that of high △K region. The crack occurs to the notch in heat affected zone first, but the final fracture surface is the surface of welded zone. It is abvious that heat affected zone of friction welded dual phase steel superior to welded zone and base metal zone in fatigue strength and resistance of fatigue crack propagation. The above mentioned facts are contrary to theory that heat affected zone of general metals is weaker than welded zone and base metal zone.
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複合組織鋼이 腐蝕疲勞크랙傳播特性에 미치는 食鹽水 濃度의 影響
吳世旭,姜鎬珉,兪在煥 東亞大學校 海洋資源硏究所 1989 硏究論文集 Vol.2 No.1
The corrosion fatigue test was performed under rotated bending in salt solution having a concentration from 0.01 wt percent to 3.5 wt percent to investigate the corrosion fatigue crack propagation characteristics for dual phase steel. Corrosion pits were occurred in martensite, and microcracks occurred at corrosion pits were selectively propagated towared martensite because slip bands were not found in ferrite which were encapsulated by martensite. The number of corrosion pits, the diameter of corrosion pits and the length of corrosion cracks for M.E.F. and raw material(R.M)were increased with the concentration of salt solution and the number of cycles. The initiation of crack was almost same regardless of the concentration of salt solution; approximately N??/N??=20% in case of M.E.F. and N??/N??=14% in case of raw material (R.M.) The difference of corrosion fatigue crack propagation was increased with an increase in concentration of salt solution. The corrosion resistance effect of dual phase steel was smaller than that of raw material (R.M.) in corrosion fatigue crack propagation rate.
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