• 한국재료학회지
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• 제13권12호
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• pp.812-818
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• 2003

The aim of this study is to investigate the fabrication processes on the microstructual changes and mechanical properties of large 7175 aluminum die forgings. The billets range from 370 to 720 mm in diameter were homogenized and hot forged after direct chill casting. The strength and elongation of the homogenized cast billets were revealed nearly same level independent of the billet diameter. However, these properties of ø370 mm cast billet were superior to those of $\Pie720$ mm billet under$ T_{6}$ / condition. The tensile strength of die forged specimens under $T_{6}$ condition increased up to 20% than that of solution treatment, however, the elongation was reduced to 50%. The fracture toughness of die forged specimens under $T_{6}$ condition was 35.6∼39.0 MPa$.$$m^{1}$2 irrespective of the billet size and free forging processes, but this property increased up to 10% by$V_{74}$ treatment. The fracture toughness of die forged specimen manufactured with ø370 mm cast billet showed nearly same level of ø720 mm billet which was processed using MF or Cog free forging followed by die forging.

• 소성∙가공
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• 제16권2호
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• pp.126-131
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• 2007

Voids in a large rotor are formed in solidification process of a cast ingot. The voids have to be eliminated from the rotor by a forming process, because they would became stress-intensity factors which suddenly fracture the rotor in the operation. Previous studies on void-elimination of a large rotor have mainly focused on finding the process variables affecting the void-closure. But the study on the amount of void closure in a large rotor has been very rare. This study was performed to obtain an equation which predicts the amount of void-closure in a forging process of a large rotor and to evaluate the availability of the void-closure equation through finite element analyses. Firstly, 2D FE analysis was carried out to find effects of time integral of hydrostatic stress and effective strain on void volume rate of a large rotor in the upsetting process for various diameters and shapes of void, and material temperature. From the 2D FE analysis, we found that effective strain was suitable for predicting the void-closure of a large rotor, because there was a constant relationship between void volume rate and effective strain. And a void-closure equation was proposed fur predicting void-closure of a large rotor in the upsetting process. Finally, ken the 3D FE analysis, the proposed void-closure equation was verified to be useful for upsetting and cogging processes.

• 한국재료학회지
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• 제14권3호
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• pp.168-174
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• 2004

The aim of this study is to investigate the effect of process conditions on the microstructual changes and mechanical properties of large 7175 aluminum die forgings. The cast billets of 370 and 720 mm in diameter were homogenized and die forged after direct chill casting. The size and volume fraction of second phase particles in 720 mm billet were larger than those of 370 mm billet. The interdendritic sites containing the second phase particles was considered to be a crack initiation region in the process of cold upsetting. The tensile and yield strength of die forged specimens of 720 mm billet in the direction of Land L T were higher than those of 370 mm billet. However, the tensile strength of these specimens were 5 to 10% lower than that of American military specification. The plane strain fracture toughness of die forged specimens of 370 mm cast billet showed almost the same level of 720 mm billet, which was die forged after free forging.

• 열처리공학회지
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• 제17권1호
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• pp.10-16
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• 2004

The aim of this study is to investigate the process conditions on the microstructual changes and mechanical properties of large 7175 aluminum ring roll forgings. The billets range from 370 mm to 720 mm in diameter were homogenized and ring roll forged after direct chill casting. The tensile properties of ring roll forged specimen manufactured with ${\Phi}370mm$ billets were superior to those of ${\Phi}720mm$ billets under $T_6$ condition. Also, these properties showed better than those of military specification under $T_{74}$ treatment. The impact value of ring roll forged specimen under $T_{74}$ treatment increased up to 20% than that of $T_6$ condition. The fracture toughness of ring roll forged specimen manufactured with ${\Phi}370mm$ cast billet showed nearly same level of ${\Phi}720mm$ billet which was processed using MF or Cog free forging followed by ring roll forging.

• 한국주조공학회지
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• 제39권1호
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• pp.7-13
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• 2019

Few studies of large blooms over 700 mm thick among those used for the forging of raw materials have been reported. The cooling rate difference between the surface and the center of a large bloom is large, and the degradation of the mechanical properties is likely in cases involving excessively coarse precipitates resulted from the slow cooling rate of a large bloom after casting. Therefore, a schematic investigation of the growth behaviors of precipitates while varying their locations in blooms is necessary. The dissolution behaviors of precipitates were investigated by simulating a reheating process during which the bloom is heated to a high temperature. The segregation behavior of the as-cast large bloom was also investigated. Reheating specimens were obtained after an isothermal heat treatment at $1150^{\circ}C$ with various holding times to simulate the reheating process, with the samples undergoing a subsequent water quenching step. The precipitates were extracted using an electrolytic extractor and a particle size analysis was conducted with the aid of SEM, EDS, and TEM. In the present work, Al oxide, MnS and Nb carbide were mainly observed.

• 헤리티지:역사와 과학
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• 제53권1호
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• pp.168-183
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• 2020

기원전 5세기부터 중국에서는 주조철기의 사용을 늘리기 위해 주조철기 특유의 취성(脆 性)을 제거하려 노력해 왔다. 그중 한 가지는 주조철기를 생산한 다음 별도의 가마에서 풀림열처리를 하는 기술이다. 이 기술은 조직 내 탄소를 응집하거나 제거해, 주조철기의 취성을 제거하고 단조(鍛造)를 가능하게 만드는 것이다. 풀림열처리 기술의 실체를 명확히 하기 위해 현재의 제철 기술 및 중국·일본의 연구 성과를 바탕으로 한반도 출토 고대 주조철기 가운데, 풀림열처리 기술이 확인된 가단주철제 주조철기를 검토하였다. 그 결과 원삼국시대 이전까지 가단주철제 주조철기는 모두 외부로부터 수입된 것으로 추정된다. 또한 일본 이시가미신궁에 소장 중인 칠지도가 한반도에서 제작된 것으로 본다면, 4세기에 들어서야 백제에서는 풀림열처리 기술을 확보했을 가능성이 있다. 하지만 당시에는 괴련철을 중심으로 철 생산이 이루어졌으며, 괴련철을 정련한 괴련강을 소재로 다량의 단조철기가 제작되었다. 또한 기존의 주조철기 중 주조괭이를 제외한 대부분의 주조철기가 단조철기로 대체되었다. 따라서 삼국시대까지 풀림열처리 기술은 많이 사용되지 못하였다. 하지만 통일신라시대에 들어서 철 생산 및 유통의 지역 거점인 장안리 유적에서 확인될 정도로 기술이 확산된 것을 확인하였다.

• Journal of Welding and Joining
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• 제32권3호
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• pp.95-101
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• 2014

Joining of grey cast iron by fusion welding has much difficulties for its extremely low ductility and low toughness because of the flake form of the graphite. And the brittle microstructure, i.e. ledeburite may be formed during fusion welding by its rapid cooling rates. By these kinds of welding problem, preheat and post heat treatment temperature must be increased to avoid weld crack or welding problems. In order to avoid these fusion welding problem, friction welding of cast iron was carried out for improving joint soundness, establishing friction welding variables. There is no factor for evaluating friction weldability in continuous drive type friction welding. In this point of view, this study proposed the parameters for calculating friction weld heat input. The results obtained are as follows ; 1. There was a close relationship between tensile strength and flash appearance of friction welded joint. 2. Tensile strength was decreased and flash was severely oxidized as increasing frictional heating time. 3. As increased forging pressure $P_2$, flash had a large crack and tensile strength was decreased. 4. As powdered graphite by rotational frictional force induced flat surface and hindered plastic flow of metal, tensile strength of welded joint was decreased. 5. Heat input for continuous drive type friction welding could be calculated by the factors of $P_1$, $P_2$ and upset distance(${\delta}$).