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1.5 wt % C 고탄소 워크롤용 잉곳의 기공압착 및 기계적 특성 분석
강성훈(S. H. Kang),이승윤(S. Y. Lee),현도빈(D. B. Hyun),김창호(C. H. Kim) 한국소성가공학회 2011 한국소성가공학회 학술대회 논문집 Vol.2011 No.10
In this study, numerical and experimental works carried out to analyze the effects of void size, reduction ratio, forming temperature and specimen rotation on void closure of 1.5% C high carbon steel un upsetting and cogging processes. After these processes, tensile test and microstructure analysis were carried out to check whether the void closure takes place and to analyze the mechanical properties according to the forging conditions. It was found from tensile test and microstructure analysis that the void closure takes place at the reduction ratio of about 30% regardless of forming temperature and void size in upsetting process. In cogging process, the opening and closing of void occurs repeatedly due to the rotation of specimen. It was observed in cogging process that the cylindrical void was changed to dumbbell shape when the reduction ratio increases. It means that the initial void closure at the positioned in the center of the speciment takes place and then final void closure happens near to surface of the specimen. In addition, the void closure occurred at the reduction ratio of about 30% as well. It swas also experimentally revealed that the higher elongation and strength were obtained at the forging temperature of 800 ℃ due to the smallyer and dispersed cementities.
스테인레스계 자유단조품의 공냉 중 크랙발생에 관한 유한요소해석
강성훈(S. H. Kang),김현수(H. S. Kim) 한국소성가공학회 2011 한국소성가공학회 학술대회 논문집 Vol.2011 No.5
In this work, heat treatment simulation was carried out to investigate the reason for crack generation during air cooling of stainless seal housing and housing cover manufactured by open-die forging. These two forged parts were sequently made by upsetting, cogging and piercing processes. After forging, two forged products were heat-treated at the temperature of 1040℃ for 12 hrs and then were cooled at the rate of 30℃/hr for 31 hrs within the furnace. Successive cooling was carried out in the air just after the surface temperature of the forged part reached 100 ℃ in the furnace. However, the large cracks were observed in the longitudinal and hoop directions of the forged seal housing during air cooling. Thus, numerical analysis of air cooling process was carried out in order to find the reason for crack generation. From the simulation results, it was found that the stress state is strongly dependent on the temperature variation and phase transformation. Especially, the tensile and compressive stresses were repeatedly generated within the forged products during cooling process. It was also found that temperature decrease near the surface is strongly related with generation of tensile stress, on the contrary, compressive stress near the surface is resulted from the phase transformation. The predicted tensile stresses in both the longitudinal and hoop directions of seal housing were more than two times than those of housing cover. Thus, it can be concluded that the higher tensile stress led to the crack generation in case of seal housing due to volume effect related with temperature difference between inside and outside of the forged product.